Embed Size (px)
Citation preview
Basic Concepts for Genetic Testing in Common Hereditary Colorectal
Cancer SyndromesKristina Markey, MS, Lisen Axel, MS, and Dennis Ahnen, MD
AddressDenver Department of Veterans Affairs Medical Center and University of Colorado Hospital Hereditary Cancer Clinic, 1055 Clermont Street, Denver, CO 80220, USA. E-mail: [email protected] Gastroenterology Reports 2002, 4:404–413Current Science Inc. ISSN 1522-8037Copyright © 2002 by Current Science Inc.
IntroductionThe last decade has brought compelling advances in theunderstanding of how genetic predisposition leads to dis-ease. This understanding has greatly facilitated the identifi-cation of families with hereditary cancer. This articleidentifies the cardinal features of hereditary cancer syn-dromes and highlights the complex nature of genetic test-ing. To illustrate these concepts, the usual strategy forgenetic testing and result interpretation for the two mostcommon forms of hereditary cancer—hereditary nonpoly-posis colorectal cancer (HNPCC) and familial adenoma-tous polyposis (FAP)—is outlined. This article does not
attempt to review the medical management of these syn-dromes or detail the genotype–phenotype correlations;rather, it offers a practical background for the practitionerwho is involved with cancer risk assessment.
Hereditary CancerIt is estimated that about 5% to 10% of all cancers arecaused by a germline single gene mutation. Such suscepti-bility mutations are inherited in an autosomal dominantfashion, so that each offspring (male or female) of a muta-tion carrier has a 50% chance of inheriting that cancer sus-ceptibility. Thus, most individuals with hereditary canceralso have affected relatives in multiple generations, but thisis not always the case, because of small family size, variablepenetrance, and the possibility that new mutations canoccur in these genes. The highly penetrant cancer suscepti-bility mutations cause an increased risk for cancer at ayoung age and multiple primary cancers, sometimes in theform of synchronous tumors. If a practitioner is able to rec-ognize the clues of hereditary cancer susceptibility from apatient’s personal and family history, subsequent modifi-cations to the patient’s cancer screening and treatment reg-imen can have lifesaving potential for the patient and at-risk relatives.
The patient’s personal history of cancer and a carefullyperformed family history of cancer are the cornerstones offamilial cancer risk assessment. For patients with coloncancer, information about age at diagnosis, location, his-tology, and stage, as well as history of concurrent or previ-ous colonic adenomas and all other cancers in the family,should be routinely obtained. Some practitioners havefound that a family history screening worksheet is helpfulin this process. For those with a suspicious family history, amore detailed pedigree can be obtained that includesinformation on all close relatives. For all relatives it isimportant to ask about whether they have had any cancer,the cancer type, ages at cancer diagnosis, current age, his-tory of chronic diseases that predispose to cancer (particu-larly Crohn’s disease and ulcerative colitis for colon cancerrisk), potential occupational and environmental expo-sures, age at and cause of death, and ethnic background.Other screening points that can be helpful for colon cancer
Approximately 5% of colorectal cancers are associated with one of the autosomal dominant hereditary cancer syndromes. The two most common familial colon cancer syndromes are familial adenomatous polyposis (FAP) and hereditary nonpolyposis colorectal cancer (HNPCC). The causative mutation can be identified in many families with these syndromes by genetic testing of an affected individual. If an affected individual tests positive for a disease-causing mutation, genetic testing of unaffected, at-risk family members can be performed to determine whether they have inherited the cancer-susceptibility mutation, and a personalized cancer surveillance strategy can be devised. Genetic testing significantly enhances cancer risk assess-ment in these families. However, the complicated nature of result interpretation and the emotional impact of the result necessitate that testing be carried out in conjunction with patient education and informed consent by a physician who has a keen appreciation for the inherent challenges. This article describes the genetic testing strategy in HNPCC and FAP.
Genetic Testing in Colorectal Cancer • Markey et al. 405
families include unusual skin lesions or freckling, “lumpsor bumps” (such as fibromas, lipomas, cysts, or keratoses),and dental abnormalities. It is also important to notewhich pieces of the history are limited or unknown.
Although an accurate family history is critical to theassessment of familial cancer risk, it is important to under-stand the limitations of self-reported family history. Stud-ies have documented that the details of the cancer-focusedfamily history are not usually all-inclusive and sometimesinclude erroneous information [1]. Often the details ofcancer diagnosis are unknown, incomplete, or misunder-stood. For these reasons, confirming critical cases in thepedigree through review of pathology records is often nec-essary for accurate assessment of the family history.
The family history information can be assembled intoa pedigree to assess the likelihood of a familial cancersyndrome. The questions asked about the pedigreeinclude the following: Is there an autosomal dominantpattern of cancer in the family? Do the types of cancer fitinto a known cancer syndrome? Do the cancers in thefamily occur at an unusually young age? Is there anyonewith a bilateral cancer or multiple primary cancers? Arethere any rare cancers in the family? Are there several rela-tives who have the same type of cancer? An evaluation ofthe ages at death can provide important informationabout unaffected relatives, because sometimes deathoccurs before the age that cancer would have become evi-dent. In addition to defining family risk, family historycan also be the only way to identify unaffected familymembers who need special cancer surveillance.
Prior to the availability of genetic testing, the diagnosisof a hereditary cancer syndrome in a family would bemade on the basis of established clinical criteria alone (asdescribed for HNPCC and FAP in the following sections),and all at-risk relatives would be advised to have increasedscreening and surveillance for the development of cancer.However, not all close relatives have a high risk for cancer.Each first-degree relative of an affected individual has a50% risk of having inherited the cancer susceptibilitymutation, so there is an equal chance that the mutationwas not inherited and the general population cancer sur-veillance would be adequate. Genetic testing allows clini-cians to identify the 50% of at-risk family members whohave inherited the disease-causing gene so that they can bescreened appropriately without excessive screening ofunaffected family members. When a cancer susceptibilitymutation is identified for a family, predictive genetic test-ing for unaffected relatives can be done inexpensively, withclose to 100% accuracy. Because many cancer-prone fami-lies do not meet the strict criteria for a clinical diagnosis ofa cancer-susceptibility syndrome, genetic testing, if posi-tive, can clarify whether a genetic syndrome is present andidentify the cancer risks. It is important to note, however,that some families that meet the clinical criteria of a famil-ial cancer syndrome may not have a detectable geneticmutation, either because their cancer syndrome is caused
by mutations in other undiscovered genes or because thecurrent testing method is not able to detect the type ofmutation that is present. Nonetheless, in families with anidentified mutation, genetic testing can define cancer riskand allow for a personal cancer surveillance program to betargeted to gene carriers within the family.
