Editing Androgen insensitivity syndrome (section)

==Pathophysiology==
[[File:Human androgen receptor and androgen binding.svg|thumb|Normal function of the androgen receptor: Testosterone (T) enters the cell and, if 5-alpha-reductase is present, is converted into dihydrotestone (DHT). Upon steroid binding, the androgen receptor (AR) undergoes a conformational change and releases heat shock proteins (hsps). Phosphorylation (P) occurs before or after steroid binding. The AR translocates to the nucleus where dimerization, DNA binding, and the recruitment of coactivators occur. Target genes are transcribed (mRNA) and translated into proteins.<ref name="1995 quigley 16" /><ref name="2005 gottlieb 10" /><ref name="1998 choong 21" /><ref name="2003 meehan 8" />]]

===Androgens and the androgen receptor===
{{Main|Androgen receptor}}
The [[Androgen#Functions|effects]] that [[androgens]] have on the human body ([[Puberty#Physical changes in boys|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.<ref name="1998 wang 83" />  Likewise, the androgen receptor itself is generally inactive in the cell until androgen binding occurs.<ref name="1995 quigley 16" />

The following series of steps illustrates how androgens and the androgen receptor work together to produce androgenic effects:<ref name="2006 hughes 20" /><ref name="2008 galani 7" /><ref name="1995 quigley 16" /><ref name="2005 gottlieb 10" />

{{ordered list
 | Androgen enters the cell.
  {{ordered list|type=lower-alpha
    | Only certain organs in the body, such as the [[gonads]] and the [[adrenal glands]], produce the androgen [[testosterone]].
    | Testosterone is converted into [[dihydrotestosterone]], a chemically similar androgen, in cells containing the [[enzyme]] [[5-alpha reductase]].
    | Both androgens exert their influence through binding with the androgen receptor.
  }}
 | Androgen binds with the androgen receptor.
  {{ordered list|type=lower-alpha
    | The androgen receptor is expressed ubiquitously throughout the tissues of the human body.
    | Before it binds with an androgen, the androgen receptor is bound to [[heat shock proteins]].
    | These heat shock proteins are released upon androgen binding.
    | Androgen binding induces a stabilizing, [[Chemical structure|conformational]] change in the androgen receptor.
    | The two [[zinc fingers]] of the [[DNA-binding domain]] are exposed as a result of this new conformation.
    | AR stability is thought to be aided by type II [[Transcription coregulator|coregulators]], which modulate [[protein folding]] and androgen binding, or facilitate NH2/carboxyl-terminal interaction.
  }}
 | The hormone-activated androgen receptor is [[Protein phosphorylation|phosphorylated]].
  {{ordered list|type=lower-alpha
    | Receptor phosphorylation can occur before androgen binding, although the presence of androgen promotes hyperphosphorylation.
    | The biological ramifications of receptor phosphorylation are unknown.
  }}
 | The hormone-activated androgen receptor [[Protein targeting#Protein translocation|translocates]] to the nucleus.
  {{ordered list|type=lower-alpha
    | Nucleocytoplasmic transport is in part facilitated by an [[amino acid]] [[Nucleic acid sequence|sequence]] on the [[androgen receptor|AR]] called the [[nuclear localization signal]].
    | The AR's nuclear localization signal is primarily encoded in the hinge region of the AR gene.
  }}
 | [[Protein dimer|Homodimerization]] occurs.
  {{ordered list|type=lower-alpha
    | Dimerization is mediated by the second (nearest the 3' end) [[zinc finger]].
  }}
 | DNA binding to regulatory [[Hormone response element|androgen response elements]] occurs.
  {{ordered list|type=lower-alpha
    | Target genes contain (or are flanked by) [[Transcription (genetics)|transcriptional]] enhancer nucleotide sequences that interact with the first zinc finger.
    | These areas are called androgen response elements.
  }}
 | [[Coactivator (genetics)|Coactivators]] are recruited by the AR.
  {{ordered list|type=lower-alpha
    | Type I coactivators (i.e., coregulators) are thought to influence AR transcriptional activity by facilitating DNA occupancy, [[chromatin remodeling]], or the recruitment of general [[transcription factor]]s associated with [[RNA polymerase II]] holocomplex.
  }}
 | Target [[gene transcription]] ensues.
}}

In this way, androgens bound to androgen receptors [[Regulation of gene expression|regulate the expression]] of target genes, thus produce androgenic effects.<ref>{{Cite journal |last1=Jin |first1=Hong-Jian |last2=Kim |first2=Jung |last3=Yu |first3=Jindan |date=September 2013 |title=Androgen receptor genomic regulation |url=https://tau.amegroups.com/article/view/2705 |journal=Translational Andrology and Urology |language=en |volume=2 |issue=3 |pages=15877–15177 |doi=10.3978/j.issn.2223-4683.2013.09.01 |issn=2223-4691 |pmc=4165347 |pmid=25237629}}</ref>

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.<ref name="1991 jenster 5" /><ref name="1991 simental 266" />  Conversely, the steroid-binding domain may act to repress the AR [[transactivation]] domain, perhaps due to the AR's [[Ligand (biochemistry)|unliganded]] conformation.<ref name="1995 quigley 16" />

[[File:Human sexual differentiation.gif|thumb|Sexual differentiation: The human embryo has indifferent sex accessory ducts until the seventh week of development.<ref name="2000 gilbert" />]]

