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Mild androgen insensitivity syndrome | |
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Other names | Undervirilized male syndrome |
AIS results when the function of the androgen receptor (AR) is impaired. The AR protein (pictured) mediates the effects of androgens in the human body. | |
Specialty | Endocrinology |
Mild androgen insensitivity syndrome (MAIS) is a condition that results in a mild impairment of the cell's ability to respond to androgens. [1] [2] [3] The degree of impairment is sufficient to impair spermatogenesis and / or the development of secondary sexual characteristics at puberty in males, but does not affect genital differentiation or development. Female genital and sexual development is not significantly affected by the insensitivity to androgens; [3] [4] as such, MAIS is only diagnosed in males. [1] The clinical phenotype associated with MAIS is a normal male habitus with mild spermatogenic defect and / or reduced secondary terminal hair. [1] [5] [6] [7] [8] [9]
MAIS is one of three types of androgen insensitivity syndrome, which is divided into three categories that are differentiated by the degree of genital masculinization: complete androgen insensitivity syndrome (CAIS) is indicated when the external genitalia is phenotypically female, mild androgen insensitivity syndrome (MAIS) is indicated when the external genitalia is phenotypically male, and partial androgen insensitivity syndrome (PAIS) is indicated when the external genitalia is partially, but not fully masculinized. [1] [2] [5] [6] [7] [10] [11] [12] [13]
Androgen insensitivity syndrome is the largest single entity that leads to 46,XY undermasculinization. [14]
Individuals with mild (or minimal) androgen insensitivity syndrome (grade 1 on the Quigley scale) are born phenotypically male, with fully masculinized genitalia; this category of androgen insensitivity is diagnosed when the degree of androgen insensitivity in an individual with a 46,XY karyotype is great enough to impair virilization or spermatogenesis, but is not great enough to impair normal male genital development. [1] [5] [6] [9] MAIS is the mildest and least known form of androgen insensitivity syndrome. [5] [16]
The existence of a variant of androgen insensitivity that solely affected spermatogenesis was theoretical at first. [17] Cases of phenotypically normal males with isolated spermatogenic defect due to AR mutation were first detected as the result of male infertility evaluations. [1] [13] [18] [19] Until then, early evidence in support of the existence of MAIS was limited to cases involving a mild defect in virilization, [15] [20] although some of these early cases made allowances for some degree of impairment of genital masculinization, such as hypospadias or micropenis. [21] [22] [23] It is estimated that 2-3% of infertile men have AR gene mutations. [6] It is also estimated that MAIS is responsible for 40% of male infertility. [24]
Examples of MAIS phenotypes include isolated infertility (oligospermia or azoospermia), [5] [7] mild gynecomastia in young adulthood, decreased secondary terminal hair, high pitched voice, or minor hypospadias repair in childhood. [1] [25] The external male genitalia (penis and scrotum) are otherwise normal in individuals with MAIS. [1] [5] [6] [9] Internal genitalia, including Wolffian structures (the epididymides, vasa deferentia, and seminal vesicles) and the prostate, is also normal, although the bitesticular volume of infertile men (both with and without MAIS) is diminished; [6] male infertility is associated with reduced bitesticular volume, varicocele, retractile testes, low ejaculate volume, male accessory gland infections (MAGI), and mumps orchitis. [6] The incidence of these features in infertile men with MAIS is similar to that of infertile men without MAIS. [6] MAIS is not associated with Müllerian remnants.
Spinal and bulbar muscular atrophy (SBMA), also known as Kennedy's disease, is a severe neurodegenerative syndrome that is associated with a particular mutation of the androgen receptor's polyglutamine tract called a trinucleotide repeat expansion. [26] [27] SBMA results when the length of the polyglutamine tract exceeds 40 repetitions. [28]
Although technically a variant of MAIS, SBMA's presentation is not typical of androgen insensitivity; symptoms do not occur until adulthood and include neuromuscular defects as well as signs of androgen inaction. [26] Neuromuscular symptoms include progressive proximal muscle weakness, atrophy, and fasciculations. Symptoms of androgen insensitivity experienced by men with SBMA are also progressive [26] and include testicular atrophy, severe oligospermia or azoospermia, gynecomastia, and feminized skin changes [29] despite elevated androgen levels. [1] Disease onset, which usually affects the proximal musculature first, occurs in the third to fifth decades of life, and is often preceded by muscular cramps on exertion, tremor of the hands, and elevated muscle creatine kinase. [30] SBMA is often misdiagnosed as amyotrophic lateral sclerosis (ALS) (also known as Lou Gehrig's disease). [27]
The symptoms of SBMA are thought to be brought about by two simultaneous pathways involving the toxic misfolding of proteins and loss of AR functionality. [1] The polyglutamine tract in affected pedigrees tends to increase in length over generations, a phenomenon known as "anticipation", [31] leading to an increase in the severity of the disease as well as a decrease in the age of onset for each subsequent generation of a family affected by SBMA. [26]