Several medical organizations have offered guidelinesthat support genetic testing for cancer risk. Some of theseinclude the American Gastroenterological Association [2],American Society of Clinical Oncology [3], American Col-lege of Medical Genetics [4], and American Society ofHuman Genetics [4]. Within the statements of thesegroups, the following concepts are stressed: First, pretestgenetic counseling and written informed consent shouldproceed genetic testing. Second, an affected individualshould be the first in the family to be tested, whenever pos-sible. Third, genetic testing should be restricted to appro-priate families (those that have a substantial pretestprobability of having a germline mutation) and used onlywhen the results will alter clinical management. Also, thetesting should be initiated at the age when specialized can-cer screening would begin. Finally, the practitioner must beable to interpret the results adequately, given the clinicalscenario. Genetic testing offers many benefits, but becauseof the inherent challenges, it should be facilitated andinterpreted by well-informed practitioners.
Informed consentInformed consent is essential for genetic testing, and theissues surrounding gene testing can be quite complex. TheAmerican Society of Clinical Oncology has identified somecritical elements to be discussed in conjunction with writ-ten informed consent for genetic testing [3]. First, informa-tion should be provided about the specific test beingperformed, and the implications of a positive, a negative,and an uninformative result should be discussed. Second,education about the mode of inheritance and the optionsand limitations of medical surveillance and screening fol-lowing testing should be provided. Third, the risks ofgenetic testing should be discussed, including risk of psy-chological distress, risk of insurance and employment dis-crimination, and patient confidentiality. Fourth, thetechnical accuracy of the test and the cost of genetic coun-seling and genetic testing should be explained. Also ofnote, the patient should understand that risk assessmentcan occur without genetic testing and that DNA banking isanother possibility for future testing. In addition to manyof these points, the Cancer Genetics Studies Consortiumemphasizes that practitioners should assist patients inexploring their related personal values and facilitate shareddecision making about genetic testing [5].
Genetic counselingNot all practitioners have the time or the experience toprovide adequate risk assessment and genetic counseling.Specialized programs have been developed to provide
406 Large Intestine
comprehensive services in cancer genetics, and many canbe located at http://www.cancer.gov/search/genetics_services. If such a center is not located within areasonable vicinity, a referral to a genetic counselor shouldbe considered. Genetic counselors are Master’s-degree-levelgraduates with special expertise in medical genetics and thepsychosocial and ethical elements of genetic disease; theycan act as physician extenders. Many cancer genetic coun-selors are available nationwide and can be easily identifiedat http://www.nsgc.org/resourcelink.asp.
Hereditary nonpolyposis colorectal cancerHereditary nonpolyposis colorectal cancer, formerlycalled Lynch syndrome, is the most common hereditarycolorectal cancer syndrome. Early evidence suggestedthat 5% to 8% of all colon cancers could be caused byHNPCC [6], but the actual frequency appears to be lessthan 5%. HNPCC is an autosomal dominantly inheritedsyndrome associated with a high risk of colorectal,endometrial, and selected other cancers. The colon can-cers in HNPCC tend to occur at a younger age, are morecommonly proximal, and are more commonly multiplethan sporadic colorectal cancers.
The clinical diagnosis of HNPCC is usually made onthe basis of a set of clinical criteria such as the Amsterdamor Bethesda criteria (Table 1) and appropriate genetic test-ing. The syndrome should be considered in individualswith a personal or family history of colorectal cancers thatoccur at an early age, have a proximal location in thecolon, and are multiple, and if there is an autosomal dom-inant pattern of cancer inheritance in the family. The pres-ence of endometrial cancer in the family is another clue tothe diagnosis of HNPCC. The mean age of diagnosis of col-orectal cancer is around 45 years in HNPCC comparedwith about 67 years for sporadic colorectal cancers. Abouttwo thirds of cases of colon cancer in HNPCC occur proxi-mal to the splenic flexure, whereas half to two thirds ofsporadic colorectal cancers are distal to the splenic flexure.About 35% to 50% of patients with HNPCC have synchro-nous or metachronous colorectal cancers, whereas the rateis less than 10% in patients with sporadic colorectal can-cers. By the age of 70 years, individuals with HNPCC have a70% to 82% risk for colorectal cancer and a 42% to 60%risk for endometrial cancer [7–9]. New germline mutationscausing HNPCC are rare, so that almost all patients withHNPCC will have a suggestive family history if the familysize is not too small.
Extracolonic cancer is an integral part of HNPCC. Inaddition to the very high risk of endometrial cancer inwomen with HNPCC, an increased risk of ovarian, stom-ach, urinary tract, and biliary tract cancers has beenobserved in HNPCC families. The cumulative risk for theseother HNPCC cancers ranges from around 10% to around20% [10,11]. The association between familial colon cancerand brain tumors has been classified historically as Turcotsyndrome, which initially was thought to be a variant of
FAP. Genetic studies of families with clinical Turcot syn-drome, however, have identified families with HNPCC andbrain tumors (mostly glioblastomas) and other familieswith FAP and brain tumors (mostly cerebellar medullo-blastomas). Muir-Torre syndrome includes the typicalHNPCC features as well as sebaceous gland tumors(benign and malignant) and keratoacanthomas.
The histologic features of colon cancers may provideuseful clues to the diagnosis of HNPCC in some families.Usually, HNPCC colon tumors are poorly differentiatedand grow in a solid pattern, showing abundant mucin pro-duction caused by the presence of signet ring cells. Tumorinfiltrating lymphocytes, similar to those seen in Crohn’sdisease, have also been reported. Evidence so far has notsuggested that the endometrial cancers in HNPCC have
Table 1. Criteria for diagnosis and genetic testing in HNPCC
Amsterdam IAll of the following must be met
3 relatives with colorectal cancer involving at least2 generations, 1 a first-degree relative of the
other 2 and1 colorectal cancer diagnosed before age 50
FAP should be excludedAmsterdam II
All of the following must be met3 relatives with an HNPCC-associated tumor (CRC,
endometrial, small bowel, ureter, or renal pelvis) involving at least
2 generations, 1 a first-degree relative of the other 2 and 1 or more cases diagnosed before age 50
Bethesda FAP should be excludedAny one of the following is sufficient to fulfill criteriaAny one of the first three is referred to as “modified
Bethesda” and is considered sufficient to proceed with germline DNA testing
1. Individuals with cancer in families who meet the Amsterdam criteria
2. Two HNPCC-related cancers in one individual (synchronous and metachronous CRC or associated extracolonic cancers, endometrial, ovarian, gastric, hepatobiliary, small bowel cancer, or transitional cell carcinoma of the renal pelvis or ureter
3. One individual with CRC and one first-degree relative with CRC and/or colorectal adenoma and/or HNPCC related cancer; one of the cancers diagnosed before age 45 and the adenoma before age 40
4. Individuals with colorectal cancer or endometrial cancer diagnosed before age 45
5. Individuals with right-side CRC with an undifferentiated pattern on histopathology diagnosed before age 45
6. Individuals with signet ring cell type CRC diagnosed before age 45
7. Individuals with adenomas diagnosed before age 45
CRC—colorectal cancer; FAP—familial adenomatous polyposis; HNPCC—hereditary nonpolyposis colorectal cancer.
