Androgen insensitivity, Turner's and Klinefelter's syndrome, chromosomes, gene loci

Sex differentiation occurs as a sequential process during the first trimester of fetal life. Genetic sex (Fig. 1) is established at the time of fertilization, leading to development of gonadal sex and culminating in formation of sex phenotypes (Fig. 2). Under normal circumstances, chromosomal sex agrees with phenotypic sex; however, occasionally chromosomal sex differs or ambiguity occurs in the sex phenotype. Abnormalities of sex development are usually not life-threatening and occur at many levels. The clinical consequences of abnormalities occurring early in sex development may result in conditions of intersex whereas defects in more terminal phases of male development may be represented by isolated cryptorchidism (failure of testes to descend into the scrotum) or microphallus (normally formed, but abnormally small penis). Disorders of sex differentiation are often inherited as single gene mutations, and the analysis of these disorders has been especially informative in defining the molecular and genetic determinants of normal sex development.

Gonadal Disorders
True Hermaphroditism: The diagnosis of true hermaphroditism is predicated upon the presence of both testicular and ovarian tissue in the same individual. Oocytes should be present within the ovarian tissue. The majority of subjects have a testis or ovary on one side and a contralateral ovotestis containing both ovarian and testicular tissue (50%), or have a testis on one side and an ovary on the other (30%), or bilateral ovotestes, or even bilateral ovary and testis combinations (20%). The amount of functional testicular tissue determines the internal duct structures. Secretion of Mullerian inhibiting substance (MIS) by testicular tissue causes variable degrees of bilateral Mullerian duct regression, whereas local secretion of high concentrations of testosterone are required for ipsilateral development of the Wolffian ducts. If a uterus is present on one side, an associated fallopian tube is often also present. Approximately half of subjects develop a uterus, but the cervix may be absent. The external genitalia are usually ambiguous, although relatively normal male or female appearance is possible. Hypospadias, cryptorchidism or an inguinal hernia containing a gonad or Mullerian remnant may also occur. The majority of true hermaphrodites are raised as males due to the external appearance of the genitalia, even though over 50% of subjects have a 46XX karyotype. Other karyotypes, such as 46XY, 46XX/XY chimerism or various forms of mosaicism, may be present. Many 46XX true hermaphrodites do not possess the SRY gene suggesting that the etiology of 46XX true hermaphroditism differs from that of 46XX males who have a translocation of the Y chromosomal SRY gene locus.

Klinefelter Syndrome: Seminiferous tubular dysgenesis occurring in 47XXY subjects with Klinefelter syndrome represents the most common cause of testicular failure, with an incidence of 1:1000 males. Prior to puberty, arm span is increased and upper-to-lower body segment ratio is decreased for age in affected subjects. They are often diagnosed as a result of personality disorders and mental retardation. Prepubertal subjects have small testes but the histology is generally normal for that age, except for a progressive tendency toward decreased numbers of spermatogonia. With the onset of puberty, gonadotropin (luteinizing hormone and follicle-stimulating hormone) concentrations in the serum increase but testosterone levels remain relatively suppressed in accordance with the degree of testicular failure. Whereas the onset of puberty often occurs at a normal age, secondary sexual changes may not progress to the normal adult stage. Gynecomastia occurs, probably due to the increased estradiol:testosterone ratio. In all cases, seminiferous tubular function is impaired and spermatogenesis is absent.

Turner Syndrome: Turner syndrome, does not strictly qualify as a disorder of sex differentiation. In the classic case, the phenotype is female, but with an absence of secondary sexual characteristics typical of puberty. Subjects lack a normal X chromosome (45X karyotype) and their ovaries degenerate into streak structures. The common anomalies of Turner syndrome include short stature, epicanthal folds, high arched palate, low nuchal hair line, webbed neck, shield-like chest, coarctation of the aorta, ventricular septal defect, renal anomalies, pigmented nevi, lymphedema, hypoplastic nails and inverted nipples.

XX Males: Subjects with an apparent 46XX karyotype but male phenotype result from the translocation of a fragment of the Y chromosome containing the testicular determining, SRY, gene to another chromosome, usually the X chromosome. Subjects may have undescended testes (15%) and hypospadias (10%) and usually have small testes that may be soft early in life but become firm with increasing age. Testicular histology reveals no spermatogonia, a decrease in the diameter of the seminiferous tubules, and Leydig cell hyperplasia, similar to that in Klinefelter syndrome. 46XX male subjects are shorter than those (47XXY) with Klinefelter syndrome. Testosterone production is low whereas gonadotropin levels are elevated.

