Androgen Insensitivity Syndrome

Etiology

Androgen insensitivity syndrome arises from loss-of-function mutations in the androgen receptor gene (AR). The androgen receptor gene is located on chromosome Xq11-12 and contains 8 exons. These mutations in the androgen receptor gene on the X chromosome result in androgen receptor dysfunction and hormone resistance. These mutations lead to a lack of virilization with or without infertility in 46,XY individuals with functional testes and adequate testosterone production. Complete and partial androgen insensitivity syndromes exhibit phenotypic variability; however, both conditions share similar genetic, endocrine, and pathophysiologic mechanisms.[2] Additionally, minimal androgen insensitivity syndrome has also been described by some authors, with no overt genital atypia and possible gynecomastia, suboptimal fertility, or both.

Epidemiology

The Office of Rare Diseases at the National Institutes of Health classifies androgen insensitivity syndrome and its subtypes as rare diseases, defined as conditions affecting fewer than 200,000 people in the US. The prevalence of individuals with a 46,XY karyotype and female phenotype is low, limiting the available data on age and clinical presentation at diagnosis in AIS. Results from a nationwide study in Denmark showed that analysis of the medical records of all known individuals with a 46, XY karyotype and female phenotype since 1960 yielded a prevalence of 6.4 per 100,000 live-born females. The prevalence of AIS was 4.1 per 100,000 live-born female individuals.[3] The prevalence of CAIS, confirmed by molecular diagnosis, is estimated to range from 1 in 20,400 to 1 in 99,100 46, XY individuals.[4]

Pathophysiology

The androgen receptor is a member of the nuclear receptor superfamily. Androgen insensitivity syndrome results from loss-of-function variants in the androgen receptor gene (AR), causing a lack of response to androgens. The AR gene has a 6-kilobase 3′ untranslated region (UTR). The gene encodes a 920-amino-acid protein that can be organized into 4 key domains: the N-terminal domain (NTD), which contains a transactivation region and ligand-independent activation function 1 (AF-1); the DNA-binding domain (DBD); a short hinge region; and a C-terminal ligand-binding domain (LBD). Another transactivation region, ligand-dependent activation function 2 (AF-2), is located in the LBD. The androgen receptor functions as a transcription factor, with AF-1 and AF-2 serving as binding sites for coactivators that modulate receptor activity (see Image. Genomic Organization and Protein Domain Architecture of the Human Androgen Receptor). More than 600 variants are known to cause the condition, distributed across all exons.[5] Relatively frequent mutations localize to the hormone-binding domain and cause androgen receptor dysfunction.[6]

The androgen receptor is kept inactive in the cytoplasm. Upon androgen binding in the LBD, the androgen receptor translocates to the nucleus and regulates transcription of its target genes. The NTD has 2 polymorphic regions, the polyglutamine (CAG) and polyglycine (GGC) repeats. Expansion of the CAG repeat beyond 38 residues leads to androgen receptor protein misfolding and spinal and bulbar muscular atrophy, also known as Kennedy disease. Spinal and bulbar muscular atrophy is a neuromuscular condition that shares some features with androgen insensitivity syndrome. CAIS represents the severe end of the phenotype, while milder phenotypes include PAIS and MAIS. The CAIS phenotype yields more variants (90% to 95%) than less severe phenotypes (approximately 30%). In addition to variants in the coding region, variants in noncoding regions, including the 5′ UTR and deep intronic regions, are now known to cause disorders within the androgen insensitivity syndrome spectrum.[7][8]

Clinical features likely depend not only on the severity of receptor dysfunction but also on other factors that contribute to varied phenotypes. In addition to noncoding region variants, non–AR gene variants affecting androgen receptor function are another possible explanation for the lower yield in the androgen insensitivity syndrome phenotype. Recently, the AR gene-variant–negative androgen insensitivity syndrome phenotype has been linked to decreased coactivator activity, leading to decreased androgen receptor function. An apolipoprotein D gene (APOD) assay has been used as a validated functional readout of androgen receptor activity in genital skin fibroblasts. Testing genital skin fibroblasts from patients with AR gene-variant–negative, clinically diagnosed androgen insensitivity syndrome showed that about one-third of the fibroblasts exhibited reduced APOD induction. Reduced APOD induction indicates diminished androgen receptor activity. This presentation has been termed androgen insensitivity syndrome type 2.[9] A formin protein and actin nucleator, disheveled-associated activator of morphogenesis 2 gene (DAAM2), and, more recently, nuclear myosin VI, have been found to play an essential role in androgen signaling and may act as coactivators affecting the androgen insensitivity syndrome phenotype.[10][11]

