CAG-repeat expansion in androgen receptor in Kennedy's disease is not a loss of function mutation

Expansion of CAG trinucleotide repeats in androgen receptor gene is present in patients with a rare X-linked inherited form of motor neuron disorder termed Kennedy's disease or spinal and bulbar muscular atrophy (SBMA). This is a late onset progressive disease often associated with mild signs of androgen insensitivity. Defects in androgen receptor (AR) action have been linked to the expansion of the CAG trinucleotide repeats and postulated to be the cause of the disease. We have identified a trinucleotide repeat of 45 in the N-terminus of the AR in two brothers with SBMA and several members in their family (range in the general population is 11-35). Treatment of the patients with androgens failed to improve their clinical symptoms and provided no hint of an anomalous function of the AR. Consistently, functional analysis of the mutant receptor showed hormone binding, transactivation and transrepression potentials identical to that of the wild-type receptor. These results together argue against SBMA being a loss of function mutation of the AR.