Normal Sexual Development

The normal development of the reproductive organs in humans begins with the differentiation of primordial structures and ends with the completion of secondary sexual characteristics that begins during puberty (see Chapter 67 for a discussion on puberty). Around the fifth week of gestation, the genital ridge develops into the appropriate gonads based on the chromosomal sex (XX or XY) established at conception. A complex series of genetic events must occur to establish the appropriate differentiation to an ovary or testis. It was clear that genes on the Y chromosome were involved in this differentiation because the presence of a Y chromosome resulted in male sexual characteristics, such as in XY or XXY individuals. Before these genes were known, they were referred to as “testis-determining factor” (TDF) because their presence led to testis formation. Ovaries and other female-specific reproductive organs were believed to develop by “default” because of the absence of Y chromosome material or TDF. Accordingly, research in mice and humans with X-Y translocations determined that the SRY gene (sex-determining region, Y) encoded for a transcription factor that is necessary for male reproductive organ development. Additional genes have also been identified for testis formation (Figure 119-1), and mutations in any of these genes (e.g., SOX9, DMRT1) have been associated with sex-reversal in XY patients. It is also important to note that more recent data have demonstrated an active role of several genes (FOXL2, WNT4, RSPO1) in ovary development in XX individuals, contradicting the older “default” female development notion.

Figure 119-1 Genes associated with testicular and ovarian differentiation.

After the development of the gonads, the genital ducts must differentiate into the appropriate internal structures necessary for reproduction (Figure 119-2). In the undifferentiated stage, both mesonephric (Wolffian) and paramesonephric (Müllerian) ducts are present. In males, the newly formed testes secrete anti-Müllerian hormone (AMH), inhibin B, and testosterone between weeks 7 and 11, causing the Müllerian ducts to regress and the Wolffian ducts to persist, eventually becoming the seminal vesicles, vas deferentia, and epididymis. The opposite occurs in females, in whom the absence of testicular secretions causes the mesonephric ducts to regress and the Müllerian ducts to persist, forming the fallopian tubes, uterus, and upper third of the vagina

Figure 119-2 Gonad and genital duct formation.

The undifferentiated genital tubercle, labioscrotal swellings, and urethral folds are present around gestational week 4, and the external genitalia begin to become more distinguishable during weeks 9 to 13. In males, the presence of testosterone and the testosterone derivative dihydrotestosterone (converted by 5-αreductase) causes the genital tubercle to form the glans penis, stimulating the urethral folds to fuse ventrally and form the spongy urethra and resulting in part of the penile shaft and labioscrotal swellings to fuse and form the scrotum (Figure 119-3). The absence of testosterone and dihydrotestosterone in females causes the genital tubercle to form the glans clitoris and the urethral folds and labioscrotal swelling to remain unfused (except posteriorly) to form the labia minora and labia majora, respectively. In males, testicular descent begins during the twelfth week of gestation and finishes during the third trimester.

Figure 119-3 Differentiation of external genitalia.

Etiology, Pathogenesis, and Clinical Presentation

Because of the myriad genes associated with urogenital development, many genetic abnormalities may alter the structure and function of the reproductive organs. As noted above, these DSDs have been recently subtyped into sex chromosome DSDs, 46,XY DSDs, and 46,XX DSDs (Table 119-1). Sex chromosome DSDs are caused by various mechanisms (Figure 119-4) and include Klinefelter syndrome (47,XXY) and variants, Turner syndrome (45,X) and variants, and other types of XX/XY mosaicism, as discussed in Chapter 118, and typically result in largely normal prenatal sexual development but reduced function of the gonads during and after puberty. Most other genetic abnormalities may include point mutations, frameshift mutations, or deletions of specific genes that cause the protein product to be nonfunctional and thereby affect gonad differentiation, leading to a disparity between the chromosomal sex (XX or XY) and gonad development (ovary or testis).

Table 119-1 Classification of Disorders of Sexual Differentiation

DSD, disorders of sexual development; LH, luteinizing hormone; MURCS, Müllerian duct aplasia, renal agenesis or ectopia, and cervical somite dysplasia; MODY5, maturity-onset diabetes of the young.

Adapted from Hughes IA: Disorders of sex development: a new definition and classification. Best Pract Res Clin Endocrinol Metab 22:119-134, 2008.

Figure 119-4 Errors in chromosomal sex.

46,XY Disorders of Sexual Development

The clearest example of 46,XY DSDs is still SRY, which is important for testis development, and when mutated, results in the undifferentiated gonad to develop into an ovary. Other abnormalities involving SRY include balanced chromosomal translocations between the SRY-containing portion of the Y chromosome (Yp11.31) and another chromosome in a father that could lead to an imbalance in the child, causing either a 46,XY DSD caused by SRY deficiency or a 46,XX DSD if there is ectopic SRY expression.

Other genes that may be associated with various 46,XY DSDs are listed in Table 119-1. SOX9 is a transcription factor that along with SF1 regulates transcription of AMH. SOX9 is also involved in chondrocyte differentiation, and mutations of this gene cause a skeletal disorder called camptomelic dysplasia (Figure 119-5), where about 67% of XY patients develop as phenotypic females with abnormalities that include gonadal dysgenesis; the presence of Müllerian structures such as a uterus, fallopian tubes, and vagina; and external female genitalia. WT1 is another transcription factor involved in testis formation and synergizes with SF1 to induce AMH. Mutations of WT1 are seen in patients with Denys-Drash syndrome, in which XY patients have various urogenital anomalies, including ambiguous genitalia. In addition, larger deletions that include WT1 cause WAGR syndrome (Wilms’ tumor, aniridia, genitourinary anomalies, and mental retardation). DHH is an important signaling molecule involved in various areas of morphogenesis, including male gonadal development, and mutations in DHH are associated with XY gonadal dysgenesis.

