Download Malattie XL, YL e Mitocondriali

Malattie XL, YL e Mitocondriali
Corso integrato di Genetica e Biologia Molecolare
a.a 2016-2017
Giovanni Malerba
Sex Chromosomes
●
Determine gender
●
X and Y chromosomes
●
Females are XX
●
Males are XY
●
Y chromosome is much smaller and does not have
the same genes as the X
●
Most “x-linked” traits are “X-linked”
Females
●
X chromosome inherited from both parents
●
Egg will have only one X chromosome
●
All of children will inherit one of the 2
X-chromosome with equal probability
Males
●
X chromosome inherited from the mother
●
Y chromosome inherited from the father
X-linked disorders
X chromosome
●
●
One of the larger chromosomes
~1100 functioning genes
Chr_X
Chr_Y
Length (bps)
155,270,560
59,373,566
Coding genes
830
72
Non coding genes
722
136
Pseudogenes
840
346
Short Variants
5,557,325
208,790
X linked Traits
●
Females have 2 alleles for the trait.
●
Males have only one allele for the trait.
Whatever alleles males have on the X
chromosome they will be expressed
X-chromosome inactivation - Lyonization
●
Random process
Certain situations can produce a skewed X-chromosome inactivation
●
Determinant of the degree of phenotypic expression
●
Functionally, women are mosaic for the expression of X-linked genes
●
The process of inactivation occurs early enough in the embryogenesis that
pattern of expression is actually clonal (in patches – sometimes it can be
mapped)
Random inactivation of X chromosome
Random (early embryo) permanent (mitosis) process
In all somatic cells
Normal females are a mosaic of 2 populations
X-inactivation profile of the human X chromosome
In female mammals, most genes on one X chromosome are silenced
as a result of X-chromosome inactivation
The inactivated X chromosome then condenses into a compact
structure called a Barr body, and it is stably maintained in a silent
state (Boumil & Lee, 2001).
Some genes escape X-inactivation and are expressed from both the
active and inactive X chromosome.
XIST gene
●
The XIST gene does not encode a protein but rather produces a 17
kilobase (kb) functional RNA molecule. Hence, it is a noncoding RNA
(Costa, 2008).
●
XIST RNA is only expressed in cells containing at least two Xs and is not
normally expressed in male cells. Higher XIST expression can be seen in
cells with more X chromosomes, as a counting mechanism dictates that
only one X per cell can remain active. In such cells, XIST is expressed
from all supernumerary Xs.
●
XIST RNA remains exclusively in the nucleus and is able to "coat" the
chromosome from which it was produced.
●
XIST RNA is expressed from an otherwise inactive X chromosome.
http://www.nature.com/scitable/topicpage/x-chromosome-x-inactivation-323
X-linked recessive disorders
●
●
●
Affect males. No male-to-male transmission
Female carriers are generally spared
In female carriers ~50% of cells (on average)
have the normal allele on the active X
chromosome – generally sufficient to spare
females from clinical effects
X-linked recessive disorders
Affected males
Carrier females
1/3600
1/1800
1/12
1/7
Lesch-Nyhan Syndrome
Duchene Muscular Dystrophy
Hunter's Disease
Menkes Disease (Kinky hair syndrome)
Glucose 6 Phosphate Dehydrogenase Deficiency
Hemophilia A
Hemophilia B
Fabry's Disease
Wiskott-Aldrich Syndrome
Bruton's Aggamaglobulinemia
Color Blindness (red-green)
Complete Androgen Insensitivity
Congenital Aqueductal stenosis (hydrocephalus)
Inherited Nephrogenic Diabetes Insipidus
X-linked Recessive (XLR) Inheritance
Recurrence risks
Affected male: 25%
Unaffected male: 25%
Unaffected carrier female: 25%
Unaffected noncarrier female: 25%
Affected child: 25%
Recurrence risks
All males unaffected
All females obligated carriers
Affected child: 0%
Carrier female: 50%
X-chromosome inactivation - Lyonization
●
Random process
Certain situations can produce a skewed X-chromosome inactivation
●
Determinant of the degree of phenotypic expression
●
Functionally, women are mosaic for the expression of X-linked genes
●
The process of inactivation occurs early enough in the embryogenesis that
pattern of expression is actually clonal (in patches – sometimes it can be
mapped)
X-chromosome inactivation - Lyonization
●
The higher the degree of skewing the closer the phenotype
approaches that of an affected male.
●
Duchenne muscular dystrophy (DMD): progressive degeneration
of muscle leading to weakness (problems with ambultion,
respiration – death at ~20yrs)
Marker: ++ creatine phosphokinase (CPK or CK).
