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Huntington Disease l: Introduction01:21

Huntington Disease l: Introduction

Huntington disease or HD is a progressive, fatal neurodegenerative disorder inherited in an autosomal dominant pattern.PathophysiologyIt is caused by expansion of the CAG trinucleotide repeat in the HTT gene on chromosome 4 (4p16.3), producing an abnormal huntingtin protein with an expanded polyglutamine tract. This misfolded protein disrupts cellular function, leading to neuronal death. Normal alleles have ≤26 repeats, 27–35 are intermediate (risk of expansion), 36–39 show reduced penetrance,...
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Alzheimer disease is a chronic, progressive, and irreversible neurodegenerative disorder and the most common cause of dementia in older adults. It leads to gradual neuronal loss, causing cognitive decline, behavioral changes, and loss of functional independence.Risk Factors and EtiologyThe disease is multifactorial. Age is the strongest risk factor, with prevalence doubling every 5 years after age 65. Genetic factors include mutations in genes such as APP, PSEN1, and PSEN2, which are associated...
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Alzheimer disease involves structural changes in the brain that begin long before symptoms appear. The most distinctive features are extracellular neuritic plaques and intracellular neurofibrillary tangles.Neuritic plaques form in the cerebral cortex and around blood vessels. These plaques contain a dense core of beta-amyloid (Aβ)—a toxic protein fragment that clumps outside neurons. The core is surrounded by damaged neuronal extensions, as well as reactive astrocytes and microglia. Abnormal...
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C9orf72 hexanucleotide repeat expansions in clinical Alzheimer disease.

Matthew Harms1, Bruno A Benitez, Nigel Cairns

  • 1Department of Neurology, Washington University School of Medicine, St Louis, Missouri, USA.

JAMA Neurology
|April 17, 2013
PubMed
Summary

Hexanucleotide repeat expansions in the C9orf72 gene are a common cause of frontotemporal dementia and ALS. This study found these expansions in a small subset of Alzheimer disease patients, suggesting genetic overlap.

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Area of Science:

  • Neuroscience
  • Genetics
  • Neurology

Background:

  • Hexanucleotide repeat expansions in the chromosome 9 open reading frame 72 (C9orf72) gene are a known cause of frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS).
  • The genetic overlap between FTD, ALS, and Alzheimer disease (AD) is complex and not fully understood.

Purpose of the Study:

  • To determine the frequency of C9orf72 repeat expansions in individuals diagnosed with late-onset Alzheimer disease (AD).
  • To investigate the clinical presentation and genetic segregation of C9orf72 repeat expansions in AD families.

Main Methods:

  • A case-control study genotyped 872 familial late-onset AD cases and 888 controls using repeat-primed polymerase chain reaction to detect C9orf72 repeat expansions.
  • Analysis included determining repeat length, segregation with disease, and clinical features of carriers.

Main Results:

  • C9orf72 repeat expansions were identified in 5 families, with 3 showing large expansions and segregation with disease.
  • One carrier presented with neuropathology consistent with AD.
  • C9orf72 repeat expansions were the second most common pathogenic mutation in this AD cohort, after PSEN1 A79V.

Conclusions:

  • C9orf72 repeat expansions account for a small percentage of clinically diagnosed AD cases, particularly those with a strong family history.
  • These findings underscore the importance of screening for FTD-associated genes, like C9orf72, in AD patients presenting with familial clustering.