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Androgen Receptor Transactivation Domain and Control of Spermatogenesis

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Androgen Receptor Transactivation Domain and Control of Spermatogenesis Reviews of Reproduction (1998) 3, 141–144 Androgen receptor transactivation domain and control of spermatogenesis Eu L. Yong, Farid Ghadessy, Qi Wang, Amparo Mifsud and Soon C. Ng Department of Obstetrics and Gynaecology, National University of Singapore, Lower Kent Ridge Road, Singapore 119 074 Male sex steroids (androgens) are important for maintaining sperm production and growth of the accessory sex organ, the prostate gland. This article examines the role of the androgen re- ceptor (AR) in the control of spermatogenesis and focusses on the N-terminal transactivation domain of the receptor, a poorly studied region that is essential for receptor function. This domain is of great interest because of its causative relationship to a fatal neuromuscular disease, spinal bulbar muscular atrophy (Kennedy’s syndrome). Genetic screening of the trans- activation domain of the AR gene of 153 patients presenting solely with defective spermato- genesis and male infertility, and of over 72 healthy fertile controls was performed. Up to 20% of infertile males have reduced androgenicity caused by an increase in length of a poly- morphic trinucleotide (CAG) repeat segment, encoding a polyglutamine tract, of the androgen receptor. The increased risk of male infertility associated with long CAG lengths is associated with reduced risk of prostate cancer. Conversely, short polyglutamine tracts are associated with increased risk of prostate cancer but a reduced risk of male infertility. Thus depressed sper- matogenesis and prostate cancer represent opposite ends of the spectrum of androgen receptor transactivation function. Improved understanding of androgen receptor action in these two important public health concerns could lead to rational and effective prevention and therapy. Male sex steroids (androgens) sculpture the human embryo composed of three main functional domains: the trans- resulting in development of the male internal and external activation domain, the DNA-binding domain and the ligand- genitalia. At puberty, a surge of androgens leads to the in- binding domain (Fig. 1). itiation of spermatogenesis and growth of accessory sex organs, Since the definition of the exon–intron boundaries of the including the prostate gland. Sperm production is highly de- androgen receptor gene, over 300 androgen receptor mutations pendent on androgens (Zirkin et al., 1989) and it has long been (Gottlieb et al., 1998) have been reported in the ligand-binding suspected that suboptimal androgen action is one of the aetio- and DNA-binding domains, making it the most naturally mu- logical factors in idiopathic male infertility (Aiman and Griffin, tated transcription factor known. Most mutations are identified 1982). Since most infertile males have normal amounts of serum because they disrupt receptor function totally, causing com- androgen (Niechlag et al., 1979), it is possible that defective plete androgen insensitivity syndrome, wherein a healthy 46XY spermatogenesis could be the result of impaired androgen re- individual is born with female external genitalia (Lim et al., sponsiveness. The cloning of the androgen receptor gene 1997). Mutations that do not completely disrupt androgen re- (Lubahn et al., 1988) and elucidation of its signal transduction ceptor function cause the partial syndrome, in which various pathway (Jenster et al., 1991) have provided new methods for degrees of ambiguous genitalia, including partial labioscrotal investigating the role of androgen receptor mutations in male fusion, hypospadias, bifid scrotum and gynaecomastia, occur infertility. All androgens transmit their multifaceted effects (Yong et al., 1998a). Mutations of the ligand-binding and DNA- through a single receptor: the androgen receptor (AR). The AR, binding domains that cause complete and partial androgen in- a member of the steroid receptor superfamily, is a nuclear tran- sensitivity syndromes have been reviewed comprehensively by scription factor that, when activated by androgens, switches on Quigley et al. (1995) and will not be discussed here. androgen-regulated genes that lead to spermatogenesis and Relatively few mutations have been reported in the trans- prostate gland development (for review see Quigley et al., 1995). activation domain because its large size (it comprises half of the Mutations of the androgen receptor are tolerated because the receptor) and high GC content make genetic screening difficult. receptor is not essential for life and, in males, mutations of The transactivation domain is essential for receptor function the X-linked receptor are expressed, providing a natural ‘knock- and its deletion leads to a non-functional protein in