Gene Regulation and Epigenetics
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Direct answer
Prader-Willi and Angelman syndromes can both arise from the same 15q11–13 deletion, and which disease the child has depends entirely on which parent's copy was lost — the clearest proof that gene expression is regulated by heritable marks layered on DNA rather than by sequence alone. Epigenetics comprises exactly those marks: CpG methylation by DNA methyltransferases, histone tail modification (acetylation, methylation, phosphorylation) governing chromatin openness, and non-coding RNAs such as XIST and microRNAs. On this sit the classic regulation systems from bacterial operons to X-inactivation and RNA interference.
What you must remember
- Lac operon (Jacob and Monod): negative control by the repressor plus positive control by catabolite activator protein–cAMP; lactose (allolactose) induces, glucose represses by lowering cAMP.
- Eukaryotic control layers: chromatin accessibility, transcription factors binding promoters and enhancers, processing, translation and protein stability — operon-style polycistronic units do not exist in humans.
- Histone acetylation by histone acetyltransferases loosens the nucleosome (open, active chromatin); deacetylation compacts it — hence histone deacetylase inhibitors like vorinostat as anticancer drugs.
- DNA methylation at CpG islands (by DNMT3a/b de novo, DNMT1 maintenance) generally silences genes; global hypomethylation plus local promoter hypermethylation (for example MLH1) is a hallmark of cancer.
- Imprinting at 15q11–13: loss of the paternal contribution gives Prader–Willi (hypotonia, obesity, hypogonadism, intellectual disability); loss of the maternal UBE3A gives Angelman (puppet-like laughter, seizures, absent speech) — uniparental disomy and imprinting defects produce the same phenotypes without any deletion.
- X-inactivation (Lyonisation) is initiated by the XIST long non-coding RNA; the Barr body is the inactivated X; happens early in every female embryo and explains manifesting heterozygotes and calico cats.
- MicroRNAs (about 22 nucleotides) processed by Drosha and Dicer guide RISC to degrade or repress target mRNAs — the basis of RNA interference, a Nobel-winning tool and therapeutic platform.
- Transcription factor motifs worth naming: zinc fingers (steroid receptors), leucine zippers, helix-loop-helix.
- Epigenetic drugs in use: azacitidine and decitabine (DNMT inhibitors) for myelodysplastic syndromes; the hydatidiform mole (all-paternal genome) and ovarian teratoma (all-maternal) show that both parental contributions are irreplaceable.
Working through an imprinting referral
A two-year-old with floppiness in infancy, now insatiable and obese with small hands and feet, raises Prader–Willi syndrome. Methylation-specific PCR of 15q11–13 shows only a maternal methylation pattern — the paternal contribution is absent (deletion, uniparental disomy or an imprinting centre defect; about a quarter of Indian referrals are uniparental disomy, for which recurrence risk is low). The child's genes are intact on the mother's side; they are simply silenced by methylation marks set in her germline, and no amount of sequencing the base pairs would reveal the difference. Contrast the Angelman child, laughing inappropriately with jerky ataxic movements and severe epilepsy: the maternal UBE3A is missing, and in neurons only the maternal allele works because the paternal copy is imprinted off. The lesson generalises: deletion size may be identical; the parent of origin decides the disease. Counselling then hinges on mechanism — a paternal deletion carries up to 1 per cent recurrence (imprinting-centre mutations more), while disomy is usually sporadic.
Where students slip
Students describe methylation as a mutation; it is not — the base sequence is unchanged, the mark is copied after each division, and (crucially) it is erased and reset in the germline, which is why cloned animals and assisted-reproduction offspring show imprinting disturbances. The second slip is treating X-inactivation as complete and permanent: some genes on the Barr body escape, and skewing can manifest a carrier (for example, an X-linked Duchenne carrier with mild weakness). Finally, examiners pair the operon with catabolite repression: glucose lowers cAMP, CAP cannot bind, and the lac promoter stays off even with lactose present — the famous diauxic growth curve.
Frequently asked questions
What mechanism produces Prader-Willi versus Angelman syndrome?
Imprinting at 15q11–13: paternal gene loss (deletion, disomy or imprint defect) gives Prader–Willi; maternal UBE3A loss gives Angelman; parent of origin, not deletion size, determines the phenotype.
How does histone acetylation switch genes on?
Histone acetyltransferases neutralise the positive charge on lysine residues of histone tails, weakening their grip on DNA; chromatin opens and transcription factors gain access — reversed by histone deacetylases, themselves drug targets in lymphoma and myelodysplasia.
What is XIST and what does it achieve?
A long non-coding RNA transcribed from the X-inactivation centre that coats the chromosome in cis, recruiting silencing complexes; the coated X condenses into the Barr body, achieving dosage compensation for X-linked genes.
How do microRNAs regulate gene expression?
Primary miRNA is processed by Drosha then Dicer into about 22-nucleotide duplexes; one strand loads into RISC and guides it to complementary mRNAs, which are cleaved or translationally repressed — a natural post-transcriptional control layer exploited by siRNA therapeutics.
Which epigenetic drugs are used in clinical practice?
DNA methyltransferase inhibitors azacitidine and decitabine reactivate silenced genes in myelodysplastic syndromes; histone deacetylase inhibitors such as vorinostat are used in cutaneous T-cell lymphoma.