Repeat Expansion Disorders PDF

Complications of mendelian inheritance patterns Repeat expansion disorders  Lecture overview Repeat expansion disorders Huntington disease: clinical findings, genetics, huntingtin function, pathogenesis Anticipation Somatic instability Overview of molecular mechansims W...

Complications of mendelian inheritance patterns Repeat expansion disorders  Lecture overview Repeat expansion disorders Huntington disease: clinical findings, genetics, huntingtin function, pathogenesis Anticipation Somatic instability Overview of molecular mechansims WGS for diagnosis of repeat expansion disorders  Repeat expansion disorders At least 50 monogenic disorders caused by expansion of tandem repeats – typically triplets (triplet repeat disorders). Mostly neurologic disorders of CNS and neuromuscular disorders; different genes, repeat sequences, location within gene, and molecular mechanisms SCA = spinocerebellar ataxia Images: https://quizlet.com/415149465/trinucleotide-repeat-expansions-flash-cards/ https://geneticeducation.co.in/trinucleotide-repeat-expansion-disorders-101/?utm_content=cmp-true Huntington disease (HD) Progressive neurodegenerative disease Motor, cognitive, and psychiatric disturbances; increasing in severity with age Adult onset (age-dependent penetrance), mean age 44y 5-20% of patients with onset at age 50y Complete penetrance over life span Median survival 15-18y after onset Inheritance: autosomal dominant Global prevalence: 5 per 100 000 individuals Clinical findings Natural history of clinical Huntington disease Bates et al. Nat Rev Dis Primers. 2015;1:15005. Abnormalities of movement chorea - irregular jerking of limbs, face or trunk; twisting and writhing motions; disjointed gait Cognitive decline impairment of executive functions; memory deficit, impaired visuospatial abilities Psychiatric disturbances. personality changes, depression, hostility, obsessive-compulsiveness, anxiety, apathy, psychosis  Atrophy of corpus striatum (caudate and putamen) and cortex Preferential loss of neurons involved in control of movement www.alamy.com Image ID 2G57F23 Normal Huntington disease Left: [18F]Fluoro-deoxyglucose (FDG) positron emission tomography (PET) A scan from (a) a normal control and (b) a patient with Huntington's disease showing reduced striatal and cortical metabolism. Andrews et al Mol Med Today. 1998 Dec;4(12):532-9 Genetics Gene: HTT Pathogenic variant: expansion of CAG triplet repeat in exon 1 normal range 11-35 CAG repeats pathogenic range >36 CAG repeats with full penetrance * Pre-mutation: risk of expansion to pathogenic * range during gametogenesis → risk of disease in progeny https://www.nature.com/articles/gim2014146  Molecular diagnosis of Huntington disease PCR amplification with primers flanking repeat region Electrophoretic separation by gel (left) or capillary (right) electrophoresis Normal Heterozygote (affected) Prenatal diagnosis Left: Strachan and Read HMG4 Figure 18.9 Right: https://www.nature.com/articles/gim2014146 Age-of-onset is inversely correlated with CAG repeat size Adult onset, typically 36-55 repeats Juvenile onset, typically >60 repeats Longer repeat size associated with earlier onset, increased severity and shorter survival. Nevertheless, a given repeat size is associated with variable expressivity across patients Holmans et al. Hum Mol Genet. 2017;26(R2):R83-R90. Anticipation Earlier age-of-onset and increasing severity from one generation to the next. Due to expansion of triplet repeats during gametogenesis. Adapted from Ranen et al. Am J Hum Genet. 1995 Sep;57(3):593-602. Repeat expansion due to DNA slippage Triplet repeats can expand during gametogenesis due to slippage of DNA strands during replication and inefficient mismatch repair. Backward slipping of new strand and loop formation Replication of expanded strand  Somatic instability Triplet repeats can undergo expansion in somatic cells throughout lifetime and in different cell types → influences disease progression Expansion can occur in nondividing cells (e.g. neurons), as well as in dividing cells suggests replication-independent mechanism for somatic expansion Degree of expansion differs substantially among somatic tissues (somatic mosaicim), and in different brain regions (greater in striatum and cortex). Brain Inherited (e.g. 60 repeats) (e.g. 36 repeats) Blood (e.g 40 repeats) Somatic expansion Muscle (e.g 36 repeats) Image: Arning et al. (2022).. Medizinische Genetik. 33. 293-300. 10.1515/medgen-2021-2101. Huntingin (HTT) No homology to other proteins Broad tissue expression; highest levels in CNS, enriched in striatal and corticostriatal neurons Wide subcellular distribution: mainly cytoplasm, also in nucleus and organelles throughout neuron (cell body, axon, synaptic terminal) PolyQ tract HEAT domains tandem repeats of 40aa antiparallel hydrophobic a-helices interact with other neuronal proteins Huntingtin function Important for development of CNS; essential for survival and function of corticostriatal network note: HTT knockout is lethal in mice Multifunctional protein → functions by interacting with other effector proteins (>400) involved in numerous neuronal processes: transcriptional regulation nuclear import/export mitochondrial function axonal transport synaptic vesicle recycling receptor endocytocis postsynaptic signaling Mutant huntingtin (mHTT) Expansion of polyQ tract Change in conformation: transformation of a- helices into b-folded chains leading to cross- linking & missfolding Aberrant interaction with other proteins Aberrant cleavage into toxic fragments Fomation of insoluble protein aggregates (inclusions) in cytoplasm and nucleus Gain-of-function: neurotoxicity  Immunohistochemical staining of Fluorescence microscopy of striatal neurons the motor cortex of an HD brain transfected with mutant huntingtin with an anti-polyQ antibody INI = intranuclear inclusions The spiny neuron in the center (yellow) C = cytoplasmic inclusions shows an inclusion body (orange) Ross Nat Rev Mol Cell Biol. 2005;6(11):891-8. https://en.wikipedia.org/wiki/Huntington%27s_disease Pathogenesis BDNF, Brain-derived neurotrophic factor; ROS, reactive oxygen species; NMDAR, N-methyl-D- aspartate receptor. Jimenez-Sanchez et al, Cold Spring Harb Perspect Med. 2017 Molecular mechanisms of nucleotide repeat expansion pathogenesis 1 2 3 4 1 Hypermethylation of DNA at gene promoter regions → transcriptional silencing of gene harbouring repeats Active transcription through repeats triggers → formation of RNA–DNA hybrids (R-loops) → triggers DNA damage response, potentially exacerbating somatic instability of repeats Example: Fragile X syndrome X-linked dominant developmental delay, intellectual disability, learning problems, social and behavioral Full mutation (>200) problems, craniofacial characteristics Premutation (55-200) Intermediate (45-54) expansion of CGG repeats in 5’UTR of FMR1 gene Normal (