Figure 119-5 Campomelic dysplasia.

Other 46,XY DSDs involve abnormalities with androgen synthesis or action. Smith-Lemli-Opitz syndrome is a multiple congenital anomaly disorder of cholesterol biosynthesis caused by mutations of the sterol δ-7-reductase (DHCR7) gene and resulting in decreased androgen production. Abnormalities typically involve ambiguous genitalia but may involve the presence of other female reproductive organs. Other mutations along the cholesterol biosynthesis pathway that cause a DSD in XY individuals include those of 17-αhydroxylase (CYP17A1) and cholesterol side-chain cleavage (CYP11A1). Defects in these genes present with a spectrum of abnormalities such as ambiguous genitalia, female internal and external genitalia, and adrenal hyperplasia and insufficiency. Patients with 5-α reductase 2 deficiency have normal-appearing female genitalia at birth because testosterone is not converted to dihydrotestosterone, an important effector hormone in male external genital development. However, at puberty, when testosterone levels surge, these patients develop clitoromegaly and virilization. Androgen insensitivity syndrome is caused by a defect in the androgen receptor gene (AR) that is unresponsive to testosterone binding. Patients have female external genitalia, breast development, inguinal or abdominal testes, and a blind vaginal pouch but no uterus or fallopian tubes.

Evaluation and Management

Although the introduction of routine prenatal ultrasound screening has allowed for the discovery and evaluation of ambiguous genitalia in a fetus, most DSDs present at birth in a newborn with ambiguous genitalia. Unfortunately, as difficult as it might be for the parents and health care team, gender assignment should be postponed until a thorough assessment has been completed. The health care team may consist of several members beyond the nursery or neonatal care unit, including endocrinology, surgery, urology, genetics, psychology, and social work. Input from all involved is critical in providing not only a methodical approach to the diagnosis and treatment of the child but also to offer appropriate family support during this difficult period.

Diagnosis

A comprehensive evaluation begins with a detailed history and physical examination. Historical information should focus on the gestational age at the time of birth (preterm neonates may have some degree of phallic ambiguity); discordance between a known prenatal karyotype and phenotypic features; maternal exposures, medications, or ingestions; and family history of any consanguinity, genital or urologic abnormalities, infant deaths, and amenorrhea or other issues with puberty or fertility. Examination findings that suggest the presence of a DSD include ambiguous genitalia in which neither male or female sex assignment can be made; apparent female genitalia with clitoromegaly; posterior labial fusion; an inguinal or labial mass; or male genitalia with nonpalpable testes, hypospadias, or micropenis. Other physical findings may focus on the presence of dysmorphic features, such as short stature, a broad neck, and wide nipples in Turner’s syndrome, syndactyly between the second and third toes in Smith-Lemli-Opitz syndrome, aniridia in WT1 deletions, or abnormal curvature of the limbs as seen in camptomelic dysplasia.

Laboratory and radiologic studies vary for each patient and are based on the historical and physical findings (Figure 119-6). The genetic investigation focuses on determining the number of X and Y chromosomes by karyotype, fluorescence in situ hybridization, or microarray analysis to determine the chromosomal sex (e.g., 46,XY, 46,XX, 45,X, 46,XX/46,XY) and any evidence of mosaicism. Specific genes can also be analyzed for mutations based on other associated findings on the physical examination or serum chemistries. Serum electrolytes are necessary to evaluate sodium concentration in cases of suspected CAH. Other CAH studies include the levels of 17-OH-progesterone (elevated in CAH) and 11-deoxycortisol (high in 11-β hydroxylase deficiency; low in 21-hydroxylase deficiency). If 17-OH-progesterone is normal, testosterone and dihydrotestosterone levels (after human chorionic gonadotropin [hCG] stimulation) may help elucidate the cause of some 46,XY DSDs (e.g., high testosterone to low dihydrotestosterone is seen in 5-α reductase 2 deficiency). hCG stimulation may also determine anorchia if there is no significant change in androgen levels combined with elevated luteinizing hormone and follicle-stimulating hormone levels. An ultrasound should be the first radiologic procedure performed because it may determine the Müllerian anatomy and possibly the presence and location of the gonads. Computed tomography or magnetic resonance imaging may also assist in clarifying these structures further. Finally, infants with intraabdominal or nonpalpable gonads may require exploratory laparoscopy with biopsy to determine if dysgenetic gonads, ovotestes, or streak testes are present.

Figure 119-6 Sexual ambiguity.

Future Directions

Patients with DSDs often present a difficult and challenging array of historical and clinical findings, diagnostic possibilities, and treatment options. Further research into the various causes of DSDs at the genetic and molecular levels will likely increase not only our understanding of these conditions but also improve the diagnostic yield and therefore risk counseling available to our patients and their families. Last, a focus on maintaining open communication between the patient (if older), family, and health care team needs to continue to provide the best possible outcomes.