Alternative titles; symbolsGAP-ASSOCIATED TYROSINE PHOSPHOPROTEIN, 62-KDSRC-ASSOCIATED PROTEIN IN MITOSIS, 68-KD; SAM68HGNC Approved Gene Symbol: KHDRBS1Cytogene...
Alternative titles; symbols
HGNC Approved Gene Symbol: KHDRBS1
Cytogenetic location: 1p35.2 Genomic coordinates (GRCh38): 1:32,013,867-32,060,849 (from NCBI)
KHDRBS1 belongs to the evolutionarily conserved signal transduction activator of RNA (STAR) family of RNA-binding proteins. These proteins play key roles during cell differentiation and development (summary by Bianchi et al., 2010).
▼ Cloning and Expression
SAM68 is a tyrosine-phosphorylated, SRC (190090)-associated protein in mitotic cells. Wong et al. (1992) purified the p62/SAM68 protein based on its binding to GTPase-activating protein (GAP; 139150). Subsequent cloning of the corresponding cDNA from a human placenta library revealed that the p62 gene encodes a 443-amino acid polypeptide with homology to a putative ribonuclear protein, GRP33.
Barlat et al. (1997) stated that SAM68 contains functional KH and RGG domains, both characteristic of RNA-binding proteins. Using RT-PCR from placenta RNA, Barlat et al. (1997) isolated and cloned an alternatively spliced variant of SAM68, termed SAM68(delta)KH, which lacks 39 amino acids of the KH domain. RT-PCR analysis revealed that both SAM68 and its alternatively spliced variant were expressed in brain, heart, kidney, lung, skeletal muscle, and liver, with intensified expression of SAM68(delta)KH in skeletal muscle and liver, and lower expression of SAM68 in brain, skeletal muscle, and liver.
Lee and Burr (1999) stated that the 443-amino acid SAM68 protein contains 3 N-terminal proline-rich sequences, followed by a KH domain embedded within a STAR domain, 3 more proline-rich sequences, a C-terminal tyrosine-rich region, which contains a domain similar to the SRGY domain of RNA-binding proteins, and a putative nuclear localization signal. SAM68 has a calculated molecular mass of 48.2 kD, but it migrated as a 68-kD protein on SDS-PAGE gels. Northern blot analysis detected a 3.1-kb SAM68 transcript expressed at moderate levels in heart, brain, placenta, skeletal muscle, kidney, and pancreas, and at lower levels in lung and liver.
By genomic sequence analysis, Lee and Burr (1999) mapped the SAM68 gene to chromosome 1p32.
Stumpf (2020) mapped the KHDRBS1 gene to chromosome 1p35.2 based on an alignment of the KHDRBS1 sequence (GenBank BC010132.2) with the genomic sequence (GRCh38).
▼ Gene Function
Expression studies by Wong et al. (1992) showed that p62/SAM68 bound to DNA and mRNA. The p62/SAM68 gene was phosphorylated when expressed in the presence of v-src, and only the phosphorylated form bound to GAP.
Barlat et al. (1997) found that the SAM68(delta)KH variant was expressed at growth arrest upon confluency in normal cells. Transfected SAM68(delta)KH inhibited serum-induced DNA synthesis. They suggested that SAM68 may have a role in cell cycle control, particularly at the G1/S transition.
Lee and Burr (1999) found that SALP (KHDRBS3; 610421) downregulated expression of SAM68 following coexpression in chicken embryo fibroblasts.
Matter et al. (2002) showed that SAM68 is an extracellular signal-regulated kinase (ERK; 600997) target. SAM68 binds exonic splice-regulatory elements of an alternatively spliced exon that is physiologically regulated by the RAS signaling pathway, namely exon v5 of CD44 (107269). Forced expression of SAM68 enhanced ERK-mediated inclusion of the v5-exon sequence in mRNA. This enhancement was impaired by mutation of ERK-phosphorylation sites in SAM68, whereas ERK phosphorylation of SAM68 stimulated splicing of the v5 exon in vitro. Finally, Matter et al. (2002) showed that RAS-pathway-induced alternative splicing of the endogenous CD44-v5 exon was abolished by suppression of SAM68 expression. Matter et al. (2002) concluded that their data define SAM68 as a prototype regulator of alternative splicing whose function depends on protein modification in response to extracellular cues.
Cote et al. (2003) found that SAM68 was methylated in vivo by protein arginine N-methyltransferase-1 (PRMT1; 602950). They identified asymmetric dimethylarginines near proline motif-3 of SAM68, and deletion of these methylation sites and the use of methylase inhibitors resulted in accumulation of SAM68 in the cytoplasm. SAM68 was also detected in the cytoplasm of Prmt1-deficient mouse embryonic stem cells. Cote et al. (2003) noted that SAM68 can export unspliced human immunodeficiency virus (HIV) RNAs, and they found that treatment of cells with methylase inhibitors prevented the ability of SAM68 to export unspliced HIV RNAs from transfected COS-7 cells.
▼ Molecular Genetics
Associations Pending Confirmation
In a Chinese mother and daughter with premature ovarian failure (POF; see 311360), Wang et al. (2017) performed whole-exome sequencing and identified a heterozygous missense mutation (c.490A-G, M154V, NM_006559) in the KHDRBS1 gene that was not found in an unaffected sister or in the father. Analysis of the KHDRBS1 gene in 215 patients with POF revealed 1 woman who was heterozygous for a P88L (c.2636C-T) mutation in KHDRBS1. Neither mutation was found in 400 healthy control women or in the 1000 Genomes Project, ExAC, or gnomAD databases. Western blot analysis showed M154V expression levels similar to those of wildtype KHDRBS1. However, transcriptome analysis after overexpression in KGN cells revealed 66 genes that were expressed significantly differently in mutant cells compared to wildtype cells. Analysis of alternative splicing events showed a total of 237 such events in 145 genes that were significantly different between wildtype- and M154V-expressing cells, and gene ontology analysis showed that alternatively spliced genes related to DNA replication and repair were more prevalent in M154V-expressing cells. Wang et al. (2017) concluded that differential expression and alternative splicing of genes induced by the M154V variant might explain the underlying mechanism of KHDRBS1-associated POF.
Carlosama et al. (2018) reanalyzed exome data from a woman with POF (Pt-23), who had previously been studied by Patino et al. (2017) and found to be heterozygous for an R22W missense mutation in the FGFR2 gene (176943), and identified a P296L variant in the KHDRBS1 gene. Carlosama et al. (2018) suggested that these findings supported the polygenic nature of POF.
▼ Animal Model
Bianchi et al. (2010) stated that 20 to 30% of Sam68 -/- mice die perinatally and that surviving animals show a mild defect in motor coordination and male infertility due to compromised spermatogenesis. However, Sam68 -/- animals are spared age-related bone loss. Bianchi et al. (2010) found that Sam68 -/- females were subfertile and bore a dramatically reduced number of pups during their life span. Subfertility was caused by delayed sexual maturity and permanent impairment of the estrous cycle due to defective development of follicles. Crosslinking and immunoprecipitation experiments showed that Sam68 directly bound Fshr (136435) and Lhcgr (152790) mRNAs, which were downregulated in ovaries of Sam68 -/- females. Gonadotropin stimulation of immature females or follicular cells upregulated Sam68 expression parallel to that of Fshr and Lhcgr mRNAs, and upregulation of Sam68 was required to allow accumulation of Fshr and Lhcgr mRNAs in follicular cells. Bianchi et al. (2010) concluded that SAM68 is required in ovarian follicles to insure proper follicular hormonal response and ovulation.