Defects of the
androgen receptor cause a wide spectrum of abnormalities of phenotypic male development, ranging from individuals with mild defects of
virilization to those with complete female phenotypes. In parallel with this phenotypic spectrum, a large number of different mutations have been identified that alter the synthesis or functional activity of the receptor
protein. In many instances, the genetic mutations identified lead to an absence of the intact, full-length receptor
protein. Such defects (splicing defects,
termination codons, partial or complete gene deletions) invariably result in the phenotype of complete
androgen insensitivity (complete
testicular feminization). By contrast, single amino acid substitutions in the
androgen receptor protein can result in the entire phenotypic spectrum of
androgen resistant phenotypes and provide far more information on the functional organization of the receptor
protein. Amino acid substitutions in different segments of the AR open-reading frame disturb AR function by distinct mechanisms. Substitutions in the
DNA binding domain of the receptor appear to comprise a relatively homogeneous group. These substitutions impair the capacity of the receptor to bind to specific DNA sequence elements and to modulate the function of responsive genes. Amino acid substitutions in the
hormone-binding domain of the receptor have a more varied effect on receptor function. In some instances, the resulting defect is obvious and causes an inability of the receptor to bind
hormone. In other instances, the effect is subtler, and may result in the production of a receptor
protein that displays qualitative abnormalities of
hormone binding or from which
hormone dissociates more rapidly. Often it is not possible to correlate the type of binding defect with the phenotype that is observed. Instead, it is necessary to measure the capacity of the receptor that is synthesized in functional assays in order to discern any type of correlation with phenotype. Finally, two types of
androgen receptor mutation do not fit such a categorization. The first of these--the
glutamine repeat expansion that is observed in
spinal and bulbar muscular atrophy--leads to a reduction of receptor function that can be measured in heterologous cells or in fibroblasts established from such patients. The expression of ARs containing such expanded repeats in men is associated with a degeneration of motor neurons in the spinal cords of affected patients. Likewise, the alterations of
androgen receptor structure that have been detected in advanced forms of
prostate cancer also behave as gain-of-function mutations. In this latter type of mutation, the exquisite specificity of the normal
androgen receptor is relaxed and the mutant receptors can be activated by a variety of steroidal and non-steroidal
ligands.