Differences from mendelian heredity

Imprinting, dynamic mutations

RNDr Z.Polívková

Lecture No 438 - course: Heredity

Genomic imprinting

Mendelian principle: autosomal genes have the same

expression if transmitted from father or mother

Imprinted genes: maternal and paternal alleles have

different expression (activity) according to parental origin

Gene imprinting = epigenetic form of regulation

of gene expression

Imprinted genes:• Monoallelic expression – expression of only one

parental allele (parent-of-origin expression)• Imprinted genes - functionally haploid• Active allele – transcribed• Inactive allele – imprinted – nontranscribed –

silent• Imprinting connected with DNA methylation

and other changes of chromatin

Imprinted genes normally involved

in embryonic growth, cell division

Abnormality of imprinting = human pathologies

Examples of human pathologies :

• Human triploidies:

Additional paternal set of chromosomes = partial

mole = hyperplasia of trophoblast

Additional maternal set of chromosomes =

hypotrophic placenta

• Gynogenesis and androgenesis:

• Ovarial teratoma – division of ovum without fertilization

• Complete mole – division of only male pronucleus (without maternal contribution)

∑ : Role of the imprinted genes in early human embryogenesis:

paternally expressed genes → placental proliferation and invasivness

maternally expressed genes → development of embryo

Prader- Willi sy (PWS)

MR, short stature ,obesity, hypotonia, characteristic facies, small feet and hands, hypogonadism

Angelman sy (AS)

MR, absence of speech, seizures, jerky gait, inappropriate laughter, dysmorphic features

PWS AS

PWS AS

deletion 15q11-13

on paternal chromosome on maternal chromosome

UPD (uniparental disomy)

maternal paternal

mutation maternal active allele

imprinting error maternal imprint paternal imprintOn both chromosomes on both chromosomes in PWS region in AS region

UPD = both chromosomes 15 from one parent

Deletion 15q11-13

Wysis

PWS AS

in proximal region of chromosome 15 – two groups of reciprocally imprinted genes

PWS region – active paternal elleles

AS region - active maternal allele(s)

loss of function of active alleles in PWS region (pat)

loss of function of active allele(s) in AS region (mat)

→ functional nullisomy

Imprinted genes on chromosome No 15

normal situation

pat mat

SNRPN

ZNF127 }}

PWS genes

AS gene

active paternal ellele active maternal allele

„silent“= imprinted paternal allele

„silent“=imprinted maternal allele

UBE3A

Deletion in PWS and AS

pat mat pat mat

PWS AS

Deletion of paternal active alleles in PWS

Deletion of maternal active alleles in AS

UPD - Uniparental disomy in PWS and AS

mat mat pat pat

UPD

Uniparental disomy

maternal in PWS

paternal in AS

PWS AS

Mutation in AS

AS

mutated active maternal allele in AS

Imprinting error

pat mat pat mat

PWS AS

maternal imprint of PWS genes on both chromosomes in PWS

paternal imprint of AS gene on both chromosomes in AS

Imprinting and Beckwith-Wiedeman syndrome (EMG – exomphalos - macroglossia – gigantism)

growth abnormalities : macroglossia, gigantism, hemihypertrophy, visceromegaly, abdominal wall defects (omphalocele, umbilical hernia); hypoglycemia in neonatal period, renal dysplasia, skeletal anomalies

-predisposition to embryonal tumors (Wilms´ tumor,… )

Imprinted genes on 11p15:IGF2 – expressed from paternal allele

H19 – expressed from maternal allele

p57 - expressed from maternal allele?

