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What is Androgen insensitivity syndrome and explain in detail?
Androgen insensitivity syndrome (AIS) is a difference in sex development involving hormonal resistance due to androgen receptor dysfunction.It affects 1 in 20,000 to 64,000 XY (karyotypically male) births. The condition results in the partial or complete inability of cells to respond to androgens. This unresponsiveness can impair or prevent the development of male genitals, as well as impairing or preventing the development of male secondary sexual characteristics at puberty. It does not significantly impair female genital or sexual development. The insensitivity to androgens is therefore clinically significant only when it occurs in genetic males, (i.e. individuals with a Y-chromosome, or more specifically, an SRY gene).  Clinical phenotypes in these individuals range from a typical male habitus with mild spermatogenic defect or reduced secondary terminal hair, to a full female habitus, despite the presence of a Y-chromosome.AIS 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 are those of a typical female; mild androgen insensitivity syndrome (MAIS) is indicated when the external genitalia are those of a typical male, and partial androgen insensitivity syndrome (PAIS) is indicated when the external genitalia are partially, but not fully, masculinized. Androgen insensitivity syndrome is the largest single entity that leads to 46,XY undermasculinized genitalia.Management of AIS is currently limited to symptomatic management; no method is currently available to correct the malfunctioning androgen receptor proteins produced by AR gene mutations.  Areas of management include sex assignment, genitoplasty, gonadectomy to reduce tumor risk, hormone replacement therapy, genetic counseling, and psychological counseling.

Genetics
The human androgen receptor (AR) is a protein encoded by a gene located on the proximal long arm of the X chromosome (locus Xq11-Xq12).  The protein coding region consists of approximately 2,757 nucleotides (919 codons) spanning eight exons, designated 1-8 or A-H.  Introns vary in size between 0.7 and 26 kb.  Like other nuclear receptors, the AR protein consists of several functional domains: the transactivation domain (also called the transcription-regulation domain or the amino / NH2-terminal domain), the DNA-binding domain, the hinge region, and the steroid-binding domain (also called the carboxyl-terminal ligand-binding domain).  The transactivation domain is encoded by exon 1, and makes up more than half of the AR protein.  Exons 2 and 3 encode the DNA-binding domain, while the 5 portion of exon 4 encodes the hinge region.  The remainder of exons 4 through 8 encodes the ligand binding domain.

Trinucleotide satellite lengths and AR transcriptional activity
The AR gene contains two polymorphic trinucleotide microsatellites in exon 1.  The first microsatellite (nearest the 5 end) contains 8  to 60  repetitions of the glutamine codon "CAG" and is thus known as the polyglutamine tract.  The second microsatellite contains 4  to 31  repetitions of the glycine codon "GGC" and is known as the polyglycine tract.  The average number of repetitions varies by ethnicity, with Caucasians exhibiting an average of 21 CAG repeats, and Blacks 18.  In men, disease states are associated with extremes in polyglutamine tract length; prostate cancer, hepatocellular carcinoma, and intellectual disability are associated with too few repetitions, while spinal and bulbar muscular atrophy (SBMA) is associated with a CAG repetition length of 40 or more.  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 male infertility and undermasculinized genitalia in men.  However, other studies have indicated no such correlation exists. A comprehensive meta-analysis of the subject published in 2007 supports the existence of the correlation, and concluded these discrepancies could be resolved when sample size and study design are taken into account.  Some studies suggest longer polyglycine tract lengths are also associated with genital masculinization defects in men.  Other studies find no such association.

AR mutations
As of 2010, over 400 AR mutations have been reported in the AR mutation database, and the number continues to grow.  Inheritance is typically maternal and follows an X-linked recessive pattern; individuals with a 46,XY karyotype always express the mutant gene since they have only one X chromosome, whereas 46,XX carriers are minimally affected.  About 30% of the time, the AR mutation is a spontaneous result, and is not inherited.  Such de novo mutations are the result of a germ cell mutation or germ cell mosaicism in the gonads of one of the parents, or a mutation in the fertilized egg itself.  In one study, three of eight de novo mutations occurred in the postzygotic stage, leading to the estimate that up to one-third of de novo mutations result in somatic mosaicism.  Not every mutation of the AR gene results in androgen insensitivity; one particular mutation occurs in 8 to 14% of genetic males, and is thought to adversely affect only a small number of individuals when other genetic factors are present.