===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.<ref name="2006 jones" />  The gonads begin as bulges of tissue called the [[gonadal ridge|genital ridges]] at the back of the [[abdominal cavity]], near the midline.  By the fifth week, the genital ridges [[sexual differentiation|differentiate]] into an outer [[Cortex (anatomy)|cortex]] and an inner [[wikt:medulla|medulla]], and are called [[Development of the gonads|indifferent gonads]].<ref name="2006 jones" />  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.<ref name="2006 achermann" /><ref name="1995 simpson" />  This process does not require the presence of androgen, nor a functional androgen receptor.<ref name="2006 achermann" /><ref name="1995 simpson" />

Until around the seventh week of development, the embryo has indifferent [[Sex determination and differentiation (human)|sex accessory ducts]], which consist of two pairs of ducts: the [[Müllerian ducts]] and the [[Wolffian ducts]].<ref name="2006 jones" />  [[Sertoli cells]] within the testes secrete [[anti-Müllerian hormone]] around this time to suppress the development of the Müllerian ducts, and cause their degeneration.<ref name="2006 jones" />  Without this anti-Müllerian hormone, the Müllerian ducts develop into the [[Female reproductive system#Internal genitalia|female internal genitalia]] ([[uterus]], [[cervix]], [[fallopian tubes]], and [[Vagina#Gross anatomy|upper vaginal barrel]]).<ref name="2006 jones" />  Unlike the Müllerian ducts, the Wolffian ducts will not continue to develop by default.<ref name="2003 yong 9" />  In the presence of testosterone and functional androgen receptors, the Wolffian ducts develop into the [[epididymis|epididymides]], [[vas deferens|vasa deferentia]], and [[seminal vesicles]].<ref name="2006 jones" />  If the testes fail to secrete testosterone, or the androgen receptors do not function properly, the Wolffian ducts degenerate.<ref name="2004 hannema 89" />

[[File:Androgen dependencies of male genital tissues.png|thumb|Masculinization of the male genitalia is dependent on both testosterone and dihydrotestosterone.<ref name="2000 gilbert" />]]
Masculinization of the [[male external genitalia]] (the [[Human penis|penis]], penile [[urethra]], and [[scrotum]]), as well as the [[prostate]], are dependent on the androgen [[dihydrotestosterone]].<ref name="2008 oakes 21" /><ref name="1999 roy 55" /><ref name="1999 kokontis 55" /><ref name="2009 rajender 91" />  Testosterone is converted into dihydrotestosterone by the 5-alpha reductase enzyme.<ref name="2006 sobel 91" />  If this enzyme is absent or deficient, then dihydrotestosterone is not created, and the external male genitalia do not develop properly.<ref name="2008 oakes 21" /><ref name="1999 roy 55" /><ref name="1999 kokontis 55" /><ref name="2009 rajender 91" /><ref name="2006 sobel 91" />  As is the case with the [[Male reproductive system#Internal genitalia|internal male genitalia]], a functional androgen receptor is needed for dihydrotestosterone to regulate the [[gene transcription|transcription of target genes]] involved in development.<ref name="1998 wang 83" />

===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 [[Transcription (genetics)|transcriptional ability]] of the [[Protein dimer|dimerized]], androgen-AR complex.<ref name="1995 quigley 16" />  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 [[regulation of gene expression|regulate gene expression]].<ref name="1995 quigley 16" />  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 [[protein domain|functional domains]] of the AR gene have been elucidated by analyzing the effects of specific mutations in different regions of the AR.<ref name="1995 quigley 16" />  For example, mutations in the steroid binding domain have been known to affect [[Ligand (biochemistry)#Receptor/ligand binding affinity|androgen binding affinity or retention]], mutations in the hinge region have been known to affect [[Protein targeting#Protein translocation|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.<ref name="1995 quigley 16" /><ref name="2003 yong 9" />  Unfortunately, even when the affected functional domain is known, predicting the [[phenotype|phenotypical]] consequences of a particular mutation (see [[#Correlation of genotype and phenotype|Correlation of genotype and phenotype]]) is difficult.{{citation needed|date=September 2021}}

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.<ref name="2003 yong 9" />  A single mutation can affect all [[Upstream and downstream (DNA)|downstream]] functional domains if a [[nonsense mutation|premature stop codon]] or [[frameshift mutation|framing error]] results; such a mutation can result in a completely unusable (or unsynthesizable) androgen receptor protein.<ref name="1995 quigley 16" />  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.<ref name="1995 quigley 16" />

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]].<ref name="2003 lund 79" /><ref name="2001 casella 58" /><ref name="1992 wooster 2" /><ref name="1996 evans 28" /><ref name="1993 lobaccaro 2" />  The form of breast cancer seen in some men with PAIS is caused by a mutation in the AR's DNA-binding domain.<ref name="1992 wooster 2" /><ref name="1993 lobaccaro 2" />  This mutation is thought to cause a disturbance of the AR's target gene interaction that allows it to act at certain additional targets, possibly in conjunction with the [[estrogen receptor]] protein, to cause [[neoplasm|cancerous growth]].<ref name="1995 quigley 16" /> The [[pathogenesis]] of spinal and bulbar muscular atrophy (SBMA) demonstrates that even the mutant AR protein itself can result in [[pathology]].  The [[trinucleotide repeat disorder|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 [[Proteolysis|proteolyze]] and disperse properly.<ref name="1999 stenoien 8" />  These misfolded AR proteins form aggregates in the cell [[cytoplasm]] and [[Cell nucleus|nucleus]].<ref name="1999 stenoien 8" />  Over the course of 30 to 50 years, these aggregates accumulate and have a [[cytotoxicity|cytotoxic]] effect, eventually resulting in the [[neurodegeneration|neurodegenerative]] symptoms associated with SBMA.<ref name="1999 stenoien 8" />