History and Physical

Androgen insensitivity syndrome may be subclassified into the following clinical types, primarily based on phenotype: complete androgen insensitivity syndrome (CAIS), partial androgen insensitivity syndrome (PAIS), and mild androgen insensitivity syndrome (MAIS).[12] This variability is commonly attributed to differences in residual androgen receptor activity, but the same genetic defects within a family may lead to different phenotypes. Phenotypic variability may result from differences in androgen receptor protein stability, DNA-binding capacity, and interactions with coactivators, and is more commonly observed with missense variants. More than 30 genotypes are classified under more than 1 phenotype. This variability is greater with ligand-binding domain variants and less severe phenotypes, including PAIS and MAIS, compared with CAIS. Somatic mosaicism and interindividual variability in androgen biosynthesis are other possible explanations for this variability.[13] Crosstalk between the androgen receptor gene (AR) and other genes involved in sex development, including the steroidogenic factor 1 gene (SF1) and the mitogen-activated protein kinase kinase kinase 1 gene (MAP3K1), may also affect phenotype, as described in recent reports.[14][15] In general, CAIS, PAIS, and MAIS present with primary amenorrhea, genital atypia, and infertility with or without gynecomastia, respectively. Table 1 summarizes a comparative analysis of these 3 variants.

Table 1. Comparative Analysis of Complete Androgen Insensitivity Syndrome (CAIS), Partial Androgen Insensitivity Syndrome (PAIS), and Mild Androgen Insensitivity Syndrome (MAIS)

 Abbreviations: AR, androgen receptor gene; CAIS, complete androgen insensitivity syndrome; DSD, differences of sex development; FSH, follicle-stimulating hormone; LH, luteinizing hormone; MAIS, mild androgen insensitivity syndrome; PAIS, partial androgen insensitivity syndrome.

Evaluation

The diagnosis of complete androgen insensitivity syndrome (CAIS) and partial androgen insensitivity syndrome (PAIS) includes assessment of clinical and biochemical features, a 46, XY karyotype, and exclusion of defects in testosterone synthesis. Furthermore, genetic testing can confirm the diagnosis.

Hormonal Evaluation

Basal measurements of testosterone, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) during minipuberty in the first year of life, between 2 and 12 weeks after birth, or after puberty onset, may show high-normal testosterone levels, inappropriately increased LH levels, and normal FSH and inhibin levels. The expected postnatal surge in gonadotropins and testosterone is usually present in PAIS but may not occur in CAIS. The cause of this blunted minipuberty in these infants is not fully understood, but the lack of prenatal androgen exposure likely prevents priming of the hypothalamic-pituitary axis, resulting in a blunted LH surge.[16] However, Sertoli cell function is normal, as shown by normal inhibin B or anti-Müllerian hormone (AMH) levels. AMH levels may be elevated compared with those in unaffected infants because the androgen-insensitive state in CAIS amplifies the effect on Sertoli cells, resulting in relatively high AMH levels.[12][17] In prepubertal children, assessment of testosterone synthesis requires a human chorionic gonadotropin (hCG) stimulation test with measurement of serum androstenedione, testosterone, and dihydrotestosterone (DHT). In adults after puberty, basal hormone measurements are usually conclusive. Estradiol (E2) levels are significantly higher than the male reference range but lower than the unaffected female reference range because of aromatase-mediated conversion of testosterone to estradiol and LH-induced estradiol production by Leydig cells.