XY Gonadal Dysgenesis: Gonadal dysgenesis may be of the "pure'' or "mixed'' form with the former referring to the presence of an aplastic or ``streak'' gonad on both sides and the latter referring most often to a unilateral streak gonad on one side and testicular tissue, usually within a dysgenetic testis, on the other side. The pure form may occur in subjects with a 46XY karyotype, whereas the mixed form commonly involves chromosomal mosaicism (45X, 46XY), but also occurs in 46XY subjects with variable degrees of functional testicular tissue in each of the gonads. The etiology may be deletion of the Y chromosome or deletion or mutation affecting the SRY gene.

Female Pseudohermaphroditism
Female pseudohermaphroditism occurs when the external genitalia are virilized in a female subject with a 46XX karyotype. These subjects have ovaries; virilization is caused by excessive androgen of extragonadal origin. The most common etiology is increased adrenal androgen secretion as a consequence of congenital virilizing adrenal hyperplasia (CVAH). The predominant form of CVAH is 21-hydroxylase (cytochrome P450c21) deficiency which accounts for 80-90% of female pseudohermaphroditism (Fig. 3). In addition to increased adrenal androgen (dehydroepiandrosterone and androstenedione) and reduced cortisol secretion, severe deficiencies in P450c21 also result in salt-losing nephropathy due to coincident reductions in mineralocorticoid (aldosterone and its precursors) synthesis. Less frequently, neonatal genital ambiguity may result from 11beta-hydroxylase (P450c11 and P450c18) or 3beta-hydroxysteroid dehydrogenase/ triangle 5 -triangle 4 isomerase (3beta-HSD) deficiency. The former condition is often accompanied by hypertension (low aldosterone, normal/high deoxycorticosterone) and the latter may involve coincident salt-losing nephropathy (low deoxycorticosterone (right arrow)aldosterone). In rare cases, excessive transplacental passage of androgen, either from an exogenous source or from pathologic maternal production, has been reported to cause masculinization of the genitalia of a female in utero. The external genitalia of females may also appear to be virilized in association with other congenital anomalies unrelated to steroid hormone effects and most often include imperforate anus, renal agenesis and malformations of the lower intestine and urinary tract.

Male Pseudohermaphroditism
Sexual ambiguity in the presence of symmetric gonads in a 46XY individual with testes is classified as male pseudohermaphroditism. This condition may be associated with varying degrees of incomplete external and internal virilization.

Disorders of Androgen Biosynthesis: These disorders affect the virilization of the internal and external genitalia of the male embryo but do not interfere with regression of the Mullerian system. These defects may be of variable severity, partial or complete, and may present at puberty as well as in the newborn period. Variable degrees of ambiguity, from complete feminization to mild hypospadias, may be present at birth. All but one of the enzymes (Fig. 3) involved in these defects are present in both the gonad and the adrenal and the primary symptoms of hypertension and/or severe renal salt loss in an affected subject may be those of congenital adrenal hyperplasia. In both cholesterol desmolase (cholesterol side chain cleavage; P450scc) and 17alpha-hydroxylase/17,20-lyase (P450c17) deficiencies, male infants are undervirilized due to decreased testosterone synthesis. By contrast, the occurrence of these same enzyme deficiencies in a female infant would not affect the otherwise female external genitalia. In 3beta-HSD deficiency, male infants are undervirilized whereas female infants are virilized. Severe deficiencies of P450scc and 3beta-HSD lead to extreme salt-loss due to deficits in mineralocorticoid synthesis, and diminished P450c17 activity results in hypertension. Defects of 17beta-hydroxysteroid dehydrogenase (17beta-HSD), an enzyme present in the gonad but not adrenal, result in deficient male genital development and these subjects with ambiguous genitalia may virilize at puberty.

Defects in Androgen Action: Abnormalities of androgen effect can be characterized as those due to defects of the androgen receptor, both partial and complete androgen insensitivity, and to deficiency of 5alpha-reductase enzyme activity.

a. 5 alpha-Reductase Deficiency: Male pseudohermaphroditism may result from inadequate conversion of testosterone to dihydrotestosterone due to deficiency of steroid 5 alpha-reductase 2 isoenzyme activity. Inadequate concentrations of dihydrotestosterone within the genital tubercle and labioscrotal folds lead to only partial masculinization of the external genitalia. 5alpha-Reductase activity in the fetal genital area peaks between weeks 7-12 of fetal life when masculinization of the male genitalia takes place. Later androgen exposure fails to correct any defect during this period. This condition is also referred to as pseudovaginal perineoscrotal hypospadias because of the specific anatomical ambiguity most often observed. Testicular testosterone and Mullerian inhibiting substance (MIS) production is normal so that Mullerian regression occurs and internal Wolffian structures develop to varying degrees. However, the sperm carrying ducts end blindly before the prostate gland, so even if spermatogenesis occurs, the ejaculate is azoospermic. Inheritance is autosomal recessive and is common among some ethnic groups due to consanguinity. In the undiagnosed subject or those in whom orchiectomy is not accomplished by the age of puberty, the ambiguous genitalia become further virilized with phallic growth and development of a muscular male habitus and male body hair patterns. Hormonal profiles include normal or elevated testosterone levels with low DHT levels in relation to testosterone and a high ratio of 5-beta- to 5-alpha-reduced urinary steroid metabolites. Stimulation with human chorionic gonadotropin further accentuates this altered ratio.