Genetic Testing in Colorectal Cancer • Markey et al. 407
any distinguishing histologic characteristics. A recentretrospective review reported that most HNPCC-relatedovarian tumors (94%) appear to be epithelial, with 72%being well or moderately differentiated, in marked contrastwith the histology of ovarian cancer in the familial breastcancer syndromes caused by mutations in BRCA1 andBRCA2, which are usually late-stage and high-grade serouscarcinomas [12].
Genetics of HNPCCThe genetic mutations that cause HNPCC lead to inactiva-tion of one of a set of genes that are necessary for DNAmismatch repair (MMR) [10–12,13•,14]. When function-ing normally, the products of these DNA repair genes formcomplexes that identify mistakes that occur during theDNA replication process and correct them before the nextcell division occurs. It is thought that both alleles of anMMR gene are inactivated in HNPCC tumors, one by germ-line and the other by somatic mutation. In addition to sin-gle base substitution errors, mutations in MMR genes leadto gains or losses in the number of short repeat sequencesthat are scattered throughout normal DNA. The latter typeof error leads to marked differences in the length of thesesequences between normal and cancer tissue, called micro-satellite instability (MSI). More than 90% of HNPCC col-orectal cancers show MSI. MSI also has been shown tooccur in HNPCC-related endometrial and stomach cancertumors [13•]. It is thought that the increased mutation ratein MMR deficient cells can lead to mutations in oncogenesor tumor suppressor genes that then drive the processof carcinogenesis.
HNPCC can be caused by germline mutations in atleast six different MMR genes. About two thirds of familieswho meet the clinical criteria for HNPCC will have a muta-tion in one of these genes. Data from the InternationalCollaborative Group on HNPCC mutation database(http://www.nfdht.nl) indicate that the two most commongermline mutations found in HNPCC are in the MSH2 andMLH1 DNA repair genes, accounting for approximately90% of all the detectable mutations in HNPCC families.The remaining families have germline mutations in MSH6,PMS1, PMS2, and MLH3. The search is ongoing for othergenes that are involved in the one third of families whomeet the clinical criteria for HNPCC but have no detect-able mutation in the known MMR genes.
The diagnosis of HNPCC is still made clinically inmany instances, and family history is the most importanttool in identifying HNPCC families. In 1991 the Amster-dam criteria (Table 1) were set forth to improve uniformityin diagnosis; these criteria were based solely on the patternof colorectal cancer in the family. Of families meetingAmsterdam criteria, about 70% have detectable mutationsin MSH2, MLH1, PMS1, or PMS2 [6]. Concerns that theAmsterdam I clinical criteria failed to capture manyHNPCC families led to the development of the AmsterdamII criteria, which allowed noncolonic HNPCC cancers to be
included in the analysis [15••], and the Bethesda criteria(Table 1), which were designed to help identify patientswho could benefit from MSI screening. Because over 90%of HNPCC colorectal tumors are MSI positive, patientswith MSI-positive tumors would then be offered mutationanalysis for MMR genes.
Genetic testing for HNPCCIf an affected individual meets Amsterdam I or AmsterdamII criteria, proceeding directly to mutation analysis is rec-ommended. The American Gastroenterological Associa-tion also recommends proceeding with DNA testing whenany of the first three Bethesda criteria are present [2].Approximately 40% of individuals who meet either of theAmsterdam or these “modified Bethesda criteria” areexpected to have a mutation in MLH1 or MSH2 [16•]. Forindividuals who meet any one of the other Bethesda crite-ria, genetic testing is initiated by first testing MSI status of acolon tumor from an affected individual. Those individu-als who have a high level of MSI in tumor tissue are thenoffered germline DNA testing.
Multiple laboratories offer genetic testing for HNPCC,and the methodologies vary among laboratories. Withfive MMR genes and over 300 different mutations docu-mented among HNPCC families, the undertaking ofgenetic testing is complex, time consuming, and expen-sive. Clinical testing for MSH2 and MLH1 is available, butthe other MMR genes are evaluated only on a researchbasis. Many laboratories begin with a more generalscreening test (Table 2) and use those results to direct thespecific mutational analysis. Others use complete DNAsequencing without pretest screening. Because approxi-mately 80% of mutations in MSH2 and MLH1 cause atruncated protein, some laboratories use a protein trunca-tion test (PTT) to direct the mutation search. Also, single-strand confirmation polymorphism (SSCP) and confir-mation strand gel electrophoresis (CSGE) may be used asa screening step, but up to 10% to 40% of mutations thatare detected by DNA sequencing are missed with thesemethods [17•]. Denaturing gradient gel electrophoresisappears to detect up to 90% to 95% of mutations present[18]. Because most of the mutations in MMR genes aretruncating mutations, immunohistochemical staining ofMLH1 and MSH2 is used in some laboratories to screenfor decreased expression of the full-length proteins [19].However, a relatively high proportion of sporadic colorec-tal tumors exhibit MLH1 promoter hypermethylation,which causes reduced expression of the MLH1 protein, sothat loss of protein expression does not establish the pres-ence of an HNPCC gene mutation. Further research willlikely clarify the best utilization of the various screeningmethods. In the mean time, the ordering practitionershould be aware of the methods used to test a givenpatient and the potential limitations of with that mode oftesting, particularly in the event that a mutation isnot identified.
