Epigenetics + Disease Flashcards
What is Angelman Syndrome (AS)?
= happy puppet syndrome (not used now)
= developmental delay
= severe speech impairment
(can vary in severity)
= jerky movements, hand flapping
= happy disposition, laughing, smiling (very sociable)
= microcephaly (small head)
= seizures
= abnormal EEG
What is Prader-Willi syndrome (PWS)?
= Hypotonia (low muscle tone)
= poor sucking reflex and feeding in infancy
= insatiable appetite in later life
= obesity (+associated health problems)
= short stature (treated with growth hormone)
= compulsive behaviour (e.g. skin picking)
= strabismus (eye crossing)
= hypogonadism, poor sexual development
What is the chromosome region associated with PWS and AS?
= 15q11-13
= contains 4Mbp region flanked by direct repeat sequences
= same region is deleted in PWS and AS
How is PWS/AS inherited?
Deletion inherited from father
= PWS
(AS gene is inactivated)
Deletion inherited from mother
= AS
(PWS gene is inactivated)
= GENOMIC IMPRINTING
(is an epigenetic trait = involves modifications of DNA, histones or both = leading to a repressed state)
e.g. Mammalian cytosine methylation (CpG methylation)
= cytosine methylated by DNA methyl transferase at position 5
= can be de novo = will methylate unmethylated
= maintenance will hemi-methylate DNA
= epigenetic trait inherited in daughter cells
How is methylation responsible for imprinting?
= methylation is removed in primordial germ cells, renewed during meiosis
= all oocytes have methylation pattern consist with those of maternal chromosomes
= all sperm have methylation pattern consistent with this of paternal chromosomes
How does Uniparental disomy cause PWS/AS?
Both chromosomes 15 from father
= paternal imprint on both chromosomes
= equivalent to a deletion of the maternal chromosome
= Angelman syndrome
Both chromosomes 15 from mother
= maternal imprint on both chromosomes
= equivalent to a deletion of the paternal chromosome
= Prader-Willi syndrome
BUT = Karyotypes / FISH will be normal
(instead need to look at methylation patterns on chromosomes)
How to observe methylation patterns on chromosomes?
= Bisulfite treatment allows methylation-specific PCR
= DNA treated with sodium metabisulfite = has different effects on methylates vs unmethylated DNA
Methylated sequence
= has methylated DNA primer = protects cytosine from sodium metabilsulfite
Unmethylated sequence
= non-methylated DNA primer = no protection = get uracil instead of unmethylated cytosine
= thymine instead in final sequence
What is another cause of Angelman Syndrome?
= UBE3A
10% of cases caused by point mutations in maternal allele
= only imprinted (paternally) in neurons in the brain
= AS caused by loss of UBE3A expression in the brain
= codes for protein turnover enzyme
(targets specific proteins in a defined subset of neurons)
What is another cause of Prader-Willi Syndrome?
= NO single gene point mutations (like in AS)
= BUT small deletions in SNRPN cause PWS
= SNRPN is over 460kb in length with at least 140 exons
= codes for a small nuclear ribonucleoprotein (?role in RNA processing)
= large number of functional small nucleolar RNA (snoRNA) genes within introns of SNRPN
= SNRPN is maternally imprinted
= loss of paternal expression leads to defects in splicing and modification of RNAs
= imprinting centre = control expression of SNRPN itself + three other genes
SNRPN imprinting centre
= controls expression of genes on the paternal chromosome
= secondary effect due to transcriptional collision
SNRPN imprinting centre
= is methylated
= and inhibited on the maternal chromosome
= primary effect of imprinting = blocks expression of genes
What are some other disorders that involve imprinting regions?
What are the evolutionary origins of genomic imprinting?
= imprinting genes are often associated with growth or nutrient acquisition
= ?sexual conflict - male/female getting genes out of the gene pool
= male’s genes favour growth (regardless of mother’s health)
= female’s genes favour restricted growth (greater chance of surviving to have more pregnancies = not at risk of own health)
= father should silence genes restricting growth (activate growth-promoting genes)
= mother should silence genes promoting growth
= should have strong effects on the placenta
What is uniparental diploidy?
= zygotes get ALL chromosomes from 1 parent
2 maternal copies
= ovarian tetratoma
= disorganised fetal body parts, no membranes
= placenta does not develop
= leads to growth restriction
2 paternal copies
= hydatidiform mole
= vigorously growing membranes, no embryo
= lots of placental tissue, no embryo
= evolutionary basis of genomic imprinting
What is X chromosome inactivation?
Xist lncRNA
= coats inactive X and recruits silencing factors
= e.g. DNA methyltransferases
= transcribed in both X chromosomes
If female
= 1 X chromosome is inactivated = compressed in Bar body
= DNA is inactivate = methylated using Xist
= ensures the number of active genes is the same in male/female
= is inherited (becomes a problem if disease-associated)
What are the consequences of X-inactivation for carriers of X-linked diseases?
= half of the cells will lack the normal gene
= effects depend on the nature of the gene product and number/location of cells containing the inactivated X
= gene for secreted protein (e.g. blood clotting factor) - unlikely to have much effect
= cell autonomous requirement (e.g. Dystrophin) - tissues may be partially affected
(distribution in tissue of Dystrophin protein can vary = different severities of disease)
(no dystrophin = DMD)
= required for cell survival - non random X-inactivation (e.g. X-linked SCID)
