Defective androgen action caused by cellular resistance to androgens causes the androgen insensitivity syndrome (AIS) (Hiort etal. 1996; Quigley etal. 1995). The end-organ resistance to androgens results in a wide clinical spectrum of defective virilization of the external genitalia in 46,XY individuals. Mullerian duct derivatives are usually completely absent because of the normal ability of the foetal testes to produce AMH. Since the AR gene has been cloned, it became obvious that inactivating mutations of the AR gene represent the major molecular genetic basis of AIS (Lubahn et al. 1988a). Due to the X-chromosomal recessive inheritance, healthy female carriers may typically be conductors (Lubahn etal. 1988b).
In the complete androgen insensitivity syndrome (CAIS), any in vivo androgen action is abolished due to complete inactivation of in vivo AR signalling. Therefore, these patients have normal female external genitalia with a short and blind-ending vagina. At puberty, CAIS patients acquire a normal female body shape and they show normal breast development. This is caused by increasing estradiol levels due to elevated testosterone biosynthesis during puberty and its conversion to estradiol
by aromatization. Usually, no pubic or axillary hair is present (Hiort et al. 1996; Sinnecker etal. 1997).
Partial impairment of AR function is usually associated with partial androgen insensitivity syndrome (PAIS). The considerable variability in the degree of impaired AR activity accounts for a wide clinical spectrum of external underviril-ization observed in PAIS. This may range from a female habitus with only a small degree of virilization as partial fusion of labioscrotal folds and minimal enlargement of the clitoris to patients with considerable degree of virilization with male habitus, gynecomastia, female pattern of secondary hair distribution, and genital malformations such as hypospadias (Hiort etal. 1993; 1996; Holterhus etal. 1997; 2000). Phenotype of external genitalia may be graded according to the scale of Sinnecker etal. (1997) (Fig. 3.3).
The minimal androgen insensitivity syndrome (MAIS) describes individuals with a normal male habitus without genital malformation with only a slight masculin-ization deficit such as high pitched voice and gynecomastia associated with sub- or infertility (Hiort etal. 1996; 2000).
Within families, the phenotype may vary considerably between MAIS and PAIS even with the same underlying molecular abnormality (Holterhus et al. 2000; Rodien etal. 1996).
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