Known as a recessive X-linked disease (asymptomatic or nonexpressing in carrier females). HOWEVER partial expression
can occur in carrier females.
X-linked Recessive (XLR) Inheritance
●
Skewed random Lyoniziation can make female heterozygotes
showing variable degree of expression.
●
Female homozygosity may occur if alternative allele is common
enough in the population
●
Female hemizygosity can occur in women with Turner syndrome
(45, X)
●
Preferential inactivation of normal X chromosome in the case of Xchromosome/autosome translocation with deleted X-chr material. →
hemizygosity of deleted region
●
Autosomal phenocopy of X-linked disorder (locus heterogeneity)
X-linked Dominant (XLD) Inheritance
●
Male and female can be both be affected
●
Within the kindren the number of affected individuals is equal
between the
●
sexes
Clinical expression is typically more consistent and severe in
hemizygous males than in heterozygous females
●
Severity of expression in males can be that of lethaly → affected
males are not seen (for instance: Aicardi syndrome)
X-linked Dominant (XLD) Inheritance
Recurrence risks
Affected male: 25%
Unaffected male: 25%
Affected female: 25%
Unaffected female: 25%
Recurrence risks
Affected male: 0%
Unaffected male: 50%
Affected female: 50%
Unaffected female: 0%
Affected child: 50%
Affected child: 50%
Da slide Prof Pignatti 2015
Sex-linked Inheritance
●
Distinction between dominant and recessive inheritance patterns
may not always be clear.
●
Mild expression of carrier females is common (sometime referred
ad semi-dominant)
Hypohydrotic ectodermal dysplasia: from embryonic ectoderm (skin,
hair, nails, teeth)
Heterogeneous condition (EDA1 gene, X-linked)
Males: full expression
Females: if fully evaluated, subtle findings of this condition are
often detected.
Sex – Linked Inheritance
●
Inheritance patterns different from autosomal inheritance
●
Locus on chromosome either X or Y
●
Females have 2 X-chromosomes
●
Males have 1 X-chromosome (and 1 Y-chromosome)
Hemizygous for X-linked genes
●
Mutant alleles on X-chromosome are “fully” expressed in males
For almost all X-linked condition, males exhibit a more severe
phenotype than females carrying the same allele. → mental
retardation occur more often in males than in females (> ~4)
Wiskott-Aldrich syndrome
●
●
XL recessive – WASP gene
Males. Infections, eczema, thrombocytopenia with small
platelets. Immune defects involving humoral and
cellular immunity. Severity increases with age.
●
Sporadic cases of females with a clinical disorder
similar to Wiskott–Aldrich syndrom
●
A case of an affected 8yrs old female with a
spontaneous mutation on exon 4 on the paternally X-
Chr associated with nonrandom pattern of inactivation
(→ XL diseases may occur in females)
X-inactivation profile of the human X chromosome
624 genes were tested in 9 inactive X chromosome (Xi) hybrids.
pseudoautosomal genes
Blue denotes significant Xi gene
expression
About 15% of X-linked genes escape
A total of 401 X-linked transcripts
inactivation to some degree
gave completely concordant results
The proportion of genes escaping
expressed 74 transcripts (~18%)
inactivation differs between different
regions of the X chromosome,
reflecting the evolutionary history of
in all hybrids:
silenced 327 transcripts
223 genes showed heterogeneity
the sex chromosomes.
among different Xi hybrids and
Females have considerable
of the Xi hybrids assayed
heterogeneity in levels of X-linked
gene expression.
were expressed in some, but not all,
[?? counseling ??]
yellow shows silenced genes
X-LINKED MENTAL RETARDATION
X-LINKED MENTAL RETARDATION
The prevalence of mental retardation in developed countries is thought to
be on the order of 2–3%
X-linked gene defects have long been considered to be important causes
of mental retardation, on the basis of the observation that mental
retardation is significantly more common in males than in females
X-linked mental retardation (XLMR)
Highly heterogeneous condition
~ 23 XLMR genes
Syndromic (further abnormalities)
Non syndromic
Fragile X (Fra(X)): the most common form of XLMR — the
mental-
retardation syndrome - ; associated with a cytogenetic marker in the distal
region Xq (FMR1 gene)
Syndromic forms of X-linked mental retardation
Syndromic forms of X-linked mental retardation
Y chromosome – Y-linked inheritance
●
●
●
●
Holoandric inheritance
Limited clinical significance
Few functioning genes
Most of the genes are involved with sex determination
Gonadal differentiation from the default ovarian to testicular
development
●
No true Y-linked condition (?)