All forms of androgen insensitivity are associated with infertility, though exceptions have been reported for both the mild and partial forms. [4] [5] [7] [33] [34] [35] Lifespan is not thought to be affected by AIS. [1]
The androgen receptor gene contains two polymorphic trinucleotide microsatellites in exon 1. [2] The first microsatellite (nearest the 5' end) contains 8 [36] to 60 [27] [30] repetitions of the glutamine codon "CAG" and is thus known as the polyglutamine tract. [3] The second microsatellite contains 4 [37] to 31 [38] repetitions of the glycine codon "GGC" and is known as the polyglycine tract. [39] The average number of repetitions varies by ethnicity, with Caucasians exhibiting an average of 21 CAG repeats, and Blacks 18. [40] Disease states are associated with extremes in polyglutamine tract length; prostate cancer, [26] hepatocellular carcinoma, [41] and intellectual disabilities [36] are associated with too few repetitions, while spinal and bulbar muscular atrophy (SBMA) is associated with a CAG repetition length of 40 or more. [28] Some studies indicate that the length of the polyglutamine tract is inversely correlated with transcriptional activity in the AR protein, and that longer polyglutamine tracts may be associated with infertility [42] [43] [44] and undermasculinized genitalia. [45] However, other studies have indicated that no such correlation exists. [46] [47] [48] [49] [50] [51] A comprehensive meta-analysis of the subject published in 2007 supports the existence of the correlation, and concluded that these discrepancies could be resolved when sample size and study design are taken into account. [11] Longer polyglycine tract lengths have also been associated with genital masculinization defects in some, [52] [53] but not all, [54] studies.
MAIS is only diagnosed in normal phenotypic males, and is not typically investigated except in cases of male infertility. [18] MAIS has a mild presentation that often goes unnoticed and untreated; [15] even with semenological, clinical and laboratory data, it can be difficult to distinguish between men with and without MAIS, and thus a diagnosis of MAIS is not usually made without confirmation of an AR gene mutation. [5] The androgen sensitivity index (ASI), defined as the product of luteinizing hormone (LH) and testosterone (T), is frequently raised in individuals with all forms of AIS, including MAIS, although many individuals with MAIS have an ASI in the normal range. [5] Testosterone levels may be elevated despite normal levels of luteinizing hormone. [15] [20] [25] Conversion of testosterone (T) to dihydrotestosterone (DHT) may be impaired, although to a lesser extent than is seen in 5α-reductase deficiency. [3] A high ASI in a normal phenotypic male, [46] especially when combined with azoospermia or oligospermia, [5] [7] decreased secondary terminal hair, [27] and/or impaired conversion of T to DHT, [3] can be indicative of MAIS, and may warrant genetic testing.
Due to its mild presentation, MAIS often goes unnoticed and untreated. [15] Management of MAIS is currently limited to symptomatic management; methods to correct a malfunctioning androgen receptor protein that result from an AR gene mutation are not currently available. Treatment includes surgical correction of mild gynecomastia, minor hypospadias repair, and testosterone supplementation. [1] [15] [55] Supraphysiological doses of testosterone have been shown to correct diminished secondary sexual characteristics in men with MAIS, [15] as well as to reverse infertility due to low sperm count. [55] [56] As is the case with PAIS, men with MAIS will experience side effects from androgen therapy (such as the suppression of the hypothalamic-pituitary-gonadal axis) at a higher dosage than unaffected men. Careful monitoring is required to ensure the safety and efficacy of treatment. [15] [57] [58] Regular breast [57] and prostate [59] examinations may be necessary due to comorbid association with breast and prostate cancers.
Androgen insensitivity syndrome (AIS) is a condition involving the inability to respond to androgens, typically due to androgen receptor dysfunction.
Lipoid congenital adrenal hyperplasia is an endocrine disorder that is an uncommon and potentially lethal form of congenital adrenal hyperplasia (CAH). It arises from defects in the earliest stages of steroid hormone synthesis: the transport of cholesterol into the mitochondria and the conversion of cholesterol to pregnenolone—the first step in the synthesis of all steroid hormones. Lipoid CAH causes mineralocorticoid deficiency in affected infants and children. Male infants are severely undervirilized causing their external genitalia to look feminine. The adrenals are large and filled with lipid globules derived from cholesterol.
Thyroid hormone resistance (also resistance to thyroid hormone (RTH), and sometimes Refetoff syndrome) describes a rare syndrome in which the thyroid hormone levels are elevated but the thyroid stimulating hormone (TSH) level is not suppressed, or not completely suppressed as would be expected. The first report of the condition appeared in 1967. Essentially this is decreased end organ responsiveness to thyroid hormones. A new term "impaired sensitivity to thyroid hormone" has been suggested in March 2014 by Refetoff et al.
Spinal and bulbar muscular atrophy (SBMA), popularly known as Kennedy's disease, is a rare, adult-onset, X-linked recessive lower motor neuron disease caused by trinucleotide CAG repeat expansions in exon 1 of the androgen receptor (AR) gene, which results in both loss of AR function and toxic gain of function.
