Download Epigenetics - Cayetano Heredia University

OBG420
Janine LaSalle, Ph.D.
Epigenetics, Parental
Imprinting, and X
Inactivation
Epigenetics
Inherited and reversible
modifications to nucleotides or
chromosomes that do not change the
sequence but can alter gene
expression
Epigenetics
• Epigenetics can explain how multicellular
organisms can express different genes in
different tissues using the same set of
chromosomes.
• Epigenetics can explain early environmental
influences on eventual phenotype.
• Epigenetics can also explain non-Mendelian
modes of inheritance such as genetic
imprinting.
PARENTAL IMPRINTING
THE CONTRIBUTION OF BOTH PARENTS IS REQUIRED FOR NORMAL
EMBRYONIC DEVELOPMENT IN MAMMALS
EGG
SPERM
EGG
EGG
SPERM
SPERM
TRANSPLANT
FERTILIZED EGG
WITHOUT NUCLEUS
EMBRYO
MOUSE
GYNOGENETIC
ANDROGENETIC
FAILURE OF
EXTRAEMBRYONIC
DEVELOPMENT
FAILURE OF
EMBRYONIC
DEVELOPMENT
Human Parthenogenetic Tumors
• Hydatidiform moles
– Androgenetic placental tumor
• Ovarian teratoma
– Gynogenetic benign tumor
Parental Imprinting of 15q11-13
Normal
M
P
Chromosome 15
Prader-Willi Syndrome
M
P
M
M
M
del
Paternal
Deletion
Angelman Syndrome
P
P
P
M
P
del
Maternal
Disomy
Maternal
Deletion
Paternal
Disomy
Maternal
Inheritence
Inheritance
Uniparental Disomy (UPD) of Chromosome 15 in
Prader-Willi (PWS) and Angelman Syndromes (AS)
Chromosome
loss
PWS
Maternal
heterodisomy
Chromosome
duplication
Meiosis I
nondisjunction
Reciprocal
nondisjunction
products
Fertilization
with normal
sperm
AS
Paternal
isodisomy
Chromosome UPD in humans
MATERNAL
PATERNAL
NO PHENOTYPE
15
PWS
15
AS
13
6
IUGR
11 BWS
17
7
IUGR
14
MR
21
16
IUGR
21
22
IUGR
14
GR IUGR
Intrauterine growth retardation
Growth retardation
20
GR GR
BWS
AS
PWS
Beckwith-Weidemann syndrome
Angelman syndrome
Prader-Willi syndrome
Characteristic features of
imprinted loci
• Allele specific differences in:
– Transcription
– Methylation
– DNA replication
Parental Imprinting
Allele-specific Transcription
• Transcription is silenced or imprinted on
one parental allele
– ex) H19 is paternally imprinted, expressed only
from maternal allele
• Imprinted monoalleleic expression can be in
all tissues (SNRPN, H19) or limited to
certain tissues (UBE3A) or developmental
stages (INS)
Parental Imprinting
Allele-specific Transcription
Paternal specific expression of SNRPN
DNA
DNA
RNA
DNA
Examples of Imprinted Genes
Chromosomal region
15q11-13
Maternal Imprint
SNRPN, IPW, NDN,
PAR1, PAR5
Paternal Imprint
UBE3A
11p15.5
IGF2
H19, p57KIP
KvLQT1, IPL
11p13
6q26-27
7q32
19q13.4
20q13
Xq13.2
WT1
IGF2R/M6P
PEG1/MEST
PEG3
GNAS1
XIST
Parental Imprinting
Allele-specific Methylation
• A covalent modification to cytosine in
higher eukaryotes (animals, plants, fungi)
Parental Imprinting
Allele-specific Methylation
• CpG (5'-C-G-3') occurs less
frequently in the mammalian
genome than any other dinucleotide.
• CpG islands are clusters of CpG
sites near the 5' ends of genes and
are the target of DNA methylation.
What role does
methylation have on
gene expression?
• Usually results in gene
silencing, but not always
• Multiple methylation sites
at the 5’ end and within
introns of a gene may
regulate transcription
• Methylation may
involved in determining
and/or maintaining genes
in inactive state
Parental Imprinting
Allele-specific Methylation
• Parental allele-specific methylation found
around imprinted genes
• Erasure and re-establishment in gametes for
each generation
• Some methylation imprints are inherited
from gametes, but most are erased and reestablished in early development
Parental Imprinting
Allele-specific Methylation
Parental Imprinting
Allele-specific DNA Replication
• Most homologous chromosomal regions
replicate synchronously in S phase
• Parental allele-specific asynchronous
replication of imprinted regions
• Asynchronous replication is established in
the gametes and maintained throughout
development
Parental Imprinting
Allele-specific DNA Replication
Theories for parental imprinting
• Parental “tug of war” in evolution
– Igf2 is paternally expressed, Igf2r is maternally
expressed
• Protection against trophoblastic disease
– Lack of trophoblast in parthenogenetic embryos
– Molar pregnancy, high rate of malignancy
• Biparental exchange or cross-talk
– Different replication timing of UPDs
– Homologous association
Parental Imprinting and Mammalian
Reproductive Cloning
• Many cloned livestock exhibit “large
offspring syndrome” due to dysregulated
expression of Igf2.
• Cloned mice and ES cells have many
epigenetic defects in imprinted genes.
• Human children from in vitro fertilization
(IVF) have increased rates of Angelman and
Beckwith-Wiedemann syndromes.
X chromosome inactivation
 In females (46,XX), X inactivation serves to
reduce the amount of transcriptionally
active X chromatin in somatic cells.
 In female somatic cells, one X chromosome
becomes inactive and is cytologically
detected as a Barr body.
X inactivation
• The number of X chromosomes are counted
prior to X inactivation.
• X inactivation follows the "n-1" rule so
that only one X chromosome remains active
in each cell, regardless of X chromosome
copy number.
X inactivation
 X inactivation is mediated by transcription
of the gene XIST from the X inactivation
center (XIC).
 XIST is only expressed from the inactive X
chromosome and does not encode a protein
but rather functions as an RNA molecule.
X inactivation: Role of XIST
X inactivation
• The inactive X chromosome in female cells
is more heavily methylated and later
replicating than the active X chromosome
• There are pseudoautosomal regions of the
X chromosome that are transcriptionally
active on both active and inactive X
chromosomes.
Random parental X inactivation
in somatic cells
 Around the stage of implantation, there is an
erasure and re-establishment of X inactivation
patterns.
 X inactivation in females becomes random,
meaning that roughly 50% of the cells have
inactivated the paternal X chromosome and the
other half are inactive on the maternal X
chromosome.
Random parental X inactivation in somatic cells
Random parental X inactivation
in somatic cells
• Female somatic cells can be mosaic for expression
of alleles or mutations of a gene.
 Non-random X inactivation can be observed in
some women and can be associated with increased
frequency of spontaneous abortion.
• Female carriers of X-linked disorders often show
skewed or non-random X inactivation patterns.
Human diseases involving methylation
Rett Syndrome