Y chromosome
●
●
One of the larger chromosomes
~1100 functioning genes
Chr_X
Chr_Y
Length (bps)
155,270,560
59,373,566
Coding genes
830
72
Non coding genes
722
136
Pseudogenes
840
346
Short Variants
5,557,325
208,790
Y Linked genes
●
●
●
Y-linked genes are passed
from father to sons
SRY gene (sex
determining region of Y
chromosome)
Gene other than SRY,
that are on Y
chromosome code for
proteins that are unique
to males and are nor
expressed unless testes
develop
Simplified Tree of Y-Chromosme Haplogroups
Y Haplpogroups of the World
Y Haplogroups in Europe
MtDNA
Mitochondrial genome
Mitochondria play a central role in cellular energy provision
Genome with a modified genetic code.
Double-stranded, circular molecule of 16,569 bp
37 genes coding for:
2 rRNAs, 22 tRNAs and 13 polypeptides
(~1000 nuclear gene encoding mitochondrial proteins)
The mtDNA-encoded polypeptides are all subunits of enzyme complexes of
the oxidative phosphorylation system.
The replication, transcription, translation, and repair of mtDNA are
controlled by proteins encoded by nuclear
The mammalian mitochondrial genome is transmitted exclusively through
the female germ line.
Human Genome
~99.9995%
0.0005%
Nuclear genome
~3300 Mb
~20,000 genes
~25%
Genes and generelated sequences
Mitocondrial genome
16.6 kb
37 genes
~75%
Extragenic DNA
2 rRNA genes
~60%
~10%
22 tRNA genes
13 polypeptideencoding genes
~40%
~90%
Unique or low copy number
Coding DNA
Noncoding DNA
pseudogenes
Gene fragments
Introns,
untranslates sequence
Moderate to high repetitive
DNA mitocondriale e nucleare
Genoma Nucleare
dimensioni
Numero di molecole
di DNA differenti
Proteine associate
Genoma Mitocondriale
~3.300 Mb
16.6 Kb
23 (XX) o 24 (XY)
lineari
1
circolare
Diverse classi (istoni e non istoni)
~ libero da proteine
~20.000
37
Numero di geni
codificanti
Densità genica (geni
codificanti)
DNA ripetitivo
trascrizione
~ 1/165 Kb
1/0.45 Kb
abbondante
individuale
Introni
In molti geni
molto poco
Più geni in una trascrizione
continua
assenti
ricombinazione
Ereditarietà
Almeno 1 per ogni coppia di
omologhi (alla meiosi)
Mendeliana per X ed autosomi
paterna per Y
Non evidente
materna
Pathogenic mitochondrial DNA
- Mitochondrial DNA mutations cause disease in >1 in 5000
( mutations in 228 protein-encoding nuclear DNA genes and 13
mtDNA genes have been linked to a human disorder
- Pathogenic alleles are present in >1 in 200 live births
- de novo at least every 1000 births
- Many mtDNA mutations are transmitted down the maternal
line and cause progressive, disabling multi-system disease
- A progressive decline in
the expression of mitochondrial genes
is a central feature of normal human aging.
Pathogenic mitochondrial DNA
MtDNA is almost exclusively maternally inherited in mammals
Probably sperm mtDNA is tagged with ubiquitin and actively degraded in the
early preimplantation embryo.
Paternal transmission of mtDNA is likely to be an extremely rare event in
humans: in practical terms, men with mtDNA disease cannot pass the
disorder on to their offspring.
A single case of paternal transmission of a pathogenic mtDNA mutation has
been described in a patient with a myopathy
[ Engl. J. Med. 347 (2002) 576–580 ]
Homoplasmic mtDNA
Women harbouring homoplasmic mtDNA mutations can transmit mutated mtDNA to
their children.
Homoplasmic mutations are typically associated with a mild biochemical phenotype,
and cause organ-specific mitochondrial disease.
- the three point mutations (11778G>A, 14484T>C; 3460G>A) in complex I (MTND)
genes that cause Leber hereditary optic neuropathy (LHON) preferentially target the
retinal ganglion cell and cause blindness;
- the 1555A>G mtDNA 12S rRNA gene mutation causes isolated sensori-neural
deafness.
In both cases the phenotypic segregation pattern implicates additional factors in the
pathophysiology, including environmental triggers and interacting nuclear genetic loci .
Heteroplasmic mtDNA
Many pathogenic mtDNA mutations are heteroplasmic, with affected individuals
harbouring varying proportions of mutated and wild-type mtDNA.
The overall mutation load broadly correlates with the clinical phenotype: the
difference in the inherited mutation load partially explains the clinical variation between
siblings in the same family.
