20.03.16 Other repeats - polyalanine - FSHD Flashcards
What are polyalanine tracts
- Runs of >4 alanines.
- 500 proteins with PAs
- Encoded by GCU, GCC, GCA, GCG
- Commonly found in transcription factors.
How do polyalanine tracts react in physiological conditions
- Hydrophobic
- Form beta-sheets
- Extremely resistant to chemical or enzymatic degradation.
- Meiotically and mitotically stable (unlike other triplet repeats)
Physiological purpose of PA tracts
Normal PA tracts thought to be flexible spacers conferring stability and flexibility to protein 3D conformation
Disease mechanism of PA tract expansions
Destabilise the native conformation of the protein and alter protein levels and activity, cause transcriptional dysregulation.
How many disorders to date are due to PA expansions
- 8 are early developmental abnormalities and OPMD.
- Only OPMD is solely caused by PA expansions.
Review of OPMD
- Oculopharyngeal muscular dystrophy
- AD (rarely AR) expansions in PA tract in exon 1 of PABPN1 gene.
- Normal= 10 PA repeats
- In disease, +1 to +7
- Encodes a nuclear poly(A)-binding protein involved in polyadenylation of mRNA precursors.
- Highly expressed in skeletal muscle
- Clinical presentation= ptosis, dysphagia and limb weakness. Presence of pathological filamentous inclusions in muscle fibre nuclei.
- Some mutations lead to addition of alanines. e.g. p.(Gly12Ala) substitutes Ala in the 11th position, followed by 2 further Ala codons, so effectively adds 3 repeats to expansion.
Review of Hand-foot-genital syndrome
- Affects distal limbs and genitounrinary tract
- Missense and LOF variants. Also a PA expansion in HOXA13 gene.
- HOXA13 is a homeobox transcription factor of the A cluster, required for limb and genital patterning.
- HOXA13 expansion results in mislocalisation of the protein to the cytoplasm as well as misfolding.
Review of X linked mental retardation
- Expansions of 2 of the 4 Ala tracts in ARX gene.
- Non-syndromic XLMR and X-linked infantile spasm syndrome.
- ARX is a paired type homeobox gene.
- LOF mutations are associated with a more severe phenotype than PA expansion, where there is abnormal genitalia and lissencephaly.
- PA expansion may be gain of function, inducing protein aggregates and cellular toxicity in vitro.
Is there a correlation between repeat length and disease severity
- Yes
- Also homozygotes can be more severe than hets
How are PA tracts thought to have arisen ,
- Unequal crossing over between two mispaired normal alleles.
- Stable in somatic and germline and stably transmitted to offspring
What causes the characteristic protein aggregation in PA expansion disorders
- Longer PA tracts form beta sheet complexes (rather than monomeric alpha helix). Complexes have stronger protein-protein interactions leading to elevated levels of dense, insoluble protein assemblies.
- Chaperone- proteasome activity is altered leading to accumulation of misfolded proteins. Eventually triggers apoptosis.
Therapeutic strategies in PA disorders
- Induction of HSP70/HSP40 to reduce PABPN1 exp-induced cellular toxicity
- Trehalose (chemical chaperone) promotes the clearance of PABPN1 expansions by proteasome
- Geldanamycin significantly increases HSP70 expression in cells with PHOX2B exp, enhancing proteasome-mediated degradation.
Review of Congenital Central Hypoventilation Syndrome
-Disorder of respiratory and autonomic regulation
-Due to AD PHOX2B mutations. Either due to PA expansions in exon 3 or LOF mutations (usually more severe phenotype).
-Normal= 20 alanine repeats. Full penetrance= 25+ repeats
Phenotype is characterised by normal ventilation whilst awake, but hypoventilation and shallow breathing during sleep (hypoventilation when both awake and asleep when most severe).
-Phenotype genotype correlation, where the longer the expansion, the more severe the respiratory symptoms.
-Meiotically stable, with many reports of stable parent-to-child transmissions.
Overview of FSHD
- Facioscapulohumeral muscular dystrophy
- 3rd most common MD (after DMD and DM1). 1 in 20,000
- Phenotype= asymmetrical progressive proximal muscle weakness, scapular winging.
- 2 subtypes (indistinguishable phenotypically) FSHD1 or FSHD2 caused by aberrant expression of DUX4 transcript, encoded by the D4Z4 repeats.
Review of FSHD1
- Contraction of D4Z4 repeats
- Autosomal dominant.
- Each D4Z4 repeat encodes a copy of DUX4 gene. All referred to as DUX4-like, apart from the most distal copy.
- DUX4-like copies are constitutively inactivated. DUX4 is also silenced apart from during early development and in testes of adult males
- DUX4 expression causes apoptosis of myoblasts.
- Disease can be modified by presence of SMCHD1 mutations (more severe if present)
