A double nucleotide insertion-induced frame-shift mutation of the androgen receptor gene in a familial complete androgen insensitivity syndrome

Small Indels in the Androgen Receptor Gene: Phenotype Implications and Mechanisms of Mutagenesis

CONTEXT

Despite high abundance of small indels in human genomes, their precise roles and underlying mechanisms of mutagenesis in Mendelian disorders require further investigation.

OBJECTIVE

To profile the distribution, functional implications, and mechanisms of small indels in the androgen receptor (AR) gene in individuals with androgen insensitivity syndrome (AIS).

METHODS

We conducted a systematic review of previously reported indels within the coding region of the AR gene, including 3 novel indels. Distribution throughout the AR coding region was examined and compared with genomic population data. Additionally, we assessed their impact on the AIS phenotype and investigated potential mechanisms driving their occurrence.

RESULTS

A total of 82 indels in AIS were included. Notably, all frameshift indels exhibited complete AIS. The distribution of indels across the AR gene showed a predominance in the N-terminal domain, most leading to frameshift mutations. Small deletions accounted for 59.7%. Most indels occurred in nonrepetitive sequences, with 15.8% situated within triplet regions. Gene burden analysis demonstrated significant enrichment of frameshift indels in AIS compared with controls (P < .00001), and deletions were overrepresented in AIS (P < .00001).

CONCLUSION

Our findings underscore a robust genotype-phenotype relationship regarding small indels in the AR gene in AIS, with a vast majority presenting complete AIS. Triplet regions and homopolymeric runs emerged as prone loci for small indels within the AR. Most were frameshift indels, with polymerase slippage potentially explaining half of AR indel occurrences. Complex frameshift indels exhibited association with palindromic runs. These discoveries advance understanding of the genetic basis of AIS and shed light on potential mechanisms underlying pathogenic small indel events.

Complete Androgen Insensitivity Syndrome due to Mutations in the DNA-Binding Domain of the Human Androgen Receptor Gene

Androgen insensitivity syndrome (AIS) is an X-linked recessive disorder with a 46,XY karyotype caused by alterations in the androgen receptor (AR) gene. We have identified 2 mutations in the AR gene that resulted in complete androgen insensitivity syndrome (CAIS) in 2 unrelated cases. This study includes cytogenetics, hormonal, molecular, and bioinformatics analysis including sequencing of the SRY (sex-determining region Y) and AR genes. Mutational analysis in the first case of primary amenorrhea revealed a novel nucleotide substitution (IVS2-2A>G) in the second intron of the AR gene. The mutation is located in the acceptor splice site (2 nucleotides before exon 3) and caused skipping of exon 3 and formation of an abnormal protein. The second mutation (g. 98762_98764delTCT) was identified in a case of oligoamenorrhea and caused the deletion of 1 amino acid (p.∆Phe583). Both identified mutations were located in the conserved P-box region of the DNA-binding domain which is responsible for base-specific contacts with the DNA major groove. Furthermore, a hormonal imbalance was also noticed in both cases with high levels of gonadotropins like FSH and LH in both cases. The present study concluded that both identified AR mutations are predicted to either abolish or decrease the binding ability of the AR to androgen response elements of its downstream genes.

Evidence that lack of ligand-binding domain correlates with nuclear distribution of unliganded human androgen receptor and loss of transactivation activity

Androgen receptor (AR) is one of the large superfamily of nuclear hormone receptors. AR consists of distinct domains including an N-terminal DNA-binding domain and a C-terminal ligand-binding domain (LBD). Regulation of AR nuclear import and subsequent transactivation activity represent essential steps in androgen action. Mutations in the AR gene are known to cause different degrees of androgen insensitivity syndrome (AIS). This study aimed to identify the possible contribution of LBD of AR to cellular distribution, ligand binding, and transactivation activities using mutant AR clone lacking the entire LBD that we previously observed in an AIS patient. Subcellular distribution was assessed by green fluorescence protein-tagged vector and transcriptional activity was analyzed by luciferase assay. Wild-type AR had ligand-dependent transcriptional activation and nuclear import activities. On the other hand, mutant AR had no transcriptional activity regardless of the presence of ligand, 5-α-dihydroxytestosterone (DHT). These mutants were presented predominantly in the nucleus even without DHT. The observation of no transactivation in the mutant receptor must be due to the loss of complex formation between androgen and AR protein. The C-terminal domain has the critical role in the cellular localization and transactivation as well as on the ligand binding.