Other causes
Some individuals with CAIS or PAIS do not have any AR mutations despite clinical, hormonal, and histological features sufficient to warrant an AIS diagnosis; up to 5% of women with CAIS do not have an AR mutation, as well as between 27 and 72% of individuals with PAIS.
In one patient, the underlying cause for presumptive PAIS was a mutant steroidogenic factor-1 (SF-1) protein.  In another patient, CAIS was the result of a deficit in the transmission of a transactivating signal from the N-terminal region of the androgen receptor to the basal transcription machinery of the cell.  A coactivator protein interacting with the activation function 1 (AF-1) transactivation domain of the androgen receptor may have been deficient in this patient.  The signal disruption could not be corrected by supplementation with any coactivators known at the time, nor was the absent coactivator protein characterized, which left some in the field unconvinced that a mutant coactivator would explain the mechanism of androgen resistance in CAIS or PAIS patients with a typical AR gene.

XY karyotype
Depending on the mutation, a person with a 46,XY karyotype and AIS can have either a male (MAIS) or female (CAIS) phenotype, or may have genitalia that are only partially masculinized (PAIS).  The gonads are testes regardless of phenotype due to the influence of the Y chromosome.  A 46,XY female, thus, does not have ovaries, and can not contribute an egg towards conception. In some cases, 46, XY females do form a vestigial uterus and have been able to gestate a children. Such examples are rare and have required the use of an egg donor, hormone therapy, and IVF.Several case studies of fertile 46,XY males with AIS have been published, although this group is thought to be a minority.  In some cases, infertile males with MAIS have been able to conceive children after increasing their sperm count through the use of supplementary testosterone.A genetic male conceived by a man with AIS would not receive his fathers X chromosome,  thus would neither inherit nor carry the gene for the syndrome.  A genetic female conceived in such a way would receive her fathers X chromosome, thus would become a carrier.

XX karyotype
Genetic females (46,XX karyotype) have two X chromosomes, thus have two AR genes.  A mutation in one (but not both) results in a minimally affected, fertile, female carrier.  Some carriers have been noted to have slightly reduced body hair, delayed puberty, and/or tall stature, presumably due to skewed X-inactivation.  A female carrier will pass the affected AR gene to her children 50% of the time.  If the affected child is a genetic female, she, too, will be a carrier.  An affected 46,XY child will have AIS.A genetic female with mutations in both AR genes could theoretically result from the union of a fertile man with AIS and a female carrier of the gene, or from de novo mutation.  However, given the scarcity of fertile AIS men and low incidence of AR mutation, the chances of this occurrence are small.  The phenotype of such an individual is a matter of speculation; as of 2010, no such documented case has been published.

Correlation of genotype and phenotype
Individuals with partial AIS, unlike those with the complete or mild forms, present at birth with ambiguous genitalia, and the decision to raise the child as male or female is often not obvious.  Unfortunately,  little information regarding phenotype can be gleaned from precise knowledge of the AR mutation itself;  the same AR mutation may cause significant variation in the degree of masculinization in different individuals, even among members of the same family. Exactly what causes this variation is not entirely understood, although factors contributing to it could include the lengths of the polyglutamine and polyglycine tracts, sensitivity to and variations in the intrauterine endocrine milieu, the effect of coregulatory proteins active in Sertoli cells, somatic mosaicism, expression of the 5RD2 gene in genital skin fibroblasts, reduced AR transcription and translation from factors other than mutations in the AR coding region, an unidentified coactivator protein, enzyme deficiencies such as 21-hydroxylase deficiency, or other genetic variations such as a mutant steroidogenic factor-1 protein.  The degree of variation, however, does not appear to be constant across all AR mutations, and is much more extreme in some.  Missense mutations that result in a single amino acid substitution are known to produce the most phenotypic diversity.