Understanding the mechanism of inheritance of heteroplasmic mtDNA mutations is a
major challenge facing clinicians and scientists working in the field.
Women harbouring heteroplasmic mtDNA mutation can transmit a wide range of
heteroplasmy levels to different offspring within the same sibship.
Mitochondrial inheritance
Heteroplasmic mtDNA
For some mutations the percentage level of mutant mtDNA tends to increase with
transmission, and for others the level seems to decrease.
The level of heteroplasmy is often markedly different between different tissues and
Organs (some mutation decreases its level in blood throughout life; for other mtDNA
mutations the level of heteroplasmy is remarkably consistent in different tissues, and
does not change during life.
The change in heteroplasmy seen during transmission does appear to differ between
mutations ( deleterious mutation is either lost from the maternal line, or reaches very
high levels and causes severe disease in childhood, thus preventing
further transmission)
Heteroplasmic mtDNA
The frequency distribution of oocyte mutation load corresponds to a binomial
distribution (although recent analysis suggests a slightly different distribution provides a
more accurate description ).
Equal numbers of oocytes might have mutation levels greater or less than the mean, in
keeping with the random genetic drift mechanism [Am. J. Hum. Genet. 68 (2001) 536–553]
Allele frequency of variants might rapidly shift and become fixed in a few generations
(bottleneck hypothesis whereby a decrease in the number of mitochondrial genomes
repopulating the offspring of the next generation causes a sampling effect during
transmission, leading to the rapid changes in heteroplasmy during one generation)
NO
Codice genetico del
mtDNA
il codice genetico mitocondriale, tranne nelle piante,
differisce in piccola parte da quello universale
il ritmo di sostituzione di sequenze nucleotidiche
nell’evoluzione dei genomi mitocondriali è 10 volte maggiore
rispetto a quello dei genomi nucleari.
mtDNA
MtDNA
il codice genetico mitocondriale, tranne nelle piante,
differisce in piccola parte da quello universale
il ritmo di sostituzione di sequenze nucleotidiche
nell’evoluzione dei genomi mitocondriali è 10 volte maggiore
rispetto a quello dei genomi nucleari.
Universal genetic code
Partolarità del codice genetico mitocondriale
MITOCONDRI
Codone
vertebrati
Drosophila
Lieviti
Piante
Superiori
UGA
Codice
Genetico
Universale
stop
Trp
Trp
Trp
stop
AUA
Ile
Met
Met
Met
Ile
AGA/G
Arg
stop
Ser
Arg
Arg
CUA
Leu
Leu
Leu
Thr
Leu
Fonte: B. Alberts e altri, Biol mol della cellula, Bologna 1995
Universal genetic code
Mitocondrio
Mitocondrio
Trp
Met
stop
stop
Gene of mitochondria-localized proteins linked to disease
MtDNA
Mutations in the mtDNA and diseases
ECM :encephalomyopathy;
FBSN familial bilateral striatal
necrosis;
LHON Leber’s hereditary optic
neuropathy;
LS Leigh’s syndrome;
MELAS mitochondrial
encephalomyopathy, lactic acidosis,
and strokelike episodes;
MERRF myoclonic epilepsy with
ragged-red fibers;
MILS maternally inherited Leigh’s
syndrome;
NARP neuropathy, ataxia, and
retinitis pigmentosa;
PEO progressive external
ophthalmoplegia;
PPK palmoplantar keratoderma;
SIDS sudden infant death
syndrome.
Aspetti della genetica mitocondriale
Ereditarietà materna: tutti i mitocondri dello zigote devivano dall’oocita
Eteroplasmia: un individuo possiede più di una popolazione di mtDNA.
La malattia si manifesta quando la percentuale di mtDNA mutante
supera una certa soglia. La proporzione di mtDNA mutante influisce
sulla gravità del fenotipo.
Alla segregazione mitotica la proporzione di mtDNA mutante nelle
cellule figlie può essere molto diversa rispetto a qualla della cellula
madre (eventi stocastici).
Aspetti della genetica mitocondriale
Ereditarietà materna: tutti i mitocondri dello zigote derivano dall’oocita
Eteroplasmia: un individuo possiede più di una popolazione di mtDNA.
La malattia si manifesta quando la percentuale di mtDNA mutante
supera una certa soglia. La proporzione di mtDNA mutante influisce
sulla gravità del fenotipo.
Alla segregazione mitotica la proporzione di mtDNA mutante nelle
cellule figlie può essere molto diversa rispetto a qualla della cellula
madre (eventi stocastici).
Simplified Tree of Mitochondrial Haplogroups
MTDNA Haplpogroups of the World
Population Genetics
S
– Giovanni Malerba @ univr.it