Minipuberty: At birth, all babies, regardless of 46, XY karyotype, experience a short period of gonadotropin-gonadal axis activation, known as minipuberty, around 1 to 3 months of age. During this time, the hypothalamic-pituitary axis stimulates the testes to produce testosterone by releasing LH and FSH. In CAIS, androgen receptors do not function normally; thus, a lack of prenatal hypothalamic-pituitary priming by androgens disrupts the loop, and the typical postnatal surge in gonadotropins and testosterone either fails to occur or remains blunted. In PAIS, androgen receptors function only partially; thus, testosterone feedback can still produce a postnatal rise in LH, FSH, and testosterone.

17β-Hydroxysteroid dehydrogenase type 3 deficiency: Individuals with CAIS and PAIS have normal testosterone responses after the hCG stimulation test.[18] Low serum testosterone synthesis implies impaired testosterone synthesis secondary to 17β-hydroxysteroid dehydrogenase deficiency. The testosterone-to-androstenedione ratio helps distinguish 17β-hydroxysteroid dehydrogenase deficiency from other 46,XY differences of sex development, with a ratio less than 0.8 suggestive of this condition.

Steroid 5α-reductase type 2 deficiency: Differentiation of CAIS and PAIS from this condition is made by analyzing the serum testosterone-to-DHT ratio. In patients with steroid 5α-reductase type 2 deficiency, the production of DHT decreases, and the plasma ratio of testosterone to DHT increases to greater than 20:1 to 30:1. In most children with PAIS, the production of DHT and the ratio of testosterone to DHT are normal. Individuals with CAIS may have secondary 5α-reductase type 2 deficiency due to decreased mass of urogenital tract tissues that normally produce DHT. Overlap in the testosterone to DHT ratio among some individuals with androgen insensitivity syndrome and steroid 5α-reductase type 2 deficiency makes this ratio imperfect for diagnosis, and molecular genetic analysis is often warranted. Other than baseline or stimulated testosterone with gonadotropin assays, sex hormone–binding globulin and AMH can be used as surrogate markers for adequate androgen action. AMH, a Sertoli cell marker, normally falls during puberty; AMH levels fall modestly during puberty in PAIS and do not fall in CAIS.[19]

More recently, the apolipoprotein D gene (APOD) assay has been used as a validated functional readout of androgen receptor activity in genital skin fibroblasts. Reduced APOD induction suggests diminished androgen receptor activity (see the pathophysiology section for more details).[9] These cell-based assays are particularly valuable in cases with inconclusive molecular genetic evaluation. 

Importance of Genetic Testing in the Evaluation of AIS

In addition to clinical features, biochemical testing may be inadequate to establish the diagnosis of CAIS or PAIS because LH and testosterone levels overlap, especially in cases of steroid 5α-reductase type 2 deficiency. Therefore, molecular genetic studies are the preferred method for establishing the diagnosis of androgen insensitivity syndrome spectrum conditions. Results from studies showed large structural alterations along with more than 600 mutations known to cause this condition. Multiplex ligation-dependent probe amplification analysis is available to detect deletions or duplications of exons or the entire gene.[20] Sequencing of the androgen receptor gene (AR) itself may confirm the diagnosis of CAIS in 90% to 95% of cases, but only in 30% to 40% of cases with suspected PAIS. Therefore, PAIS still poses a substantial challenge for clinicians attempting to predict genotype-phenotype correlations and prognosis.

Sequencing of the entire AR gene, including the 5′ UTR and intronic regions, or whole-exome sequencing can be used to look for variants in different parts of the AR gene and non–AR genes, including mitogen-activated protein kinase kinase kinase 1 gene (MAP3K1) and nuclear receptor subfamily 5 group A member 1 gene (NR5A1), that affect phenotype. Additionally, long-read sequencing may be needed in selected cases to detect triplet-repeat disorders and assess the contributions of epigenetic changes in androgen insensitivity syndrome spectrum disorders. In addition to the AR coding region, noncoding regulatory regions, deep intronic variants, and coactivator dysfunction are implicated in androgen insensitivity syndrome spectrum disorders. They should be considered when evaluating an individual suspected of having PAIS based on clinical and hormonal evaluation with negative results on initial mutation testing.[5][21] The diagnosis of CAIS and PAIS includes assessment of clinical and biochemical features, a 46XY karyotype, and exclusion of defects in testosterone synthesis. Furthermore, genetic testing can confirm.

