Abstract

Background: Intermediate repeat alleles are usually stable when transmitted to the next generation. However, in a small percentage of transmissions these repeats expand into the affected range, leading to a new mutation and a sporadic patient. Until now, expansions of intermediate alleles into the affected range have only been documented in paternal transmission. Case History: The index patient is a 40-year-old female with a clinical diagnosis of Huntington's disease. Symptoms started around the age of 30. The two older siblings of the patient are symptom free. CAG repeat analysis revealed the presence of a normal allele (17 repeats) and an expanded allele of 48 repeats, thereby confirming the clinical diagnosis. Her father died at 87-years-old father and had no signs of the disease. Her mother has died of sclerodermia and (vascular) dementia at the age of 70. DNA of both parents was available: blood of the father and paraffin embedded colon tissue of the mother. The father tested homozygous for an allele in the normal range (17 repeats), while the mother carried a normal allele (17 repeats) and an intermediate allele of 33 repeats. Paternity testing confirmed that they are the biological parents of the index case. The 6 siblings of the mother do not have Huntington's disease. Conclusions: The expanded repeat in our patient has originated from the intermediate repeat allele carried by her mother. This is the first documented case in Huntington's disease of expansion of an intermediate CAG repeat allele into the affected range that has occurred in maternal transmission.

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