Pathophysiology
Androgens and the androgen receptor
The effects that androgens have on the human body (virilization, masculinization, anabolism, etc.) are not brought about by androgens themselves, but rather are the result of androgens bound to androgen receptors; the androgen receptor mediates the effects of androgens in the human body.  Likewise, the androgen receptor itself is generally inactive in the cell until androgen binding occurs.The following series of steps illustrates how androgens and the androgen receptor work together to produce androgenic effects:

In this way, androgens bound to androgen receptors regulate the expression of target genes, thus produce androgenic effects.Theoretically, certain mutant androgen receptors can function without androgens; in vitro studies have demonstrated that a mutant androgen receptor protein can induce transcription in the absence of androgen if its steroid binding domain is deleted.  Conversely, the steroid-binding domain may act to repress the AR transactivation domain, perhaps due to the ARs unliganded conformation.

Androgens in fetal development
Human embryos develop similarly for the first six weeks, regardless of genetic sex (46,XX or 46,XY karyotype); the only way to tell the difference between 46,XX or 46,XY embryos during this time period is to look for Barr bodies or a Y chromosome.  The gonads begin as bulges of tissue called the genital ridges at the back of the abdominal cavity, near the midline.  By the fifth week, the genital ridges differentiate into an outer cortex and an inner medulla, and are called indifferent gonads.  By the sixth week, the indifferent gonads begin to differentiate according to genetic sex.  If the karyotype is 46,XY, testes develop due to the influence of the Y chromosome’s SRY gene.  This process does not require the presence of androgen, nor a functional androgen receptor.Until around the seventh week of development, the embryo has indifferent sex accessory ducts, which consist of two pairs of ducts: the Müllerian ducts and the Wolffian ducts.  Sertoli cells within the testes secrete anti-Müllerian hormone around this time to suppress the development of the Müllerian ducts, and cause their degeneration.  Without this anti-Müllerian hormone, the Müllerian ducts develop into the female internal genitalia (uterus, cervix, fallopian tubes, and upper vaginal barrel).  Unlike the Müllerian ducts, the Wolffian ducts will not continue to develop by default.  In the presence of testosterone and functional androgen receptors, the Wolffian ducts develop into the epididymides, vasa deferentia, and seminal vesicles.  If the testes fail to secrete testosterone, or the androgen receptors do not function properly, the Wolffian ducts degenerate.

Masculinization of the male external genitalia (the penis, penile urethra, and scrotum), as well as the prostate, are dependent on the androgen dihydrotestosterone.  Testosterone is converted into dihydrotestosterone by the 5-alpha reductase enzyme.  If this enzyme is absent or deficient, then dihydrotestosterone is not created, and the external male genitalia do not develop properly.  As is the case with the internal male genitalia, a functional androgen receptor is needed for dihydrotestosterone to regulate the transcription of target genes involved in development.

