The identification of the gene (huntingtin) on chromosome 4p now allows for direct mutation analysis. The gene contains a trinucleotide repeat (CAG) that is found to be expanded in length in affected patients. The normal allele size ranges from 11 to 34 triplet repeat units, while 42 repeats or greater is considered diagnostic of huntington disease.
as in other trinucleotide repeat disorders, the phenomenom of anticipation has been observed, in this case expressed as earlier age of onset in offspring, particularly with paternal transmission. A further complication is the presence of two neighboring trinucleotide repeats (both CCG) which can expand independently without causing the disease.
Careful attention must be paid to the psychosocial support structure of prospective test subjects in huntington disease genetic counseling. Established protocols require systematic neuropsychiatric assessment and informed consent prior to dna testing.
Inheritance: autosomal dominant.