Treatment / Management

An interdisciplinary team comprising an endocrinologist (pediatric or adult), a urologist, a gynecologist, and a psychologist should guide treatment. Treatment of androgen insensitivity syndrome involves a holistic approach to the psychological, physiological, and social well-being of the individual with this disorder. Treatment considerations include sex of rearing in the context of genital atypia, need for hormonal replacement therapy and surgical procedures, fertility prospects, risk of tumor development if gonads are retained, and psychological assessment of gender identity, gender role, and sexual orientation. After initial assessment, individuals with androgen insensitivity syndrome require counseling and support throughout life stages. Genetic counseling is also integral to the treatment of this condition, given its genetic basis.

Complete Androgen Insensitivity Syndrome

Individuals with CAIS usually have a female sex of rearing because they have very low or no risk of gender dysphoria. CAIS may present as an incidental finding of gonads at the time of other procedures, predominantly during inguinal hernia repairs, or as primary amenorrhea in adolescents with a female phenotype. Clinicians should consider obtaining a biopsy of the gonad at the time of hernia repair and replacing the gonad subcutaneously or within the abdomen while further discussion with the parents about the diagnosis and plans for future treatment is pending.[22]

Parents can choose early gonadectomy to prevent tumorigenesis when CAIS presents in infancy, and the child is unaware of the issues surrounding the diagnosis. In this scenario, puberty induction can be performed later via estrogen replacement. Induction protocols may be similar to those used for other conditions requiring puberty induction, such as Turner syndrome, starting near 11 years of age and advancing gradually over 2.5 to 3 years. Please see StatPearls' companion reference, "Turner Syndrome," for further information. Additionally, because individuals with CAIS do not have a uterus, estrogen-induced endometrial cancer risk is absent. Therefore, unopposed estrogen therapy can be continued, unlike in other conditions.

Alternatively, gonadectomy can be delayed until early adulthood because the risk of a gonadal tumor in childhood is very low.[23][24] In this situation, puberty induction via external estrogen replacement is not needed. Puberty occurs spontaneously, manifesting as typical breast development and an appropriately timed growth spurt; however, menarche does not occur. Formal tumor surveillance is needed in individuals who choose to retain the gonads until puberty completion or later. Surgical treatment in CAIS includes vaginal dilation and, rarely, vaginoplasty for sexual function and well-being. Adoption or use of donor oocytes and a surrogate with the partner’s sperm is an option for parenthood.

Partial Androgen Insensitivity Syndrome 

Unlike CAIS, infants with partial androgen insensitivity syndrome are usually born with genital atypia, which requires an accurate diagnosis and a decision on sex assignment after a holistic discussion with family and caregivers. After sex assignment, the clinician can address early treatment issues. Ongoing counseling should include discussion of expected pubertal development, fertility, gonadal tumor risk, and possible need for staged hormonal or surgical treatment.

In infants assigned male sex, medical treatment includes androgen supplementation at the time of puberty. Surgical treatment includes correction of hypospadias and undescended testes. These procedures are preferable during the second to third year of life. At the time of puberty, gynecomastia may develop, which should be corrected with reduction mammoplasty. The incidence of breast cancer in males with PAIS is low.[25]

Treatment of PAIS in infants assigned female sex includes estrogen supplementation at the time of puberty, along with genitoplasty with gonadectomy before the onset of puberty. Sometimes, puberty suppression is needed until parents and the child decide on the sex of rearing to prevent irreversible virilization. Individuals raised as male require high-dose androgen supplementation for virilization and possible fertility, though no standard regimen can be recommended for all patients. A short course of testosterone or topical dihydrotestosterone may help assess the phallus's responsiveness to androgens, and the external masculinization score at birth or presentation may be useful in predicting potential virilization during puberty and in guiding a decision regarding the sex of rearing. Surgical procedures and hormonal puberty blockade should conform to the local legal and cultural framework. Individuals with mild androgen insensitivity syndrome need treatment of gynecomastia with surgical intervention and evaluation for infertility. In addition to surgical and hormonal treatment, psychological support is needed in nearly all individuals with the androgen insensitivity syndrome spectrum.