Changes in BWS:• paternal duplication of 11p (2 x IGF2)

• paternal UPD (2 x IGF2)

• deletion or translocation of maternal active allele H19→ activation of maternal IGF2 allele („enhancer“ competition model for expression control of IGF2 and H19)

• abnormal imprint = biallelic expression of IGF2

Pathogenesis of disease – increased dose of IGF2 (growth factor)

role of H19 and other genes = ?

normal situation BWS region on

11p15

pat mat

pat mat

1. paternal duplication

active and silent paternal alleles

active and silent maternal alleles

IGF2

IGF2

IGF2H19

H19

H19

IGF2

H19

pat pat pat mat

2. paternal UPD

3. del,, transl. of maternal allele H19, expression of paternal IGF2

pat mat

4. imprinting error

biallelic expression of IGF2

IGF2

H19

IGF2

IGF2

H19

Imprinting and cancer

Genetic changes in cancer:inherited or spontaneous mutations that are not corrected

by repair mechanisms – irreversible changes in protooncogenes, tumour suppressor genes

Epigenetic changes: (do not affect the primary sequences of genome) – changes in methylation (imprinting) of these genes→ activation of protooncogenes, silencing of tumour suppressor genes by aberrant methylation

Wilms´ tumor (WT)

Locus 11p13 – connected with WAGR syndrome (Wilms´tumor, aniridia, urogenital anomaly, mental

retardation)

WT1 gene- transcription factor (tumour suppressor) – biallelic expression in kidney and other organsBut! in some persons, in some tissues – WT1 is

imprinted

= polymorphism of imprinting = predisposition to cancer

Locus 11p15 – connected with BWS

IGF2 – growth factor – monallelic expression in kidney

biallelic expression in Wilms tumor and other tumors (LOI = loss of imprinting) oncogen

H19 – oncofetal RNA – expressed in embryogenesis-

function = ?, loss of H19 expression in WT with biallelic IGF2 expression

H19 - expressed in some tumors (lung, oesophageal, bladder carcinoma)

p57 – inhibitor of cyclin dependent kinase

reduced expression in WT and other tumors (lungs)

Imprinting connected with methylation

Imprinted allele = methylated

Imprinted protooncogenes – loss of imprinting (LOI) = activation of imprinted allele = biallelic expression

= oncogenes

Imprinted tumor supressor genes = predisposition to tumors – loss of only one allele = loss of gene function

Knudson two-hit hypothesis of inactivation

of tumor suppressor genes :

1st step: germ mutation or somatic

mutation, or imprinting of one allele

2nd step: loss of heterozygosity (LOH) –by

mutation in somatic cell

Polymorphism of imprinting of some genes in population:tumour supressor genes WT1 (11p13), IGF2R (=IGF2 receptor on 6q26 - inactive in different tumours, role of IGF2R in extracellular IGF2 degradation)

In population – biallelic expression of these genes

in some people monoallelic expression (imprinted)

= predisposition to cancers

methylation = reversible process – possibility of therapy of

tumours caused by aberrant methylation ???

Imprinting• stage-, tissue-, species- and strain-specific• polymorphism of imprinting (inter-individual differences)• imprinted genes – function in positive or negative regulation of embryonal growth, cell division and differentiation• imprinted genes: receptors, growth factors, regulation

proteins, transcription factors, proteins in splicing

= protooncogenes, tumour suppressor genes• role in embryonal development• biallelic expression in some stages of ontogenesis ?• imprinting is reversible

Imprinting connected with methylation,

histone deacetylation and with

remodelation of chromatine structure to

inactive state

Abnormality of imprinting = growth abnormalities, abnormality of development, behaviour and cancers

• Lack of expression (because of mutation, deletion, deficiency) of a gene usually expressed monoallelically from a specific parent

• or overexpression (because of duplication, relaxation of imprinting or loss of control) of a normally monoallelically expressed gene

Uniparentalní disomy=inheritace of both homologs from the same parent

Mechanisms of UPD origin:• Loss of one chromosome from trisomic zygote

=„correction“ of initial trisomy (trisomy rescue)

• gametic complementation = fertilization between nullisomic and disomic gametes (for the same chromosome)

• duplication of the single chromosome in monosomic zygote

• postfertilisation error – nondisjunction and reduplication of the single chromosome or mitotic recombination

Origin of uniparental disomy from trisomic zygote

trizomic zygote

loss of chromosome loss of chromosome

uniparental disomy

normal

Fertilization of disomic and nullisomic

gametes

Origin of uniparental disomy by gamete complemetation

Origin of uniparental disomyby duplication of chromosome of monosomic zygote

monozomic zygote

duplication of chromosome

Origin of partial isodisomy by postfertilisation error

mitotic recombination

nondisjunction and

duplication

normal zygote-dizomic

Evidence for UPD: • trisomy 15 in CVS, normal karyotype in fetal blood child with PWS