vitro out system’ for studying the phenotypic effects of the gene. The (Jenster et al., 1991). This domain is of particular interest be- AR gene encodes a receptor protein of apparent molecular cause of its causative relationship to a fatal neuromuscular mass 110–114 kDa including 910–919 amino acids. The full- disease, spinal bulbar muscular atrophy (Kennedy’s syn- length AR, like other steroid receptors, is a single polypeptide drome). This review will focus on the novel hypothesis that © 1998 Journals of Reproduction and Fertility 1359-6004/98 $12.50 Downloaded from Bioscientifica.com at 09/28/2021 07:02:11AM via free access 142 E. L. Yong et al. patients have ≥ 40 CAG repeats compared with ≤ 35 in the normal population. The aetiology of trinucleotide expansions 5′ 123 4 5 6 7 8 3′ and the molecular mechanism of pathogenesis of neurological disease in SBMA is not known but is a subject of intense CAG GGC research. 1TAD DBD LBD 919 Patients with SBMA also have evidence of reduced andro- gen receptor function in the form of gynaecomastia, oligo- spermia (reduced sperm production) or azoospermia (no sperm production), testicular atrophy and reduced fertility. Fig. 1. Simplified representation (not to scale) of the structure of the Investigators have also found an inverse relationship between androgen receptor gene and protein. The gene consists of eight polyglutamine length and androgen receptor function by exons that when translated result in a protein with three main do- expressing androgen receptor constructs of different CAG mains: the transactivation domain (TAD), the DNA-binding do- lengths in cell lines and measuring the androgen receptor main (DBD) and the ligand-binding domain (LBD). The TAD makes function by using reporter genes containing androgen response up half the receptor and has two polymorphic regions (orange elements. When androgen receptors with deleted polyglut- boxes) consisting of the trinucleotide repeats, (CAG) and (GGC) . n n amine tracts were assayed for their abilities to activate tran- scription of several different androgen receptor-responsive polymorphisms and mutations affecting the transactivation reporter genes, it was found that elimination of the tract in domain of the androgen receptor can cause subtle disruption both human and rat androgen receptors resulted in increased of receptor function and impairment of human spermato- transcriptional activation activity (Chamberlain et al., 1994). genesis without any associated defect in secondary sexual Progressive expansion of the CAG repeat in the human characteristics. The transactivation domain of the androgen re- androgen receptor caused a linear decrease in transactivation ceptor gene of 191 patients with male infertility due to defec- function but did not completely eliminate androgen receptor tive spermatogenesis and 101 healthy fertile controls was activity. A polyglutamine tract of normal size represses andro- screened for mutations. Genetic screening has revealed triplet gen receptor transactivation function and polyglutamine repeat polymorphisms that increase the risk of male infertility, expansion increases that effect. The relationship between and a point mutation in the transactivation domain that causes polyglutamine tract and androgen receptor transactivation subtle impairment of receptor activity. was inverse and linear from 0 to 50 glutamines (Kazemi- Esfarjani et al., 1995). Triplet repeat polymorphisms in the transactivation domain Short CAG repeats associated with increased risk for prostate cancer Exon 1 of the androgen receptor gene, which encodes the transactivation domain, contains several regions consisting of Besides the pathological expansions of polyglutamine lengths repeats of three nucleotides (Fig. 1). Two of these segments are found in SBMA, alleles of different sizes within the normal polymorphic, varying in size among normal individuals. The polymorphic range were found to be associated with the most interesting of these is the segment consisting of multiple androgen-dependent tumour, prostate cancer. The prevalence copies of the three nucleotides (CAG) that code for glutamine. of short CAG alleles (< 22 repeats) was highest (75%) in The CAG repeat segment contains an average of 21 ± 2 repeats African–American males with the highest risk of prostate with a range of 14–31 in the normal fertile male population (Tut cancer, intermediate (62%) in intermediate-risk non-Hispanic et al., 1997). Further downstream is an invariate stretch of eight whites, and lowest (49%) in Asians at very low risk of prostate (CCG)s, coding for eight proline residues. Closer to the DNA- cancer (Irvine et al., 1994). There was also an association of binding domain is the other polymorphic region consisting of short CAG repeats with increasing severity of disease. Thus, multiple repeats of codon GGC, which codes for the amino
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