Differential Diagnosis

The differential diagnosis of androgen insensitivity syndrome includes a range of differences in sex development that may present with undervirilization or a predominantly female phenotype in individuals with a 46, XY karyotype. These conditions can be broadly categorized into defects in androgen production and disorders of gonadal development. Complete androgen insensitivity syndrome has a more distinct phenotype and a higher yield on molecular genetic analysis, while partial androgen insensitivity syndrome has a more variable presentation and a lower yield on genetic analysis, resulting in a wider differential diagnosis.

Defects in Androgen Biosynthesis

Defects in androgen biosynthesis result from abnormalities in enzymes of the steroidogenic pathway or from dysfunction of the luteinizing hormone receptor. In these conditions, testes are present but unable to produce adequate testosterone, leading to undervirilization despite a 46, XY karyotype. Unlike the AIS spectrum, serum testosterone levels are typically low.[23] Steroid 5α-reductase type 2 deficiency is an important differential diagnosis because it presents with undervirilization and adequate testosterone levels during puberty or after human chorionic gonadotropin stimulation, unlike other androgen biosynthesis defects. 17β-Hydroxysteroid dehydrogenase type 3 deficiency is another condition that presents with female or atypical genitalia with a 46, XY karyotype, but affected individuals usually have low testosterone levels.

Disorders of Gonadal Development

Mixed gonadal dysgenesis (MGD), a chromosomal DSD, is most commonly associated with a mosaic karyotype of 45, X/46, XY. Typical findings include a dysgenetic testis on one side and a streak gonad on the other, with variable development of internal and external genitalia. Asymmetric external genitalia, with a palpable testis on one side and no palpable gonad on the other, is the typical phenotype, although presentation may vary. These individuals may share some features of Turner syndrome due to 45, X/46, XY mosaicism.

Partial gonadal dysgenesis occurs in individuals with a 46, XY karyotype and incomplete testicular development. Partial gonadal dysgenesis is characterized by reduced production of testosterone, AMH, and inhibin B, persistence of Müllerian structures, and a broad clinical spectrum ranging from ambiguous genitalia to an undervirilized male phenotype. Known genes associated with gonadal dysgenesis spectrum disorders include sex-determining region Y gene (SRY), zinc finger protein, FOG family member 2 gene (ZFPM2), nuclear receptor subfamily 5 group A member 1 gene (NR5A1), desert hedgehog signaling molecule gene (DHH), and Wnt family member 1 gene (WNT1).

Complete gonadal dysgenesis presents with absent pubertal progression and primary amenorrhea in an individual with a 46, XY karyotype. The presence of Müllerian structures in a 46, XY individual suggests impaired anti-Müllerian hormone production or action and is more consistent with gonadal dysgenesis than with AIS.

Ovotesticular DSD: In an individual with variable genital atypia with breast development, a 46, XX karyotype is more common, but a 46, XY karyotype can also occur.

Müllerian agenesis, also known as Mayer-Rokitansky-Küster-Hauser syndrome, should be considered in individuals with a female phenotype presenting with primary amenorrhea, a blind vaginal pouch, and an absent uterus.[24] The key distinction is that individuals with Müllerian agenesis have a 46, XX karyotype with normal ovarian function and normal estrogen and androgen levels. Consequently, these individuals exhibit normal pubertal development and normal axillary and pubic hair growth, unlike patients with CAIS who lack androgen action.[25]

Pertinent Studies and Ongoing Trials

In addition to genital skin fibroblast studies and androgen receptor knockout mouse studies used to elucidate androgen receptor function in affected individuals, testicular organoids originating from human induced pluripotent stem cells may be used to study the effects of androgen receptor insensitivity in a more representative model.[26]