• increased parental age in UPD

• transmission of hemphilia from father to son (zygote XXY and loss of maternal X chromosome

• transmission od balanced translocation 22/22 in balanced form to healthy child (trisomic zygote and loss of single chromosome 22)

• pericentric inversion was present on one homologue in mother and on both homologues in one offspring

• maternal UPD in PWS, paternal UPD in AS

• maternal UPD of chromosome No 7 in a patient with cystic fibrosis and growth retardation (first detection of UPD)

UPD abnormal development if

imprinted genes are present

Dynamic mutations

Fragile X syndrome= X-linked mental retardation - 1:1500 of malescytogenetic manifestation – fragile site Xq27.3 = FRAXA

Clinical signs: mental retardation, macroorchidism (large testicles), long face, large mandible, large everted ears

mothers od affected males = carriersbut: 30% of women = carriers - mentally retarded 20% fraX men mentally normal

deterioration of manifestation through generation = (Sherman paradox)

Unstable triplet repeats (CCG)n in FMR1 gene

in normal population 6-50 copies

premutation (without MR) 50-200 copies

full mutation (with MR) 200-2000 copies

DNA methylation (promoter region) FMR1 is

not transcribedabsence of proteinMR

Premutation=unstable

premutationfull mutation = only through mother

carrier (in oogenesis or early in embryonal life)

man with premutation length of element is not

increased in the next generation

length of amplification in correlation with cytogenetic expression

gene function ?? – protein expressed in tissues, higer levels in brain and testes

gradual origin of mutation = dynamic mutation

Dynamic mutations =initial change of DNA produce another

change = expansion of triplet repeats

Main features od dynamic mutations:• homogenity – no more alleles

• somatic variability- different numbers of copies in

different tissues

• effect of parental origin on manifestation

• difference from mendelian principles (low penetrance)

• no new mutations – gradual arise through premutation,

familiar

• expresivity depends on number of copies

• anticipation=deterioration of clinical signs through

generations

Two groups of dynamic mutations:

amplification in noncoding (nontranslated) region of gene (promoters, introns) loss of function

fra X(CCG/GGC), myotonic dystrophy (CTG),

Friedreich ataxia (GAA)

amplification in exons (usually CAG repeats) genes

are transcribed abnormal protein

Huntington disease-HD- (abnormal protein

huntingtin inactivates associated proteins), spinocerebellar

ataxia type 1

Expansions depends on the sex of transmitting parent

Fra X, myotonic dystrophy – expansion if

disease is inherited from mother

HD - expansion- if inherited from father

Postzygotic origin of amplification on chromosome of specific parental origin - determined in gametogenesis

Dynamic mutation Fragile X Myotonic dystrophy Huntington disease

FRAXA DM HDheredity XD with reduced AD with different age AD with different age

penetrance of onset of onsetparental origin maternal=full mutation maternal –congenital forms paternal – early onsetmechanism abnormal DNA metylation different mRNA level abnormal protein

transcription of FMR1 toxic for neurons? is stoppedamplification (CCG)n (CTG)n (CAG)n

normal number 10-50 5-35 9-34abnormal number 50-200 premutation 50-80 30-1000

200-2000 full mutation 80-2000 lower number=reduced

penetrance

gene FMR1 IT15

http://dl1.cuni.cz/course/view.php?id=324 presentation

http://dl1.cuni.cz/course/view.php?id=324 supplementary text to cytogenetics

Thompson &Thompson: Genetics in medicine, 7th ed.

Chapter 5: Principles of clinical cytogenetics: Parent-of-origin effects: Genomic imprinting

Chapter 7: Patterns of single gene i heritance: Imprinting in pedigrees, Unstable repeat expansions

Chapter 12: The molecular, biochemical and cellular basis of genetic disease: Diseases due to the expansion of unstable repeat sequences: Biochemical and cellular mechanisms