Pathogenesis of AIS
Mutations in the androgen receptor gene can cause problems with any of the steps involved in androgenization, from the synthesis of the androgen receptor protein itself, through the transcriptional ability of the dimerized, androgen-AR complex.  AIS can result if even one of these steps is significantly disrupted, as each step is required for androgens to activate the AR successfully and regulate gene expression.  Exactly which steps a particular mutation will impair can be predicted, to some extent, by identifying the area of the AR in which the mutation resides.  This predictive ability is primarily retrospective in origin; the different functional domains of the AR gene have been elucidated by analyzing the effects of specific mutations in different regions of the AR.  For example, mutations in the steroid binding domain have been known to affect androgen binding affinity or retention, mutations in the hinge region have been known to affect nuclear translocation, mutations in the DNA-binding domain have been known to affect dimerization and binding to target DNA, and mutations in the transactivation domain have been known to affect target gene transcription regulation.  Unfortunately, even when the affected functional domain is known, predicting the phenotypical consequences of a particular mutation (see Correlation of genotype and phenotype) is difficult.Some mutations can adversely impact more than one functional domain.  For example, a mutation in one functional domain can have deleterious effects on another by altering the way in which the domains interact.  A single mutation can affect all downstream functional domains if a premature stop codon or framing error results; such a mutation can result in a completely unusable (or unsynthesizable) androgen receptor protein.  The steroid binding domain is particularly vulnerable to the effects of a premature stop codon or framing error, since it occurs at the end of the gene, and its information is thus more likely to be truncated or misinterpreted than other functional domains.Other, more complex relationships have been observed as a consequence of mutated AR; some mutations associated with male phenotypes have been linked to male breast cancer, prostate cancer, or in the case of spinal and bulbar muscular atrophy, disease of the central nervous system.  The form of breast cancer seen in some men with PAIS is caused by a mutation in the ARs DNA-binding domain.  This mutation is thought to cause a disturbance of the ARs target gene interaction that allows it to act at certain additional targets, possibly in conjunction with the estrogen receptor protein, to cause cancerous growth. The pathogenesis of spinal and bulbar muscular atrophy (SBMA) demonstrates that even the mutant AR protein itself can result in pathology.  The trinucleotide repeat expansion of the polyglutamine tract of the AR gene that is associated with SBMA results in the synthesis of a misfolded AR protein that the cell fails to proteolyze and disperse properly.  These misfolded AR proteins form aggregates in the cell cytoplasm and nucleus.  Over the course of 30 to 50 years, these aggregates accumulate and have a cytotoxic effect, eventually resulting in the neurodegenerative symptoms associated with SBMA.

Diagnosis
The phenotypes that result from the insensitivity to androgens are not unique to AIS, thus the diagnosis of AIS requires thorough exclusion of other causes.  Clinical findings indicative of AIS include the presence of a short vagina  or undermasculinized genitalia, partial or complete regression of Müllerian structures, bilateral nondysplastic testes, and impaired spermatogenesis and/or virilization.  Laboratory findings include a 46,XY karyotype and typical or elevated postpubertal testosterone, luteinizing hormone, and estradiol levels.  The androgen binding activity of genital skin fibroblasts is typically diminished, although exceptions have been reported.  Conversion of testosterone to dihydrotestosterone may be impaired.  The diagnosis of AIS is confirmed if androgen receptor gene sequencing reveals a mutation, although not all individuals with AIS (particularly PAIS) will have an AR mutation (see Other Causes).Each of the three types of AIS (complete, partial, and mild) has a different list of differential diagnoses to consider.  Depending on the form of AIS suspected, the list of differentials can include:

Classification
AIS is broken down into three classes based on phenotype: complete androgen insensitivity syndrome (CAIS), partial androgen insensitivity syndrome (PAIS), and mild androgen insensitivity syndrome (MAIS).  A supplemental system of phenotypic grading that uses seven classes instead of the traditional three was proposed by pediatric endocrinologist Charmian A. Quigley et al. in 1995.  The first six grades of the scale, grades 1 through 6, are differentiated by the degree of genital masculinization; grade 1 is indicated when the external genitalia is fully masculinized, grade 6 is indicated when the external genitalia is fully feminized, and grades 2 through 5 quantify four degrees of decreasingly masculinized genitalia that lie in the interim.  Grade 7 is indistinguishable from grade 6 until puberty, and is thereafter differentiated by the presence of secondary terminal hair; grade 6 is indicated when secondary terminal hair is present, whereas grade 7 is indicated when it is absent.  The Quigley scale can be used in conjunction with the traditional three classes of AIS to provide additional information regarding the degree of genital masculinization, and is particularly useful when the diagnosis is PAIS.