Prognosis

Individuals with complete androgen insensitivity syndrome (CAIS) have better outcomes in terms of satisfaction with assigned sex, contentment with their body, sexual life, and overall psychological health. Individuals with suspected partial androgen insensitivity syndrome (PAIS) need more extensive hormonal, molecular, and psychological evaluation to establish a diagnosis and guide decisions regarding sex of rearing and hormonal therapy. Even after undergoing more intensive discussions and evaluation, individuals with PAIS have poorer outcomes regarding contentment with their assigned sex, irrespective of the assigned sex. Results from studies suggested an increased tumor risk greater than 30% in late adulthood if gonadectomy is not performed.[27] Boys with genetically confirmed PAIS are likely to have poorer clinical outcomes than those with 46, XY differences of sex development, normal testosterone synthesis, and no identifiable androgen receptor gene mutation.[28] 

Gonadectomy is usually undertaken after puberty, except in individuals with PAIS who are raised as girls, for whom prepubertal gonadectomy is preferred to prevent irreversible virilization. However, local laws and practices should be taken into account when deciding the timing of gonadectomy. In individuals with PAIS, hormone replacement is usually required to support adequate virilization in males or feminization in females. In individuals with CAIS, estrogen replacement is also needed after gonadectomy. Urogenital surgical procedures to align external genitalia with sex of rearing may be needed in individuals with PAIS; the extent of surgical intervention varies by the degree of masculinization present.

Individuals with androgen insensitivity syndrome and cryptorchidism have an increased risk of tumorigenesis. Risk of tumors depends on 3 factors: (1) the presence of the gonadoblastoma on the Y chromosome region, candidate gene TSPY; (2) expression of the embryonic germ cell markers octamer-binding transcription factor 3/4 gene (POU5F1) or KIT ligand gene (KITLG) beyond the age of 1 year; and (3) gonad location, with higher gonad location associated with higher risk.[29] Cryptorchidism in PAIS should be corrected surgically soon after diagnosis to maintain testicular function and minimize the risk of malignant neoplasm. These tumors may be germ cell tumors or gonadoblastomas. They occur in approximately 1.5% to 2% of undescended testes and may develop into malignant neoplasms.[30] 

Carcinoma in situ or intratubular germ cell neoplasia unclassified is the premalignant precursor from which tumors associated with AIS arise.[31] Carcinoma in situ arises from gonocytes or primordial germ cells and is thought to result from developmental arrest of fetal germ cells. Carcinoma in situ leads to the development of gonadoblastoma in more than 50% of cases. Individuals with CAIS are advised to have gonadectomy because of tumor risk, which may increase substantially in later adulthood.[32] Results from a meta-analysis of 15 studies and 456 patients with CAIS who underwent gonadal biopsy or surgical procedure showed that 6.14% had a premalignant lesion, with the overwhelming majority being postpubertal (82.14%). A malignant lesion was found in 1.3%, and all individuals were postpubertal.[33] The risk of germ cell tumors is higher in PAIS than in CAIS, and results from studies suggested an incidence of 15% or higher if the testes are not in a scrotal position.[34] Monitoring for carcinoma is based primarily on imaging studies. Individuals with mild androgen insensitivity syndrome need care regarding fertility and gynecomastia, and have much better outcomes compared with other variants.

Complications

Complications may present at various stages of life:

Atypical genitalia may create difficulty in deciding the sex of rearing during infancy. Later, multiple surgical procedures may be required to align the external genitalia with the sex of rearing. Complications related to these surgical procedures increase morbidity in affected individuals. Eligible individuals may also require gonadectomy. Individuals with untreated or inadequately treated androgen insensitivity syndrome and their families may face severe psychological distress during puberty and adulthood. Psychological distress is more frequent in adults with partial androgen insensitivity syndrome than in those with complete androgen insensitivity syndrome, irrespective of whether they were raised as male or female. Results from a study by Bouvattier et al showed that in men with PAIS, all aspects of sexual activity were substantially impaired.[35] Additionally, gynecomastia and infertility are common in this condition. The systemic health effects of high-dose hormonal treatment in individuals raised as male remain poorly understood. The risk of gonadal tumors varies according to the androgen insensitivity syndrome subtype and the anatomic location of the gonads.