Epigenetic phenomena

• Epigenetics refers to genetic inheritance that is not coded by the DNA sequence

• It includes changes in gene expression due to modification of DNA or change in its chromatin state (facultative heterochromatin)

• Lecture will illustrate this with various examples

X chromosome inactivation• In female mammals with 2 X chromosomes,

one X is inactivated, i.e. all its genes are switched off, and it forms the Barr body

• This is to prevent a double dose of X-chromosome gene products relative to that in male cells

• Which of the 2 X chromosomes is inactive in a cell line, is usually randomly determined early in development

The calico cat• The calico cat is an example of X-

chromosome inactivation• She is heterozygous for a gene on

the X, one allele gives orange fur, the other black

• Random X-inactivation in cells early in development gives patches with either the orange or black allele active

• White patches are due to an autosomal gene, “spotting”

Consequences of X inactivation:the calico cat

Muscular dystrophy in girls• Duchenne muscular dystrophy (DMD) is an X-linked

recessive disease, usually only affects boys

• The karyotype of girls with DMD sometimes shows an X:autosome translocation

• In this case, X-inactivation is not random - the normal X is always inactivated, because the translocation interferes with the inactivation process

• Therefore, the normal DMD gene is switched off, and the other one is disrupted by the translocation

• So these girls show the symptoms of DMD

Genomic imprinting• Usually it does not make any difference from which parent you got a

particular gene

• But with some genes it does matter - this is called genomic (or genetic) imprinting

• Example:

– Prader-Willi syndrome (PWS): small stature, obesity, learning difficulties

– Angelman syndrome (AS): epilepsy, learning difficulties, unsteady gait, “happy” appearance

– PWS often caused by deletion of a gene “SNRPN” on paternal chromosome 15

– AS often caused by deletion of the same gene, but the maternally-derived one

– Therefore the gene must be expressed differently depending on which parent it came from

Father’s imprint on his daughter’s thinking?

• Why are boys more likely to have autism (and other disorders of social function) than girls?

• Turner’s syndrome (45XO) girls are of normal intelligence but often have social function problems

• Their single X can be either maternal or paternal in origin

• The ones with a maternal X are much more likely to have the social problems

• All boys have a maternally-derived X

• So, there could be imprinted gene(s) on the X, which are involved in social function

• When maternally inherited this could might contribute to disorders such as autism

Mechanism of imprinting• The mechanisms of X-inactivation

and imprinting are not fully understood but both involve DNA methylation

• DNA can be reversibly methylated on C bases - fig 11.22 in Hartl

• Methylation of a gene’s promoter tends to switch it off, due to binding of a specific protein to methylated DNA

Determination of methylation

CCGGGGCC

CCGGGGCC

CCGGGGCC

m

m m

m

Gel electrophoresis of fragments

1

23

4

1

5

Non-methylated site

Methylatedsites whichare not presentin HpaII digest

MspI HpaII

Is methylation state related to gene activity?

• Many sites within a genes are methylated• some sites only in certain tissues

• others in all tissues

• A minority of sites are methylated in tissues in which the gene is not expressed, but are unmethylated in tissues in which the gene is active

• Experiments suggest that these sites are important regulators of gene activity

Position-effect variegation (PEV)• State of chromatin (euchromatin, heterochromatin)

can affect gene expression• A gene could be moved to a heterochromatic

region by an inversion• Heterochromatin’s structure tends to switch off

gene expression

An example of PEV• A mutant allele of the w gene in Drosophila causes

eyes to be white (wild-type is red)• An inversion of part of the X chromosome causes

eyes to have red and white patches (fig 7.36 in Hartl)

• This is because of PEV switching off w gene in some cell lines in the eye

• The boundary between heterochromatin and euchromatin is not exactly the same in all cell lines, hence eyes are mosaic

Summary - epigenetic gene regulation

•Both mammalian X inactivation and Drosophila position effect variegation are examples of epigenetic gene regulation.

•The repressed state caused by the chromatin rearrangement is heritable, but importantly the decision to induce the repressed state is not encoded by the genome.