408 Large Intestine
Tab
le 2
.D
escr
ipti
on
of g
enet
ic s
cree
ning
and
dia
gno
stic
tes
t m
etho
ds
Tes
tM
ediu
mD
escr
ipti
onIn
terp
reta
tion
/stu
dy
Mic
rosa
telli
te in
stab
ility
(M
SI)
Para
ffin-
embe
dded
or
fres
h tu
mor
an
d no
rmal
tiss
ue
Scre
enin
g te
st fo
r HN
PCC
(>90
% o
f HN
PCC
col
orec
tal
tum
ors
have
MSI
; 15%
to 2
0% o
f spo
radi
c co
lore
ctal
tu
mor
s ha
ve M
SI,
MSI
hig
h: >
30%
to 4
0% o
f the
mar
kers
are
uns
tabl
e
Min
imum
ref
eren
ce p
anel
of 5
mar
kers
M
SI lo
w: <
30%
to 4
0% o
f the
mar
kers
are
uns
tabl
eA
naly
sis fo
r in
stab
ility
(exp
ansio
ns o
r co
ntra
ctio
ns) o
f sh
ort n
ucle
otid
e re
peat
seq
uenc
es (m
icro
sate
llite
s)
MSI
sta
ble:
no
mar
kers
are
uns
tabl
e
If M
SI is
hig
h, p
roce
ed to
gen
e te
stin
gIm
mun
ohist
oche
mist
ry
(IHC
)Pa
raffi
n-em
bedd
ed
tum
or a
nd
norm
al ti
ssue
Eval
uate
s exp
ress
ion
of M
LH1
and
MH
S2 g
ene
prod
ucts
in
tum
or ti
ssue
Inac
tivat
ing
mut
atio
ns a
nd h
yper
met
hyla
tion
of th
ese
DN
A r
epai
r ge
nes
lead
s to
loss
of p
rote
in e
xpre
ssio
n in
the
canc
er ti
ssue
Can
dire
ct w
hich
mism
atch
rep
air
gene
sho
uld
be
targ
eted
for
gene
seq
uenc
ing
in H
NPC
C
Use
d in
FA
P an
d so
met
imes
HN
PCC
bec
ause
the
germ
line
mut
atio
ns r
espo
nsib
le fo
r th
ese
synd
rom
es
ofte
n ar
e tr
unca
ting
mut
atio
nsm
RNA
is is
olat
ed fr
om th
e pa
tient
's ly
mph
ocyt
es a
nd
reve
rse
tran
scrib
e th
e m
RNA
into
DN
A; t
hen
in v
itro
tran
scrib
e an
d tr
ansla
te th
e cD
NA
into
pro
tien
Ofte
n fo
llow
ed b
y D
NA
seq
uenc
ing
if PT
T is
dete
cted
Det
ects
bet
wee
n 60
% an
d 95
% o
f mut
atio
ns w
ithin
shor
t D
NA
str
ands
DN
A is
che
mic
ally
den
atur
ed in
to s
ingl
e st
rand
s w
hich
th
en fo
ld u
pon
them
selv
es; t
he sh
ape
of th
e st
rand
with
m
utat
ions
may
diff
er fr
om th
ose
stra
nds
with
out
mut
atio
ns
Cau
tion:
spo
radi
c co
lore
ctal
tum
ors
that
are
MSI
hig
h of
ten
have
hy
perm
ethy
latio
n of
MLH
1, r
esul
ting
in lo
ss o
f thi
s pr
otei
n w
ithin
th
e tu
mor
cel
ls; a
n IH
C a
ssay
sho
win
g lo
ss o
f one
of t
hese
pro
tein
s do
es n
ot m
ean
the
patie
nt h
as H
NPC
C b
ut c
ould
pro
mpt
gen
e te
stin
g
Prot
ein
trun
catio
n te
stin
g (P
TT)
Bloo
dTr
unca
ted
prot
eins
(sm
alle
r siz
e) in
dica
te a
mut
atio
nW
ill n
ot id
entif
y m
issen
se m
utat
ions
or
who
le-g
ene
dele
tions
, and
m
isint
erpr
etio
n (fa
lse p
ositi
ve) c
an o
ccur
whe
n an
alyz
ed p
rodu
cts
resu
lting
from
alte
rnat
ive
splic
ing
sites
are
inte
rpre
ted
as tr
unca
ted
prot
ein
prod
ucts
[34
] Tr
unca
tions
clo
se to
the
5’ e
nd a
nd fr
ames
hift
mut
atio
ns in
the
3’ e
nd
may
go
unde
tect
ed [3
5]
Sing
le-s
tran
d co
nfor
mat
ion
poly
mor
phism
(SSC
P)
anal
ysis
Bloo
dTh
e w
ild ty
pe a
nd m
utan
t str
ands
run
diff
eren
tly o
n ge
l el
ectr
opho
resis
Iden
tifie
s po
int m
utat
ions
, sm
all d
elet
ions
and
inse
rtio
ns, a
nd s
plic
e-sit
e m
utat
ions
The
susp
icio
us r
egio
n is
then
seq
uenc
ed to
def
ine
the
mut
atio
nD
enat
urin
g gr
adie
nt g
el
elec
trop
hore
sis (D
GG
E)Bl
ood
Scre
ens
exon
s to
det
ect s
mal
l cha
nges
in D
NA
cod
e
Iden
tifie
s po
int m
utat
ions
, sm
all d
elet
ions
and
inse
rtio
ns, a
nd s
plic
e-sit
e m
utat
ions
. PC
R pr
oduc
ts a
re d
enat
ured
to s
epar
ate
the
two
DN
A
stra
nds,
whi
ch a
re r
un o
n ge
ls w
ith a
n in
crea
sing
grad
ient
of u
rea
and
form
alde
hyde
; the
nor
mal
and
m
utan
t str
ands
hav
e di
ffere
nt m
obili
ty o
n el
ectr
opho
resis
DG
GE
miss
es m
utat
ions
in p
rom
oter
or
enha
ncer
reg
ions
, cry
ptic
ch
ange
s w
ithin
non
codi
ng s
eque
nces
, and
larg
e de
letio
ns s
pann
ing
who
le e
xons
or
the
who
le g
ene
[18]
The
susp
icio
us r
egio
n is
then
seq
uenc
ed to
def
ine
the
mut
atio
n
FAP—
fam
ilial
ade
nom
atou
s po
lypo
sis;
HN
PCC
—he
redi
tary
non
poly
posi
s co
lore
ctal
can
cer;
PC
R—
poly
mer
ase
chai
n re
actio
n.