b. Complete androgen insensitivity (CAIS) is characterized by the development of female external genitalia and failure to masculinize the Wolffian system in a subject with a 46XY karyotype. Inguinal or labial testes may be palpable, although they may only be discovered during exploration of an apparent inguinal hernia. The vagina is short due to secretion of MIS by testicular Sertoli cells. Increased testicular stimulation by elevated gonadotropins at puberty results in normal or elevated testosterone levels to which the subject is nonresponsive. However, the peripheral aromatization (P450arom; Fig. 3) of testosterone and androstenedione in skin and adipose tissue leads to normal or elevated levels of estrogens (estradiol and estrone) which promote female breast development when unopposed by androgen action. Sexual and body hair is scant. If the diagnosis is not made before puberty, primary amenhorrea or infertility may be the presenting complaint. Additional studies may include in vitro androgen receptor binding measurements, hCG stimulation of testicular androgen secretion, or assessment in vivo of testosterone effect. The abnormality lies with a molecular defect in the X-chromosomal androgen receptor gene causing an abnormality in receptor function.

There is an increased risk of testicular tumors in CAIS and, therefore, orchidectomy should be performed by the end of the second decade of life, following completion of puberty. However, if there is a possibility that the subject has a partial form of androgen insensitivity (PAIS), with the risk of masculinization during puberty, the testes should be removed prior to that time. Carcinoma in situ as evidenced by abnormal morphology of germ cells has been observed in testes of a few subjects with AIS during adolescence. Later in life, adenomatous transformation of both Sertoli and Leydig cells has been reported to occur.

c. Partial androgen insensitivity presents with highly variable degrees of virilization. The phenotype ranges from slightly virilized female genitalia, to penile hypospadias, undescended testes and adolescent gynecomastia, to micropenis, and to isolated infertility. Subjects who present with ambiguous genitalia or micropenis in the neonatal period may have hormonal profiles of elevated testosterone, luteinizing hormone and follicle stimulating hormone, which are characteristic of androgen insensitivity. Further diagnostic testing involves androgen stimulation in vivo. The lack of detectable or adequate penile growth in response to androgen is consistent with the diagnosis of androgen insensitivity. Marked ambiguity and biochemical evidence of severe androgen insensitivity dictates a female sex of rearing. In addition, a partial defect may allow further masculinization at puberty in response to increased testosterone secretion and therefore, gonadectomy should be performed prior to puberty to prevent this occurrence. Mutations in the androgen receptor gene are responsible for the various presentations of PAIS.

d. Hypospadias or micropenis may occur as isolated phenotypic events or in association with the observance of sexual ambiguity. Hypospadias is defined as failure of complete development and incorporation of the penile urethra within the shaft of the penis. The urethral opening may therefore be at any position on the ventral surface of the penis from the perineum to the glans. The position of the urethral opening forms the basis of classification as a glandular, coronal, distal or midshaft, penoscrotal or perineoscotal hypospadias. Because hypospadias reflects the failure of androgen-stimulated midline fusion, it represents a form of ambiguous genitalia. Its estimated occurrence is 8 per 1000 males. Micropenis refers to the presence of a fully formed but small penis in the absence of other abnormalities of sex differentiation. The definition is statistical and refers to a penis which is 2.5 SD below the normal standards for age and stage of pubertal development. Normal stretched penile length for newborns is 2.8 to 4.2 cm. The lower limit for 2.5 SD is 1.9 cm.

Suggested Reading

Brown TR, Scherer PA, Chang Y-T, Migeon CJ, Ghirri P, Murono K, Zhou Z. Molecular genetics of human androgen insensitivity. Eur J Pediatr 1993;152:S62.

Hum DW, Miller WL. Transcriptional regulation of human genes for steroidogenic enzymes. Clin Chem 1993;39:333.

Lee MM, Donahoe PK. Mullerian inhibiting substance: a gonadal hormone with multiple functions. Endocr Rev 1993;14:152.

Migeon CJ, Berkowitz GB, Brown TR. Male sex differentiation and development. In: Kappy M, Blizzard R, Migeon CJ, eds. The Diagnosis and Treatment of Endocrine Disorders in Childhood and Adolescence. Springfield, IL: Thomas. 1994: Chap. 12.

Thigpen AE, Davis DL, Milatovich A, Mendonca BB, Imperato-McKinley J, Griffin JE, Francke U, Wilson JD, Russell DW. Molecular genetics of steroid 5alpha-reductase deficiency. J Clin Invest 1992;90:799.