Two cases of androgen insensitivity due to somatic mosaicism

Androgen insensitivity syndrome (AIS) is caused by mutations in the gene encoding the androgen receptor (AR). The incidence of AIS is estimated to be 1 in 99,000. Complete androgen insensitivity syndrome (CAIS) is characterized by a 46,XY karyotype with external genitalia that appear typically female and results from mutations that render the androgen receptor non-functional. Partial androgen insensitivity syndrome (PAIS) results from partial loss of function mutations in AR. Rarely, PAIS results from somatic mosaicism for an AR mutation and not from a hypomorphic variant. We present two cases of PAIS due to somatic mosaicism, one caused by a novel nonsense mutation and one caused by a missense mutation previously reported in CAIS. Two patients with atypical genitalia presented to our multidisciplinary clinic for disorders of sex development and sequencing of AR was performed as part of the diagnostic evaluation. In case one, AR sequencing revealed mosaicism for a nonsense mutation, c.1331T > A; p.Leu444Ter. This mutation has not previously been reported, but is presumed to be pathogenic. In case two, AR sequencing revealed a mosaic missense mutation, c.2279 C > A; p.Ser760Tyr, which has previously been reported in CAIS but not in PAIS. Similar phenotypes may result from AR mutations that are present in a mosaic state with full loss of function or hypomorphic mutations that partially impair the function of the protein in either all tissues or in a mosaic state.