Genetic Testing in Colorectal Cancer • Markey et al. 409
Con
form
atio
n-se
nsiti
ve
gel e
lect
roph
ores
is (C
SGE)
Bloo
d>9
0% s
ensit
ivity
to d
etec
t mut
atio
nsD
NA
is a
mpl
ified
by
PCR,
whi
ch is
then
hea
t den
atur
ed
and
allo
wed
to r
eann
eal a
t low
er te
mpe
ratu
res;
if a
m
utat
ion
is pr
esen
t, th
ree
diffe
rent
ban
ds w
ill fo
rm o
n th
e ge
l
Iden
tifie
s po
int m
utat
ions
, sm
all d
elet
ions
and
inse
rtio
ns, a
nd s
plic
e-sit
e m
utat
ions
CSG
E ca
n m
iss m
utat
ions
in p
rom
oter
or
enha
ncer
reg
ions
, cry
ptic
ch
ange
s w
ithin
non
codi
ng s
eque
nces
, and
larg
e de
letio
ns s
pann
ing
who
le e
xons
or
the
who
le g
ene
The
susp
icio
us r
egio
n is
then
seq
uenc
ed to
de
fine
the
mut
atio
nPr
otei
n tr
unca
ting
mut
atio
ns a
re u
sual
ly in
terp
rete
d as
del
eter
ious
; no
nsen
se, i
nser
tion,
del
etio
n m
utat
ions
are
mor
e lik
ely
path
ogen
ic
than
miss
ense
DN
A s
eque
ncin
gBl
ood
Hig
hest
sen
sitiv
ity b
ut e
xpen
sive
and
time
cons
umin
gC
omm
only
don
e ei
ther
as
the
first
gen
etic
test
or
afte
r a
posit
ive
MSI
, IH
C, P
TT, o
r SS
CP/
CSG
EPC
R am
plifi
es e
xons
and
som
etim
es th
e re
gion
s ju
st
outs
ide
thes
e co
ding
seq
uenc
esSe
quen
cer
is us
ed to
iden
tify
each
bas
e pa
ir in
the
sequ
ence
, sea
rchi
ng fo
r an
err
or
Miss
ense
mut
atio
ns a
re d
iffic
ult t
o in
terp
ret b
ecau
se th
ey m
ight
be
norm
al p
olym
orph
isms;
man
y la
bora
torie
s us
e th
e cr
iterio
n th
at if
th
e va
riant
is s
een
in m
ore
than
a c
erta
in p
ropo
rtio
n (o
ften
2% to
5%
) of t
he g
ener
al p
opul
atio
n, it
is c
onsid
ered
a p
olym
orph
ism
Fam
ily s
tudi
es m
ight
indi
cate
whe
ther
the
alte
ratio
n is
trac
king
with
th
e ca
ncer
in th
e fa
mily
Inve
rsio
ns, d
elet
ions
, and
dup
licat
ions
can
be
miss
ed
Non
e of
the
abov
e m
etho
ds w
ould
iden
tify
Larg
e de
letio
ns o
r in
sert
ions
Mut
atio
ns in
intr
on/p
rom
oter
Gen
omic
rea
rran
gem
ents
Som
e sp
lice-
site
mut
atio
nsM
utat
ions
in th
e pr
omot
er r
egio
n or
intr
onic
reg
ions
Sout
hern
blo
tBl
ood
Can
ass
ess
for
maj
or r
earr
ange
men
ts w
ithin
a g
ene
and
larg
e de
letio
ns10
% o
f MSH
2 m
utat
ions
app
ear
to b
e de
letio
ns [3
6]
Link
age
anal
ysis
Bloo
dC
onsid
ered
in F
AP
fam
ilies
if th
e sp
ecifi
c m
utat
ion
cann
ot b
e de
tect
ed
Nee
d ac
cura
te c
linic
al d
iagn
oses
of p
artic
ipat
ing
fam
ily m
embe
rs a
nd
accu
rate
ass
ignm
ent o
f bio
logi
c re
latio
nshi
psD
NA
mar
kers
clo
se o
r w
ithin
dise
ase
gene
are
co
mpa
red
betw
een
fam
ily m
embe
rs to
iden
tify
the
chro
mos
ome
linke
d w
ith th
e di
seas
e-ca
usin
g m
utat
ion
Requ
ires
will
ing
part
icip
atio
n fr
om a
t lea
st tw
o af
fect
ed fa
mily
m
embe
rs, s
o ca
nnot
be
used
in c
ases
of n
ew m
utat
ions
Som
e fa
mili
es a
re n
ot in
form
ativ
e fo
r lin
ked
mar
kers
, and
this
met
hod
cann
ot b
e us
edTh
is m
etho
d en
able
s det
ectio
n of
mut
atio
ns in
pro
mot
er r
egio
ns a
nd
othe
r m
utat
ions
that
affe
ct p
rote
in e
xpre
ssio
n
Fam
ily m
embe
rs w
ould
be
stud
ied
for
the
sam
e pa
tter
nC
linic
al u
tility
is s
till u
nder
eva
luat
ion
Con
vers
ion
tech
nolo
gy
(pre
viou
sly c
alle
d m
onoa
llelic
mut
atio
n an
alys
is [M
AM
A])
Bloo
dM
ost g
enet
ic te
chno
logi
es a
re li
mite
d in
that
the
norm
al a
llele
can
obs
cure
the
abno
rmal
alle
le, w
hich
th
en r
emai
ns u
ndet
ecte
dIs
olat
es a
sin
gle
alle
le in
a r
oden
t som
atic
cel
l hyb
ridTh
e m
ater
nal a
nd p
ater
nal a
llele
can
then
be
exam
ined
in
depe
nden
tly
Tab
le 2
.D
escr
ipti
on
of g
enet
ic s
cree
ning
and
dia
gno
stic
tes
t m
etho
ds (
Cont
inue
d)
Tes
tM
ediu
mD
escr
ipti
onIn
terp
reta
tion
/stu
dy
FAP—
fam
ilial
ade
nom
atou
s po
lypo
sis;
HN
PCC
—he
redi
tary
non
poly
posi
s co
lore
ctal
can
cer;
PC
R—
poly
mer
ase
chai
n re
actio
n.
410 Large Intestine
Familial adenomatous polyposisFamilial adenomatous polyposis (FAP), also called ade-nomatous polyposis coli (APC), is a rare (about 1/8000 to1/10,000 births) autosomal dominantly inherited syn-drome with nearly 100% penetrance. The hallmark featureof FAP is the presence of numerous adenomatous polypsof the large bowel, with most affected individuals havinghundreds to thousands of colonic adenomas. The clinicaldiagnosis of FAP is made on the basis of the histologicallyverified presence of over 100 colorectal adenomas. Approx-imately 50% of patients with FAP will develop adenomasby the age of 15, and 95% will do so by age 35. About 5%to 10% will develop cancer by age 21 and over 90% by age50. It is thought that over 90% of mutation carriers willmanifest signs of the disease, but asymptomatic individu-als in their 50s have been reported [20]. Adenomatous pol-yps with malignant potential can occur in other regions ofthe gastrointestinal tract, including the distal duodenum,particularly around the papilla of Vater, and the proximalstomach. Polyps can also occur in the jejunum and ileum,but these rarely become malignant. Other associatedmalignancies include nonmedullary thyroid cancer, hepa-toblastoma in children, adrenocortical tumors, pancreaticcancer, and gallbladder cancer. If they are examined care-fully, most individuals with FAP will also have non-intesti-nal features of FAP such as desmoid tumors; congenitalhypertrophy of the retinal pigment epithelium; cysts of thejaw; sebaceous and epidermoid cysts on the back; osteo-mas of the skull, mandible, and long bones; lipomas;fibromas; and dental abnormalities. The polyposis com-bined with osteomas and desmoid tumors is sometimesreferred to as Gardner syndrome [21]. Considerable phe-notypic variability exists among FAP families. Some ofthese variations are due to differences in the type of germ-line mutation, but some are also likely due to factors suchas genotypic differences at other loci (modifier genes), dif-ferences in environmental exposures, and chance [22].