Complete AIS
Partial AIS
Mild AIS
Management
Management of AIS is currently limited to symptomatic management; no method is currently available to correct the malfunctioning androgen receptor proteins produced by AR gene mutations.  Areas of management include sex assignment, genitoplasty, gonadectomy in relation to tumor risk, hormone replacement therapy, genetic counseling, and psychological counseling.

CAIS
PAIS
MAIS
Epidemiology
AIS represents about 15% to 20% of DSDs and affects 1 in 20,000 to 1 in 64,000 males.Estimates for the incidence of androgen insensitivity syndrome are based on a relatively small population size, thus are known to be imprecise.  CAIS is estimated to occur in one of every 20,400 46,XY births.  A nationwide survey in the Netherlands based on patients with genetic confirmation of the diagnosis estimates that the minimal incidence of CAIS is one in 99,000.  The incidence of PAIS is estimated to be one in 130,000.  Due to its subtle presentation, MAIS is not typically investigated except in the case of male infertility, thus its true prevalence is unknown.

Controversy
Preimplantation genetic diagnosis
Preimplantation genetic diagnosis (PGD or PIGD) refers to genetic profiling of embryos prior to implantation (as a form of embryo profiling), and sometimes even of oocytes prior to fertilization. When used to screen for a specific genetic sequence, its main advantage is that it avoids selective pregnancy termination, as the method makes it highly likely that a selected embryo will be free of the condition under consideration.In the UK, AIS appears on a list of serious genetic diseases that may be screened for via PGD.  Some ethicists, clinicians, and intersex advocates have argued that screening embryos to specifically exclude intersex traits is based on social and cultural norms as opposed to medical necessity.

History
Recorded descriptions of the effects of AIS date back hundreds of years, although significant understanding of its underlying histopathology did not occur until the 1950s.  The taxonomy and nomenclature associated with androgen insensitivity went through a significant evolution that paralleled this understanding.

Timeline of major milestones
1950: Lawson Wilkins administers daily methyltestosterone to a karyotype|46,XY female patient, who shows no signs of virilization.  His experiment is the first documented demonstration of the pathophysiology of AIS.
1970: Mary F. Lyon and Susan Hawkes reported that a gene on the X chromosome caused complete insensitivity to androgens in mice.
1981: Barbara Migeon et al. narrowed down the locus of the human androgen receptor gene (or a factor controlling the androgen receptor gene) to somewhere between Xq11 and Xq13.
1988: The human androgen receptor gene is first cloned and partially analyzed by multiple parties.  Terry Brown et al. reported the first mutations proven to cause AIS.
1989: Terry Brown et al. reported the exact locus of the AR gene (Xq11-Xq12), and Dennis Lubahn et al. published its intron-exon boundaries.
1994: The androgen receptor gene mutations database was created to provide a comprehensive listing of mutations published in medical journals and conference proceedings.

Early terminology
The first descriptions of the effects of AIS appeared in the medical literature as individual case reports or as part of a comprehensive description of intersex physicalities.  In 1839, Scottish obstetrician Sir James Young Simpson published one such description in an exhaustive study of intersexuality that has been credited with advancing the medical communitys understanding of the subject.  Simpsons system of taxonomy, however, was far from the first; taxonomies or descriptions for the classification of intersexuality were developed by Italian physician and physicist Fortuné Affaitati in 1549, French surgeon Ambroise Paré in 1573, French physician and sexology pioneer Nicolas Venette in 1687 (under the pseudonym Vénitien Salocini), and French zoologist Isidore Geoffroy Saint-Hilaire in 1832.  All five of these authors used the colloquial term "hermaphrodite" as the foundation of their taxonomies, although Simpson himself questioned the propriety of the word in his publication.  Use of the word "hermaphrodite" in the medical literature has persisted to this day, although its propriety is still in question.  An alternative system of nomenclature has been recently suggested, but the subject of exactly which word or words should be used in its place still one of much debate.

