Homozygosity in Huntington's disease: new ethical dilemma caused by molecular diagnosis.

Huntington's disease (HD) is a degenerative disorder of the central nervous system with autosomal dominant inheritance. Genetic counseling has always been difficult in this disorder with anguish, depression and denial being very common in both the patient and family members. The discovery of the causal gene has led to precise diagnostic procedures allowing homozygotes for the disease to be identified. Contrary to what occurs in some other autosomal dominant diseases, the course of the disease is not more severe in the homozygote than in the heterozygote. The present authors describe a family comparing two affected siblings: one is heterozygotic and the other homozygous for the HD mutation. They confirm that the age and symptoms of onset did not differ significantly between the subjects; however, the disease seemed to have a more severe progression in the heterozygote than in the homozygote. The authors discuss the ethical dilemma derived from the genetic counseling of a homozygotic patient, given the fact that all his offspring will be affected. Letting the offspring know about their 100% probability of inheriting the disorder is equivalent to delivering a non-requested predictive test, while not informing them constitutes withholding crucial information from the individual.