Several variants of FAP have been described. An attenu-ated form of FAP (AFAP) has been seen in families withmutations at either extreme end of the APC gene [23]. Indi-viduals with AFAP typically have less than 100 colon ade-nomas (usually between 1 and 50). The polyps developsubstantially later than in typical FAP, with the mean age ofdetection of polyps in AFAP about 44 years and the meanage of about 56 years in cancer. This FAP variation has alsobeen referred to as hereditary flat adenoma syndrome.These flat adenomas are less than twice the thickness of thesurrounding normal mucosa and usually less than 1 cm insize. They are reddish with central umbilication and mayshow higher-grade histology than typical adenomatouspolyps [24]. The adenomas in AFAP are often clusteredproximally to the splenic fissure of the colon, with relativesparing of the rectum. In contrast to classic FAP, which canalways be established by sigmoidoscopy, the diagnosis ofAFAP could be missed by routine sigmoidoscopy. A smallnumber of families with AFAP have been reported to have
serrated adenomas that have mixed hyperplastic and dys-plastic features [25]. Upper gastrointestinal lesions, hepa-toblastoma, breast cancer, congenital hypertrophy of theretinal pigment epithelium (CHRPE), osteomas, and des-moids have all been reported in AFAP families. Because thefindings in AFAP are variable, even among members of thesame family, the diagnosis is often delayed. AFAP shouldalways be considered in cases in which upper gastrointesti-nal or periampullary adenomas and cancers are found orin cases in which fundic gland polyps are paired with a per-sonal or family history of colon cancer [23]. The colonicfeatures of AFAP overlap with those of HNPCC. Most casesof AFAP have been associated with mutations in exons 3, 4,5, and 9 and in the 3’ end of the APC gene [26,27]. Whengenetic testing for AFAP is negative, testing for HNPCCcould be considered via MSI testing of an adenoma, andvice versa.
Brain tumors, primarily cerebellar medulloblastoma,have been described in a small subset of families with fea-tures of FAP. Families with FAP and brain cancer may alsoexhibit intracranial epidermoid cysts and meningiomas[28]. The FAP families with brain tumors had historicallybeen classified as having Turcot syndrome. However,genetic studies of families with Turcot syndrome haveidentified mutations in genes causing both HNPCC andFAP, so this classification is now rarely used.
Molecular genetics of FAPGermline mutations in the APC tumor suppressor gene areassociated with about 80% to 90% of cases that meet theclinical criteria for classic FAP. It is suspected that rare fam-ilies with FAP have a mutation in a different, as yet undis-covered, gene. In contrast to HNPCC, in which newmutations are rare, about 25% of patients diagnosed withFAP have no family history of the disease and appear tohave new mutations [29]. It is generally thought that asomatic mutation in the second APC gene is the initial stepof multiple genetic events that ultimately lead to adenomaand cancer formation. Interestingly, about 80% of sporadiccolorectal cancer tumors have somatic mutations in theAPC gene. Thus, loss of APC function is common in bothinherited and sporadic forms of colorectal neoplasia.
The APC gene codes for a large multidomain protein;there are 8538 base pairs in the coding region of the gene,contained within 15 exons. Exon 15 contains more than75% of the coding region. The gene product is a 2843–amino acid polypeptide. The APC protein is suspected toplay a role in the wnt-signaling pathway, intercellularadhesion, stability of microtubular cytoskeleton, cell-cycleregulation, and possibly apoptosis [30•].
Genetic testing for FAP, AFAP, and FAP with brain tumorsAlmost all of the known germline mutations in the APCgene are truncating mutations, and approximately 80% ofindividuals with clinical FAP will be found to have a
Genetic Testing in Colorectal Cancer • Markey et al. 411
truncated APC protein. Thus, the simplest genetic screen-ing method for FAP is the protein truncation test (PTT)(Table 2). If the PTT is negative in a clinically affected indi-vidual, consideration should be given to use of DNAsequencing, the most sensitive method for detecting muta-tions in the APC gene. Even sequencing has its limitations,however, with approximately 10% of FAP patients still hav-ing an undetectable mutation. A recent study found thatseven of 60 patients (12%) with classic FAP who had previ-ously tested negative for an APC germline mutation hadlarge deletions of the APC gene, and in many the entiregene was deleted (large or total gene deletions would beundetectable by PTT or by sequencing) [31]. Anotherrecent study using conversion technology evaluated nineunrelated FAP patients who had negative PTT; seven ofthem were found to have altered protein expression. In sixcases, the protein was absent and in one it was reduced(Table 2) [32]. Further investigation is warranted of theutility of testing alleles separately (conversion technology)in FAP families whose mutation cannot be identifiedthrough other methods.
When initial APC testing is negative, certain featuresmight prompt further testing. A few families have beenreported that have macroscopic polyposis of the proximalcolon, dermoid cysts, and CHRPE. The adenomas in thesecases may also be sessile, nonpolypoid, or flat. These fami-lies have been found to have complete deletions of theAPC gene, and none of the previously described methodswould typically identify this. A technique that evaluatesprotein expression from both alleles, like conversion tech-nology, is needed to detect mutations in such families.Also, some families have been reported to have FAP as wellas other congenital abnormalities and/or mental retarda-tion. For these families, chromosome analysis focusing onchromosome 5q21 is recommended to assess for structuralabnormalities. Linkage analysis can also be attempted inFAP families with undetectable mutations, but thisapproach requires participation of at least two affected rel-atives. If the family is informative, linkage studies can iden-tify which unaffected family members inherited thechromosome linked to the disease mutation, with about95% to 98% confidence.
