MRCPath Part 1 4 th September 2009 Val Wilson

1. Explain what is meant by anticipation and give examples.Illustrate to what extent molecular data explains the observations. MRCPath Part 1 4th September 2009 Val Wilson

2. Anticipation is a phenomenon characterised by a younger age of onset and an increased severity of disease in successive generations

3. anticipation found in many of the repeat expansion disorders tri- tetra- penta- more complex nucleotide repeats cause > 40 neurological, neurodegenerative and neuromuscular disorders repeats found in promoter, 5’UTR, intron, coding region and 3’UTR of genes • normal range repeat size is small, polymorphic, sometimes interrupted and stable in the germline (mutation rate <1%/generation) • longer normal alleles have a higher mutation rate (~5%/generation), usually restricted to expansions of 1 – 3 repeats

4. when the repeat size exceeds the disease threshold they become much more unstable • interruptions are typically lost in disease causing alleles – these have a stabilising effect • intergeneration mutation rate can be as high as 100%/generation • tendency towards further repeat gains is often >90% • repeat instability seen in somatic and germline tissues – dynamic mutations • repeat instability due to combination of aberrant DNA replication, recombination and repair • increase in repeat length correlates to increased disease severity and earlier age of onset • repeat instability between generations can lead to larger repeat length in successive generation – anticipation is observed • parental expansion bias observed – dependent on sequence of the repeat and length of the allele

5. Huntington disease (HD) • progressive neurodegenerative disorder characterised by triad of motor, cognitive and emotional disturbances • autosomal dominant disorder • CAG repeat expansion in exon 1 in IT15 gene • polyQ disorder normal allele size ≤26 repeats intermediate 27 – 35 repeats affected ≥36 repeats (full penetrance ≥40 repeats) • conversion of a normal allele into the premutation range is thought to involve the loss of stabilising interruption in long normal alleles • paternal transmission bias longer repeat length seen earlier age of onset repeat expansion shown during spermatogenesis

6. significant inverse correlation between the CAG repeat length, age of onset of disease and severity of disease adult onset 36 – 55 repeats juvenile onset (<21yr) >60 repeats • expansions >60 repeats - ~4% spectrum of mutations • frequency of juvenile HD (JHD) - ~8 – 10% cases • transmitting parent in 70 – 80% cases is male • mutations of 80 – 100 repeats are rare - ~5% of JHD cases • may cause infantile HD with onset <10yrs • increased concentration of protein aggregates are seen in neurons of JHD patients • however repeat size not the only factor influencing age of onset candidate gene modifiers are TAA polymorphic repeat in GluR6 gene adenosine A2A-receptor dysfunction

7. Dentatorubral-Pallidolusian Atrophy (DRPLA) progressive neurodegenerative disorder characterised by a varying combination of myoclonus, ataxia, involuntary muscle movements, dementia and other psychiatric disorders autosomal dominant predominantly found in Japanese, identified in other populations including European CAG repeat expansion in exon 5 of ATN1 gene (encodes atrophin 1) polyQ disorder normal allele size 6 – 35 repeats mutable normal 20 – 35 repeats affected 48 – 93 repeats (<60 repeats reduced penetrance) paternal transmission bias increased intergenerational repeat size segregation distortion – offspring of affected fathers more likely to inherit disease allele older fathers more likely to transmit disease allele with greater increase in size

8. significant inverse correlation between the CAG repeat length, age of onset of disease and severity of disease age of onset repeat size <21 63 – 79 (+ epilepsy) 21 – 40 61 – 69 >40 48 – 67 • severe infantile onset reported - 90 – 93 repeats • homozygosity - greater severity of disease • anticipation is seen 28yrs/generation with paternal transmission 15yrs/generation with maternal transmission 52 repeats very mild symptoms at 67yrs 57 repeats first symptoms at 32yrs, severely affected by mid-40s, no epilepsy seen 66 repeats symptoms at 13yrs, severely affected by 20, epilepsy

9. Autosomal Dominant Spinocerebellar Ataxias • polyQ expansion disorders • anticipation is seen in SCAs1, 2, 3 and 7 • affected allele size SCA1 39 – 91 SCA2 >32 SCA3 45 – 86 SCA7 34 – 460 • SCA7 regarded as most unstable of disorders with CAG repeats • paternal expansion bias • anticipation generally not observed in SCA6 and SCA17 • affected allele size SCA6 20 – 33 small expansion SCA17 43 – 66 complex repeat with interruptions