Pseudohermaphroditism
"Pseudohermaphroditism" has, until very recently, been the term used in the medical literature to describe the condition of an individual whose gonads and karyotype do not match the external genitalia in the gender binary sense.  For example, 46,XY individuals who have a female phenotype, but also have testes instead of ovaries—a group that includes all individuals with CAIS, as well as some individuals with PAIS—are classified as having "male pseudohermaphroditism", while individuals with both an ovary and a testis (or at least one ovotestis) are classified as having "true hermaphroditism".  Use of the word in the medical literature antedates the discovery of the chromosome,  thus its definition has not always taken karyotype into account when determining an individuals sex.  Previous definitions of "pseudohermaphroditism" relied on perceived inconsistencies between the internal and external organs; the "true" sex of an individual was determined by the internal organs, and the external organs determined the "perceived" sex of an individual.German-Swiss pathologist Edwin Klebs is sometimes noted for using the word "pseudohermaphroditism" in his taxonomy of intersexuality in 1876, although the word is clearly not his invention as is sometimes reported; the history of the word "pseudohermaphrodite" and the corresponding desire to separate "true" hermaphrodites from "false", "spurious", or "pseudo" hermaphrodites, dates back to at least 1709, when Dutch anatomist Frederik Ruysch used it in a publication describing a subject with testes and a mostly female phenotype.  "Pseudohermaphrodite" also appeared in the Acta Eruditorum later that same year, in a review of Ruyschs work.  Also some evidence indicates the word was already being used by the German and French medical community long before Klebs used it; German physiologist Johannes Peter Müller equated "pseudohermaphroditism" with a subclass of hermaphroditism from Saint-Hilaires taxonomy in a publication dated 1834, and by the 1840s "pseudohermaphroditism" was appearing in several French and German publications, including dictionaries.

Testicular feminization
In 1953, American gynecologist John Morris provided the first full description of what he called "testicular feminization syndrome" based on 82 cases compiled from the medical literature, including two of his own patients.  The term "testicular feminization" was coined to reflect Morris observation that the testicles in these patients produced a hormone that had a feminizing effect on the body, a phenomenon now understood to be due to the inaction of androgens, and subsequent aromatization of testosterone into estrogen.   A few years before Morris published his landmark paper, Lawson Wilkins had shown through experiment that unresponsiveness of the target cell to the action of androgenic hormones was a cause of "male pseudohermaphroditism".   Wilkins work, which clearly demonstrated the lack of a therapeutic effect when 46,XY patients were treated with androgens, caused a gradual shift in nomenclature from "testicular feminization" to "androgen resistance".

Other names
A distinct name has been given to many of the various presentations of AIS, such as Reifenstein syndrome (1947), Goldberg-Maxwell syndrome (1948), Morris syndrome (1953), Gilbert-Dreyfus syndrome (1957), Lubs syndrome (1959), "incomplete testicular feminization" (1963), Rosewater syndrome (1965), and Aimans syndrome (1979).  Since it was not understood that these different presentations were all caused by the same set of mutations in the androgen receptor gene, a unique name was given to each new combination of symptoms, resulting in a complicated stratification of seemingly disparate disorders.Over the last 60 years, as reports of strikingly different phenotypes were reported to occur even among members of the same family, and as steady progress was made towards the understanding of the underlying molecular pathogenesis of AIS, these disorders were found to be different phenotypic expressions of one syndrome caused by molecular defects in the androgen receptor gene.AIS is now the accepted terminology for the syndromes resulting from unresponsiveness of the target cell to the action of androgenic hormones.  CAIS encompasses the phenotypes previously described by "testicular feminization", Morris syndrome, and Goldberg-Maxwell syndrome; PAIS includes Reifenstein syndrome, Gilbert-Dreyfus syndrome, Lubs syndrome, "incomplete testicular feminization", and Rosewater syndrome; and MAIS includes Aimans syndrome.The more virilized phenotypes of AIS have sometimes been described as "undervirilized male syndrome", "infertile male syndrome", "undervirilized fertile male syndrome", etc., before evidence was reported that these conditions were caused by mutations in the AR gene