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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
On both chromosomes
in PWS region
paternal imprint
on both chromosomes
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
UBE3A
}
PWS
genes
} AS gene
active paternal ellele
„silent“= imprinted
paternal allele
active maternal allele
„silent“=imprinted
maternal allele
Deletion in PWS and AS
pat mat
pat
mat
Deletion of paternal
active alleles in PWS
Deletion of maternal
active alleles in AS
PWS
AS
UPD - Uniparental disomy
in PWS and AS
mat
mat
pat
pat
Uniparental disomy
maternal in PWS
paternal in AS
PWS
AS
UPD
Mutation in AS
mutated active
maternal allele
in AS
AS
Imprinting error
pat mat
pat mat
maternal imprint of
PWS genes on both
chromosomes in PWS
PWS
AS
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 = ?
pat
pat
mat
mat
IGF2
IGF2
IGF2
H19
H19
H19
IGF2
H19
normal situation
BWS region on
11p15
1. paternal duplication
active and silent paternal alleles
active and silent maternal alleles
pat
pat
pat
mat
IGF2
IGF2
H19
3. del,, transl. of
2. paternal UPD
pat
mat
maternal allele H19,
expression of paternal
IGF2
IGF2
H19
biallelic expression
of IGF2
4. imprinting error
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 organs
But! 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 embryogenesisfunction = ?, 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
uniparental
disomy
loss of chromosome
normal
Origin of uniparental disomy by gamete
complemetation
Fertilization of disomic
and nullisomic
gametes
Origin of uniparental disomy
by duplication of chromosome of monosomic zygote
monozomic
zygote
duplication of
chromosome
Origin of partial isodisomy by
postfertilisation error
normal zygote-dizomic
nondisjunction
and
duplication
mitotic
recombination
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 males
cytogenetic manifestation – fragile site Xq27.3 = FRAXA
Clinical signs: mental retardation, macroorchidism (large
testicles), long face, large mandible, large everted ears
mothers od affected males = carriers
but: 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 transcribedabsence of proteinMR
Premutation=unstable
premutationfull 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
heredity
parental origin
mechanism
amplification
normal number
abnormal number
gene
Fragile X
Myotonic dystrophy
FRAXA
XD with reduced
penetrance
maternal=full mutation
abnormal DNA metylation
transcription of FMR1
is stopped
(CCG)n
DM
AD with different age
of onset
maternal –congenital forms
different mRNA level
(CTG)n
10-50
50-200
premutation
5-35
50-80
200-2000
full mutation
80-2000
FMR1
Huntington disease
HD
AD with different age
of onset
paternal – early onset
abnormal protein
toxic for neurons?
(CAG)n
9-34
30-1000
lower number=reduced
penetrance
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