In genetics, trinucleotide repeat disorders, a subset of microsatellite expansion diseases, are a set of over 30 genetic disorders caused by trinucleotide repeat expansion, a kind of mutation in which repeats of three nucleotides increase in copy numbers until they cross a threshold above which they cause developmental, neurological or neuromuscular disorders. In addition to the expansions of these trinucleotide repeats, expansions of one tetranucleotide (CCTG), five pentanucleotide, three hexanucleotide, and one dodecanucleotide (CCCCGCCCCGCG) repeat cause 13 other diseases. Depending on its location, the unstable trinucleotide repeat may cause defects in a protein encoded by a gene; change the regulation of gene expression; produce a toxic RNA, or lead to production of a toxic protein. In general, the larger the expansion the faster the onset of disease, and the more severe the disease becomes.
Isolated hypogonadotropic hypogonadism (IHH), also called idiopathic or congenital hypogonadotropic hypogonadism (CHH), as well as isolated or congenital gonadotropin-releasing hormone deficiency (IGD), is a condition which results in a small subset of cases of hypogonadotropic hypogonadism (HH) due to deficiency in or insensitivity to gonadotropin-releasing hormone (GnRH) where the function and anatomy of the anterior pituitary is otherwise normal and secondary causes of HH are not present.
Corticosteroid 11-β-dehydrogenase isozyme 2 also known as 11-β-hydroxysteroid dehydrogenase 2 is an enzyme that in humans is encoded by the HSD11B2 gene.
Partial androgen insensitivity syndrome (PAIS) is a condition that results in the partial inability of the cell to respond to androgens. It is an X linked recessive condition. The partial unresponsiveness of the cell to the presence of androgenic hormones impairs the masculinization of male genitalia in the developing fetus, as well as the development of male secondary sexual characteristics at puberty, but does not significantly impair female genital or sexual development. As such, the insensitivity to androgens is clinically significant only when it occurs in individuals with a Y chromosome. Clinical features include ambiguous genitalia at birth and primary amenhorrhoea with clitoromegaly with inguinal masses. Müllerian structures are not present in the individual.
Estrogen insensitivity syndrome (EIS), or estrogen resistance, is a form of congenital estrogen deficiency or hypoestrogenism which is caused by a defective estrogen receptor (ER) – specifically, the estrogen receptor alpha (ERα) – that results in an inability of estrogen to mediate its biological effects in the body. Congenital estrogen deficiency can alternatively be caused by a defect in aromatase, the enzyme responsible for the biosynthesis of estrogens, a condition which is referred to as aromatase deficiency and is similar in symptomatology to EIS.
Gonadotropin-releasing hormone receptor is a protein that in humans is encoded by the GNRHR gene.
The human gene SRD5A2 encodes the 3-oxo-5α-steroid 4-dehydrogenase 2 enzyme, also known as 5α-reductase type 2 (5αR2), one of three isozymes of 5α-reductase.
Thyroid hormone receptor beta (TR-beta) also known as nuclear receptor subfamily 1, group A, member 2 (NR1A2), is a nuclear receptor protein that in humans is encoded by the THRB gene.
Follitropin subunit beta also known as follicle-stimulating hormone beta subunit (FSH-B) is a protein that in humans is encoded by the FSHB gene. Alternative splicing results in two transcript variants encoding the same protein.
Homeobox protein prophet of PIT-1 is a protein that in humans is encoded by the PROP1 gene.
Luteinizing hormone subunit beta also known as lutropin subunit beta or LHβ is a polypeptide that in association with an alpha subunit common to all gonadotropin hormones forms the reproductive signaling molecule luteinizing hormone. In humans it is encoded by the LHB gene.
Thyroid stimulating hormone, beta also known as TSHB is a protein which in humans is encoded by the TSHB gene.
Aromatase deficiency is a rare condition characterized by extremely low levels or complete absence of the enzyme aromatase activity in the body. It is an autosomal recessive disease resulting from various mutations of gene CYP19 (P450arom) which can lead to ambiguous genitalia and delayed puberty in females, continued linear growth into adulthood and osteoporosis in males and virilization in pregnant mothers. As of 2020, fewer than 15 cases have been identified in genetically male individuals and at least 30 cases in genetically female individuals.
Complete androgen insensitivity syndrome (CAIS) is an AIS condition that results in the complete inability of the cell to respond to androgens. As such, the insensitivity to androgens is only clinically significant when it occurs in individuals who are exposed to significant amounts of testosterone at some point in their lives. The unresponsiveness of the cell to the presence of androgenic hormones prevents the masculinization of male genitalia in the developing fetus, as well as the development of male secondary sexual characteristics at puberty, but does allow, without significant impairment, female genital and sexual development in those with the condition.
Kowarski syndrome describes cases of growth failure, despite the presence of normal or slightly high blood growth hormone by radioimmunoassay (RIA-GH) and low serum IGF1, and who exhibit a significant increase in growth rate following recombinant GH therapy.
Cytochrome P450 family 19 subfamily A member 1 is a protein that in humans is encoded by the CYP19A1 gene.