Rett syndrome is an X-linked dominant
disorder affecting heterozygous females.
Rett syndrome infants develop normally until 6
to 18 months of age but then develop a
progressive loss of neurodevelopmental
milestones.
Mutations in the methylation-specific binding
protein MECP2 on the X chromosome cause
Rett syndrome.
Human diseases involving methylation
Rett Syndrome



Females are heterozygous and mosaic for
expression of MECP2 mutations in somatic cells.
MECP2 mutation in males is rare because
mutations are more frequent on paternal allele.
MECP2 mutation in males can cause MR.
Some cases of inheritance from a female carrier
have been reported, in which the phenotypically
normal female carrier shows non-random Xinactivation.
Human diseases involving methylation
Rett Syndrome
 Methylated DNA is recognized by methylationspecific binding proteins such as MeCP2.
• MeCP2 localizes DNA to transcriptionally
inactive heterochromatin and associates with
proteins involved in histone packaging and
transcriptional silencing.
Human diseases involving methylation
ICF Syndrome
 Variable immunodeficiency centromeric
heterochromatin instability and facial abnormalities
• Cytogenetics reveals many chromosomal breaks and
interchanges
• Hypomethylation of centromeric satellite repeats
results in instability of chromosomes during mitosis
• Mutations were found in ICF patients within the
DNMT3B gene encoding a DNA methyltransferase
Human diseases involving methylation
Fragile X Syndrome
 Expansion of CGG triplet repeat in the 5’
untranslated region of FMR1 on X chromosome
• Pre-mutation (50-230 repeats) can be expanded to
full mutation (>230 repeats) by passage through a
female carrier
• Hypermethylation of the expanded CGG repeat can
result is loss of expression of the FMR-1 gene
Why does methylation exist?
• Genome defense mechanism
– Foreign DNA becomes methylated in order to
silence undesired transcription and reduce
recombination
• Developmental control of gene expression
– Imprinting in mammals, meiosis in fungi
• Development of the central nervous system
in mammals
– Multiple human neurodevelopmental disorders
have epigenetic etiologies