SLC44A2 single nucleotide polymorphisms, isoforms, and expression: Association with severity of Meniere's disease?

SLC44A2 was discovered as the target of an antibody that causes hearing loss. Knockout mice develop age related hearing loss, loss of sensory cells and spiral ganglion neurons. SLC44A2 has polymorphic sites implicated in human disease. Transfusion related acute lung injury (TRALI) is linked to rs2288904 and genome wide association studies link rs2288904 and rs9797861 to venous thromboembolism (VTE), coronary artery disease and stroke. Here we report linkage disequilibrium of rs2288904 with rs3087969 and the association of these SLC44A2 SNPs with Meniere's disease severity. Tissue-specific isoform expression differences suggest that the N-terminal domain is linked to different functions in different cell types. Heterozygosity at rs2288904 CGA/CAA and rs3087969 GAT/GAC showed a trend for association with intractable Meniere's disease compared to less severe disease and to controls. The association of SLC44A2 SNPs with VTE suggests that thrombi affecting cochlear vessels could be a factor in Meniere's disease.

Age-related changes in expression of CTL2/SLC44A2 and its isoforms in the mouse inner ear.

The membrane glycoprotein CTL2/SLC44A2 is expressed by supporting cells in the inner ear and has been identified as a target of antibodies that may induce auto-immune hearing loss. To determine if CTL2/SLC44A2 also has roles in inner ear development and to distinguish between isoform-specific roles, we assessed age-related changes in expression of CTL2/SLC44A2 isoforms and protein in the developing murine inner ear. We determined that both isoform p1 and isoform p2 (named for the upstream p1 and proximal p2 promoters that control alternate exons 1a and 1b) were robustly expressed as early as E14 and persisted during embryonic development, but after birth the p1 isoform fell to barely detectable levels while isoform p2 levels were maintained. This trend continued and became even more apparent later in post-natal development and remained in mature ears until at least 6 weeks of age. In aged (18 mo old) mice, the level of isoform p1 transcripts rose again to levels similar to the p2 isoform like that seen early in development. At the earliest stage examined, CTL2/SLC44A2 protein was expressed in both immature supporting cells and immature sensory cells, but after birth expression in the sensory cells declined in both the utricle and cochlea and by day P1 expression of CTL2/SLC44A2 was restricted to supporting cells. The changes we observed in isoform distribution are indicative of differential developmental roles and age related changes between the two isoforms of CTL2/SLC44A2 in the inner ear.

Autoantibodies to recombinant human CTL2 in autoimmune hearing loss.

OBJECTIVES/HYPOTHESIS: Choline transporter-like protein 2 (CTL2), a 68-72 kDa inner-ear membrane glycoprotein, is a candidate target antigen in autoimmune hearing loss (AIHL). The objective of this study was to test recombinant human CTL2 as a potential target for the detection of human autoantibodies in patients with AIHL. STUDY DESIGN: In vitro assay development. METHODS: Human inner ear CTL2 mRNA was cloned into baculovirus and used to infect insect cells. Immunofluorescence and western blotting were used to determine optimal expression of recombinant human CTL2 (rHuCTL2) in insect cells. AIHL patient sera of known reactivity with guinea pig inner ear were tested for antibodies to purified rHuCTL2 on western blots. Sera from normal hearing donors were used as controls. RESULTS: The rHuCTL2 protein migrated as three bands: a core protein of 62 kDa and two N-glycosylated bands at 66 and 70 kDa. Sera from 6/12 (50%) of AIHL patients with antibody to the 68-72 kDa inner-ear protein or to supporting cells also have antibody to rHuCTL2. Four of the four patients with antibody to rHuCTL2 responded to corticosteroids, whereas 4/8 that lacked antibody to rHuCTL2 did not. Among normal human sera, 80% were negative; binding was barely detectable in 3/15 (20%). CONCLUSIONS: The rHuCTL2 protein can be produced efficiently and used as a substrate for testing human sera. Antibodies to rHuCTL2 were detected in 50% of inner-ear-reactive AIHL sera. Additionally, circulating antibody to rHuCTL2 is with associated response to corticosteroids in some AIHL patients.

Galanin receptor subtype 2 suppresses cell proliferation and induces apoptosis in p53 mutant head and neck cancer cells.

PURPOSE: Galanin and its three receptors (GALR1-3) are expressed in many normal tissues, but silenced in some tumors. Contradictory roles for galanin and its receptors in various tumors have been reported. To understand their function, investigations of individual galanin receptors are necessary. In head and neck squamous carcinoma cells (HNSCC) with silenced GALR1 and GALR2, we showed that reexpressed GALR1 suppresses tumor cell proliferation via Erk1/2-mediated effects on cdk inhibitors and cyclin D1. Others showed that GALR2 could induce apoptosis in neuroblastoma cells with wild-type p53, whereas GALR2 stimulated proliferation in small cell lung cancer. In this study, we investigated the role of GALR2 in HNSCC cells that have mutant p53 and do not express GALR1. EXPERIMENTAL DESIGN: UM-SCC-1, a human oral carcinoma cell line with a splice site mutation causing a 46-bp p53 off-frame deletion, was stably transfected to express GALR2 (UM-SCC-1-GALR2). RESULTS: Galanin treatment of UM-SCC-1-GALR2 caused morphologic changes and a marked decrease in cell number that were not observed in UM-SCC-1-mock cells. Galanin and GALR2 resulted in decreased bromodeoxyuridine incorporation, p27(Kip1) and p57(Kip2) up-regulation, and decreased cyclin D1 expression. These effects were similar to GALR1 signaling in HNSCC, but GALR2 also induced caspase-3-dependent apoptosis, which was confirmed by Annexin-V staining and DNA fragmentation analysis. These were not observed with GALR1. CONCLUSION: This study shows that GALR2 reexpression can inhibit cell proliferation and induce apoptosis in HNSCC cells with mutant p53. GALR2 may be a feasible target for HNSCC therapy.

Identification and characterization of choline transporter-like protein 2, an inner ear glycoprotein of 68 and 72 kDa that is the target of antibody-induced hearing loss.

The Kresge Hearing Research Institute-3 (KHRI-3) antibody binds to a guinea pig inner ear supporting cell antigen (IESCA) and causes hearing loss. To gain insight into the mechanism of antibody-induced hearing loss, we used antibody immunoaffinity purification to isolate the IESCA, which was then sequenced by mass spectroscopy, revealing 10 guinea pig peptides identical to sequences in human choline transporter-like protein 2 (CTL2). Full-length CTL2 cDNA sequenced from guinea pig inner ear has 85.9% identity with the human cDNA. Consistent with its expression on the surface of supporting cells in the inner ear, CTL2 contains 10 predicted membrane-spanning regions with multiple N-glycosylation sites. The 68 and 72 kDa molecular forms of inner ear CTL2 are distinguished by sialic acid modification of the carbohydrate. The KHRI-3 antibody binds to an N-linked carbohydrate on CTL2 and presumably damages the organ of Corti by blocking the transporter function of this molecule. CTL2 mRNA and protein are abundantly expressed in human inner ear. Sera from patients with autoimmune hearing loss bind to guinea pig inner ear with the same pattern as CTL2 antibodies. Thus, CTL2 is a possible target of autoimmune hearing loss in humans.