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Huntington disease genetics — MRCPsych Paper A MCQ

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EasyGeneticsHuntington disease geneticsMRCPsych Paper A

A 38-year-old man develops chorea, disinhibition and progressive cognitive impairment. His father developed the same syndrome at 55 years of age. His adult daughter asks whether, if she inherited the disorder, her symptoms could begin at an even younger age. Which inheritance pattern and genetic mechanism best explain this family history?

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Correct answer: D — Autosomal dominant CAG repeat expansion with anticipation

The correct answer is D. Huntington disease is caused by a pathogenic CAG trinucleotide repeat expansion in the HTT gene and is inherited in an autosomal dominant pattern, giving each child of an affected heterozygous parent a 50% chance of inheriting the expansion. The earlier onset in the son than in his father illustrates anticipation: the repeat can expand during transmission, and a larger repeat is generally associated with earlier onset. Expansion is particularly associated with paternal transmission. Mitochondrial disorders are transmitted maternally, while autosomal recessive and X-linked recessive disorders do not fit successive affected generations with male-to-male transmission. Paternal deletion with genomic imprinting is associated with disorders such as Prader–Willi syndrome, not Huntington disease.

Reference: NHS England Genomics Education Programme. GeNotes Knowledge Hub: Huntington disease, sections 'Genomics' and 'Inheritance and genomic counselling'. Last reviewed 2025. https://www.genomicseducation.hee.nhs.uk/genotes/knowledge-hub/huntington-disease/