10. SCA17 • phenotype is complex and includes ataxia, dementia, chorea, parkinsonism, and psychiatric symptoms • polyQ repeat expansion in TATA-binding protein gene (TBP) • repeat expansion is usually interrupted 5’-(CAG)3(CAA)3(CAG)7-11CAA CAG CAA (CAG)9-21CAA CAG-3’ normal allele size 25 – 42 pathogenic >42 reduced penetrance 43 – 48 full penetrance ≥49 – 66 • stable transmission – anticipation not observed • shorter configuration has been seen on rare occasions - ~10% expansions 5’-(CAG)3(CAA)3(CAG)>32CAA CAG-3’ • unstable transmission – anticipation observed • strong correlation between CAG/CAA repeat configuration and instability found

11. 10 intergenerational transmissions studied paternal transmission – 6 maternal transmission – 4 increased length by 1 – 13 repeats/generation paternal transmission – median 4.5 repeats maternal transmission – median 1.5 repeats age of onset 1 – 35 years earlier paternal transmission – range 16 – 35yrs (mean 27yrs) maternal transmission – range 1 – 14yrs (mean 5.5yrs) severity of disease increased wheelchair bound – mean 5.3yrs compared to 12.5yrs confinement to bed – mean 5.3yrs compared to 15.3yrs anticipation correlated with parental age suggests progressive age-dependent instablility of the uninterrupted repeat tract in the germline

12. DM1 • multisystem disorder affecting skeletal and smooth muscle, eye, heart, endocrine system and central nervous system • categorised into 3 overlapping phenotypes mild – cataracts and mild myotonia, normal life span classic – above + muscle weakness and wasting, abnormalities in cardiac conduction, shortened life span congenital – hypotonia and severe generalised weakness at birth, respiratory insufficiency, early death • CTG repeat expansion in 3’UTR of DMPK gene normal allele size 5 – 35 repeats premutation 35 - 49 repeats affected 50 – >4000 repeats • maternal transmission bias woman with CTG repeat <300, 10% risk of child who has inherited an abnormal allele having congenital DM1 woman with CTG repeat >300, 59% risk of child who has inherited an abnormal allele having congenital DM1

13. significant inverse correlation between the CAG repeat length, age of onset of disease and severity of disease mild 50 - ~150 repeats onset 20 – 70yrs classic ~100 - ~1000 repeats onset 10 – 30yrs congenital >2000 repeats onset birth – 10yrs • repeat sizes 100s or even 1000s greater than parent have been seen in offspring • age of onset typically decreases by 20 – 30 years/generation

14. polyalanine disorders • cause at least 9 known disorders • 8 developmental disorders with polyalanine expansions in transcription factors essential for early development • expansion in PABPN1, encoding a polyadenylation factor is associated with late onset oculopharyngeal muscular dystrophy • encoded by the imperfect triplet (GCN)n – therefore interrupted • threshold length for expansion is low (10 – 20 repeats) • rarely expand more than 1.5 fold – largest expanded allele 33 repeats (within the normal range for most polyQ disorders) • thought expansions arise through unequal recombination which leads to stable expanded repeats • stably transmitted • no anticipation seen

15. anticipation may also occur in disorders caused by defects in telomere maintenance • familial chronic myeloproliferative disorder significant shortening of telomere length in offspring compared to parent earlier age of onset Rumi et al. (2007) J Clin Oncol 25:5630-5 Rumi et al. (2008) Blood 112:2587-8 • dyskeratosis congenita shorter telomere length in patients with TERC mutations observed earlier age of onset and more severe disease Vulliamy et al. (2004) Nature Genetics 36:447-9

16. anticipation also observed in monogenic disorders Lynch Syndrome Westphalen et al. (2005) Human Genet 116:461-5 Nilbert et al. (2009) J Clin Oncol 27:360-4 BRCA mutation carriers Peixoto et al. (2006) Fam Cancer 5:379-8 • anticipation has also been described in polygenic disorders Rheumatoid Arthritis McDermott et al. (1996) Ann Rheum Dis 55:475-7 Radstake et al. (2001) J Rheumatol 28:962-7 Graves’ Disease Vox et al. (2009) Eur J Endocrinol 161:131-8 Brix et al. (2003) Thyroid 13:447 Biopolar Disorder Swift-Scanlon et al. (2005) Psychiatr Genet 15:91-9 Crohn Disease Freeman and Hershfield (2001) Can J Gastroenterol 15:695-8 • remains to be proven

17. References Gao et al. (2008) Eur J Hum Genet 16:215-222 Gomes-Pereira and Monckton (2006) Mut Research 598:15-34 Maglione et al. (2005) Am J Med Genet 139B:101-105 Maglione et al. (2006) Neurosci Letts 393:27-30 Messaed and Rouleau (2009) Neurobiol Dis 34:397-405 Rasmussen et al. (2007) Ann Neurol 61:607-610 Squitieri et al. (2006) Mech Age and Devt 127:208-212 Vinton et al. (2005) Am J Med Genet 136A:201-204