Interpretation of genetic testing in HNPCC and FAPPractitioners should offer genetic testing or DNA bankingto patients who have suspected HNPCC or FAP if a muta-tion has not already been identified in the family. It hasbeen our experience that many patients who already have aclinical diagnosis are not offered genetic testing at all. Per-haps these practitioners feel that testing would not add tothe clinical picture. On the contrary, the detection of amutation further substantiates the diagnosis, particularlyin HNPCC, and in some cases allows more precise riskassessment. Defining the mutation also offers the patientmore choices with family planning. Perhaps most impor-tantly, the information can provide unaffected relatives
with a highly accurate test at a greatly reduced cost. Whenunaffected relatives come for risk assessment and genetictesting without the affected relative’s DNA information,they are faced with having to ask this relative to considertesting. This can further exacerbate common feelings ofguilt or blame between family members. Even more prob-lematic is the case where all of the affected relatives aredeceased. Practitioners would assist the whole family byintroducing the concept of genetic testing around the timeof initial diagnosis and treatment. It is only when a delete-rious mutation has been documented in a family that DNAtesting is most informative for unaffected, at-risk familymembers. These at-risk family members can then be giveneither a “true positive” or a “true negative” result andappropriate clinical advice about screening.
The biggest problems in interpretation of genetic test-ing for HNPCC or FAP arise when genetic testing is initi-ated first in an unaffected individual who has a familyhistory that is suspicious for HNPCC or FAP. If an unaf-fected individual is found to possess a clearly deleteriousmutation, his or her future cancer risk is clear. However, if amutation is not identified in the unaffected individual, itmeans that the test is uninformative, not that the individ-ual does not have a cancer risk. This is because it has notbeen proven whether affected members of the family havea detectable mutation. For these families, a negative resultshould not be followed by any changes in cancer surveil-lance recommendations; the medical surveillance shouldbe based solely on the patient’s personal and family his-tory. An “uninformative negative” result, if misinterpreted,could lead to reduced surveillance of people who have avery high cancer risk. Regardless of the interpretation, if apatient misunderstands the implications of an “uninfor-mative negative” result, he or she may not continue toschedule appointments for surveillance.
There is also a risk of a “false positive” gene test in bothaffected and unaffected individuals; it is important to usecaution in interpreting missense mutations, because not allsequence variations can be assumed to cause cancer. Suchan “indeterminate result,” if not adequately explained tothe patient, can also lead to erroneous interpretation ofcancer risk among the patient and family members. Thepatient’s clear understanding of the test results is critical.
A 1997 study from Johns Hopkins University School ofMedicine evaluated the use and interpretation of FAPgenetic testing. The study evaluated 177 patients testedfrom 125 different families within 1 year. Eighty-three per-cent had appropriate indications for testing, whichincluded presymptomatic testing of at-risk relatives, confir-mation of clinical diagnosis, and further evaluation ofmultiple colorectal adenomas. Approximately 10% of thetests were done to evaluate a personal or family history ofisolated colorectal cancer without clinical evidence of poly-posis; this would not usually be considered a valid indica-tion for FAP gene testing. The strategy of testing theaffected relative first was used in approximately 79% of
412 Large Intestine
patients. Nineteen percent of patients received formalgenetic counseling before testing. Seventeen percent pro-vided written informed consent. The practitioner wouldhave misinterpreted 32% of the cases if the laboratory hadnot intervened with education. The most commonly notedmisinterpretation of the APC gene test was that unaffectedindividuals with negative PTTs who did not have a knownfamilial mutation were interpreted not to be at risk [33].Practitioners who provide cancer genetics services have anobligation to educate themselves and their patients aboutthe limitations and problems this testing poses.
ConclusionsGenetic testing is clinically available for both HNPCC andFAP. Obtaining a directed and complete family history isstill the primary mode for clinical diagnosis of these twosyndromes, but genetic testing is now routinely used toestablish the diagnosis in many cases and to assess at-riskfamily members. Practitioners should recognize the valueof offering genetic testing to their affected patients, evenwhen their diagnosis is not in question. Genetic testing ismost informative to both the practitioner and the patientwhen it is carried out in conjunction with pre- and post-test genetic counseling, informed consent, and carefulinterpretation and explanation of the result. Clinics thatspecialize in hereditary cancer can greatly enhance the careof families with HNPCC and FAP.
References and Recommended ReadingPapers of particular interest, published recently, have been highlighted as:• Of importance•• Of major importance
1. Katballe N, Juul S, Christensen M, et al.: Patient accuracy of reporting on hereditary non-polyposis colorectal cancer-related malignancy in family members. Br J Surg 2001, 88:1228–1233.
2. American Gastroenterological Association: American Gastro-enterological Association medical position statement: hered-itary colorectal cancer and genetic testing. Gastroenterology 2001, 121:195–197.
3. American Society of Clinical Oncology: Statement of the American Society of Clinical Oncology: genetic testing for cancer susceptibility. J Clin Oncol 1996, 14:1730–1736.
4. Joint Test and Technology Transfer Committee Working Group: Genetic testing for colon cancer: joint statement of the American College of Medical Genetics and American Society of Human Genetics. Genet Med 2000, 2:362–366.
5. Geller G, Botkin JR, Green MJ, et al.: Genetic testing for susceptibility to adult-onset cancer: the process and content of informed consent. JAMA 1997, 277:1467–1474.
6. Lynch HT, de la Chapelle A: Genetic susceptibility to nonpoly-posis colorectal cancer. J Med Genet 1999, 36:801–818.
7. Vasen HFA, Wijnen JT, Menko FH, et al.: Cancer risk in families with hereditary nonpolyposis colorectal cancer diagnosed by mutation analysis. Gastroenterol 1996, 110:1020–1027.
8. Aarnio M, Sankila R, Pukkala E, et al.: Cancer risk in mutation carriers of DNA-mismatch-repair genes. Int J Cancer 1999, 81:214–218.
9. Mecklin JP, Jarvinen HJ: Clinical features of colorectal carcinoma in cancer family syndrome. Dis Colon Rectum 1986, 29:160–164.
10. Lin KM, Shashidharan M, Thorson AG, et al.: Cumulative incidence of colorectal and extracolonic cancers in MLH1 and MSH2 mutation carriers of hereditary nonpolyposis colorectal cancer. J Gastrointest Surg 1998, 2:67–71.
11. Scott RJ, McPhillips M, Meldrum CJ, and Hunter Family Cancer Service: Hereditary nonpolyposis colorectal cancer in 95 families: differences and similarities between mutation-posi-tive and mutation-negative kindreds. Am J Hum Genet 2001, 68:118–127.
12. Watson P, Butzoz R, Lynch HT, and International Collaborative Group on HNPCC: The clinical features of ovarian cancer in hereditary nonpolyposis colorectal cancer. Gynecol Oncol 2001, 82:223–228.
13.• Peltomaki P: Deficient DNA mismatch repair: a common etiologic factor for colon cancer. Hum Molec Genet 2001, 10:735–740.
