Androgen Insensitivity Syndrome (AIS) via the Androgen Receptor (AR) Gene

Male sexual differentiation requires the Androgen Receptor (AR) and its ligands—Testosterone and Dihydroxytestosterone (DHT) (Wilson and Davies 2007). Testosterone, which is secreted by the developing testes beginning at 9 weeks of gestation, stimulates Wolffian duct differentiation into epididemis, vas deferens and seminal vesicles. DHT, derived from testosterone via the enzymatic activity of 5α-reductase type 2, stimulates prostate development and masculinization of the primordial external genital into penis and scrotum (Brinkmann et al. 2001). Both androgens exert their effect through binding and activation of the AR. In individuals with a 46,XY karyotype, defects in AR signaling result in Androgen Insensitivity Syndrome (AIS). AIS symptoms range from complete insensitivity (CAIS), whereby patients have female external genitalia, a short, blind ending vagina and no Wolffian duct derived structures, to partial insensitivity (PAIS), whereby patients can have a predominately female appearance with mild cliteromegaly, ambiguous genitalia, or an undervirilized male appearance (Galani et al. 2008). Given the complex nature of genital anomalies, recommendations for treatment and management of AIS should rely on an elaborate and individualized approach (Dacou-Voutetakis 2007). Genetic testing for the molecular cause of AIS is recognized as an invaluable aspect of this approach.

Pathogenic variants in the Androgen Receptor (AR) gene are the most common cause of Androgen Insensitivity Syndrome (AIS). To date, over 1,000 pathogenic variants have been described throughout the AR gene (Gottlieb et al. 2012). Nonsense and frameshift pathogenic variants cause complete AIS (CAIS), while weaker pathogenic variants, such as missense and regulatory (i.e. those in the promoter, introns, and 3’ untranslated region) variants, cause more variable symptoms and partial AIS (PAIS). Importantly, all documented AR pathogenic variants, and their respective phenotype, can be found in the Androgen Receptor Gene Mutations Database (ARDB, http://androgendb.mcgill.ca/), making it possible for physicians and genetic counselors to offer a statistically relevant phenotype prognosis for most identified pathogenic variants. The AR gene is located on the X chromosome, so in most cases (~60%) the pattern of inheritance is X-linked recessive. However, an apparently high mutation rate within the AR gene also results in relatively frequent occurrence (~40%) of de novo pathogenic variants and somatic mosaicism (Audi et al. 2010). Identifying patients with de novo pathogenic variants and somatic mosaicism has important consequences for sex assignment and genetic counseling (Kohler et al.2005). Pathogenic variants in AR have been detected in 46,XY females. 46,XX carrier females are often unaffected.

This test is predicted to identify a pathogenic variant in 65% of all patients with Androgen Insensitivity Syndrome (Boehmer et al. 2001).

Although the clinical sensitivity of deletion/duplication testing is expected to be relatively low, at least eighteen gross deletions/duplications have been reported in the AR gene (Human Gene Mutation Database).

This test provides full coverage of all coding exons of the AR gene, plus ~10 bases of flanking noncoding DNA. We define full coverage as >20X NGS reads or Sanger sequencing.