This paper briefly reviews the biochemical process of DNA mismatch repair and how loss of this function is related to both HNPCC and sporadic colorectal cancer. The authors describe the full spectrum of mutations in DNA mismatch repair genes that have been found in HNPCC families, the importance of epigenetic mechanisms (methy-lation of MLH1) of defective mismatch repair in sporadic colorectal cancers, and the links between defective mismatch repair and high mutation rates in important growth regulatory genes. This paper is useful for readers who want to learn more about how mismatch repair deficiency leads to an increased cancer risk in both hereditary and sporadic colon cancer.14. Miyaki M, Konishi M, Tanaka K, et al.: Germ line mutation of
MSH6 as the cause of hereditary nonpolyposis colorectal cancer. Nat Genet 1997, 17:271–272.
15.•• Vasen HFA, Watson P, Mecklinn JP, Lynch HT, and the ICG-HNPCC: New clinical criteria for hereditary nonpolyposis colorectal cancer (HNPCC, Lynch syndrome) proposed by the international collaborative group on HNPCC. Gastroenter-ology 1999, 116:1453–1456.
This important paper describes the Amsterdam II criteria,which include the extracolonic tumors that are found in HNPCC. These criteria recommend that many HNPCC families who do not meet the original strict Amsterdam criteria should be referred for genetic counseling and offered DNA testing or targeted surveillance. The arti-cle describes the extracolonic cancers and their incidence in HNPCC families.16.• Syngal S, Fox EA, Eng C, et al.: Sensitivity and specificity of
clinical criteria for hereditary non-polyposis colorectal cancer associated mutations in MSH2 and MLH1. J Med Genet 2000, 37:641–645.
This is a key validation paper for genetic testing in HNPCC. The authors examined the value of the Amsterdam and Bethesda criteria in identifying subjects with mutations in their mismatch repair genes and found that the first three Bethesda criteria were the most sensitive for finding mutations. The paper provides an approach to more streamlined Bethesda criteria that would be easier to use in a clinical setting and makes recommendations about which patients should proceed directly to sequencing and which should have MSI screening prior to sequencing.17.• American Medical Association: Identifying and Managing Risk for
Hereditary Nonpolyposis Colorectal Cancer and Endometrial Cancer. Chicago, IL: American Medical Association; 2001.
This review of HNPCC includes easy-to-follow guidelines that explain how to approach genetic testing in families at risk for HNPCC. It discusses clinical criteria, interpretation of genetic test results, and counseling before and after testing.18. Wahlberg S, Liu T, Lindblom P, Lindblom A: Various mutation
screening techniques in the DNA mismatch repair genes hMSH2 and hMLH1. Genet Test 1999, 3:259–264.
19. Marcus VA, Madlensky L, Gryfe R, et al.: Immunohistochemis-try for hMLH1 and hMSH2: a practical test for DNA mismatch repair deficient tumors. Am J Surg Pathol 1999, 23:1248–1255.
Genetic Testing in Colorectal Cancer • Markey et al. 413
20. Evans DG, Guy SP, Thakker N, et al.: Non-penetrance and late appearance of polyps in families with familial adenomatous polyposis. Gut 1993, 34:1389–1393.
21. Gardner EJ, Richards RC: Multiple cutaneous and subcutane-ous lesions occurring simultaneously with hereditary polyposis and osteomatosis. Am J Hum Genet 1953, 5:139–147.
22. Houlston R, Crabtree M, Phillips R, Tomlinson I: Explaining differences in the severity of familial adenomatous polyposis and the search for modifier genes. Gut 2001, 48:1–5.
23. Hernegger GS, Moore HG, Guillem JG: Attenuated familial adenomatous polyposis: an evolving and poorly understood entity. Dis Colon Rectum 2002, 45:127–136.
24. Muto T, Kamiya J, Sawada T, et al.: Small ‘flat adenoma’ of the large bowel with special reference to its clinicopathologic features. Dis Colon Rectum 1985, 28:847–851.
25. Matsumoto T, Iida M, Kobori Y, et al.: Serrated adenoma in familial adenomatous polyposis: relation to germline APC mutation. Gut 2002, 50:402–404.
26. Spiroio L, Olschwang S, Groden J, et al.: Alleles of the APC gene: an attenuated form of familial polyposis. Cell 1993, 75:951–957.
27. Lamlum H, Al Tassan N, Jaeger E, et al.: Germline APC variants in patients with multiple colorectal adenomas, with evidence for the particular importance of E1317Q. Hum Mol Genet 2000, 9:2215–2221.
28. Leblanc R: Familial adenomatous polyposis and benign intracranial tumors: a new variant of Gardner’s syndrome. Can J Neurol Sci 2000, 27:341–346.
29. Bisgaard ML, Fenger K, Bulow S, et al.: Familial adenomatous polyposis (FAP): frequency, penetrance, and mutation rate. Hum Mutat 1994, 3:121–125.
30.• Fearnhead NS, Britton MP, Bodmer WF: The ABC of APC. Hum Mol Genet 2001, 10:721–733.
This is a comprehensive and very readable review of the structure and function of the APC gene. The paper briefly describes the clinical features of FAP and then focuses on the APC gene and protein. It describes the types of mutations that occur in the APC gene, the functional domains of the gene APC protein, and the biochemical and biologic functions of the normal APC protein. This review is valuable for the reader who desires a more detailed genetic, biochemical, and biologic understanding of FAP.31. Sieber OM, Lamlum H, Crabtree MD, et al.: Whole-gene APC
deletions cause classical familial adenomatous polyposis, but not attenuated polyposis or ‘multiple’ colorectal adenomas. Proc Natl Acad Sci U S A 2002, 99:2954–2958.
32. Laken SJ, Papadopoulos N, Petersen GM, et al.: Analysis of masked mutations in familial adenomatous polyposis. Proc Natl Acad Sci U S A 1999, 96:2322–2326.
33. Giardiello FM, Brensinger JD, Petersen GM: AGA technical review on hereditary colorectal cancer and genetic testing. Gastroenterology 2001, 121:198–213.
34. Bala S, Kraus C, Wijnen J, et al.: Multiple products in the protein truncation test due to alternative splicing in the adenomatous polyposis coli (APC) gene. Hum Genet 1996, 98:528–33.
35. Ballhausen WG: Genetic testing for familial adenomatous polyposis. Ann N Y Acad Sci 2000, 910:36–47.
36. Salovaara R, Loukola A, Kristo P, et al.: Population-based molecular detection of hereditary nonpolyposis colorectal cancer. J Clin Oncol 2000, 18:2193–2200.
