Characterisation of Genetic Variation in ST8SIA2 and Its Interaction Region in NCAM1 in Patients with Bipolar Disorder

Alpha-2,8-sialyltransferase 2 (ST8SIA2) is an enzyme responsible for the transfer of polysialic acid (PSA) to glycoproteins, principally the neuronal cell adhesion molecule (NCAM1), and is involved in neuronal plasticity. Variants within ST8SIA2 have previously shown association with bipolar disorder, schizophrenia and autism. In addition, altered PSA-NCAM expression in brains of patients with schizophrenia or bipolar disorder indicates a functional dysregulation of glycosylation in mental illness. To explore the role of sequence variation affecting PSA-NCAM formation, we conducted a targeted re-sequencing study of a ∼100 kb region – including the entire ST8SIA2 gene and its region of interaction with NCAM1 – in 48 Caucasian cases with bipolar disorder using the Roche 454 platform. We identified over 400 DNA variants, including 47 putative novel variants not described in dbSNP. Validation of a subset of variants via Sequenom showed high reliability of Roche 454 genotype calls (97% genotype concordance, with 80% of novel variants independently verified). We did not observe major loss-of-function mutations that would affect PSA-NCAM formation, either by ablating ST8SIA2 function or by affecting the ability of NCAM1 to be glycosylated. However, we identified 13 SNPs in the UTRs of ST8SIA2, a synonymous coding SNP in exon 5 (rs2305561, P207P) and many additional non-coding variants that may influence splicing or regulation of ST8SIA2 expression. We calculated nucleotide diversity within ST8SIA2 on specific haplotypes, finding that the diversity on the specific “risk” and “protective” haplotypes was lower than other non-disease-associated haplotypes, suggesting that putative functional variation may have arisen on a spectrum of haplotypes. We have identified common and novel variants (rs11074064, rs722645, 15∶92961050) that exist on a spectrum of haplotypes, yet are plausible candidates for conferring the effect of risk and protective haplotypes via multiple enhancer elements. A Galaxy workflow/pipeline for sequence analysis used herein is available at: https://main.g2.bx.psu.edu/u/a-shaw-neura/p/next-generation-resources.

α-2,8-唾液酸转移酶2（Alpha-2,8-sialyltransferase 2, ST8SIA2）是一种负责将多唾液酸（polysialic acid, PSA）转移至糖蛋白的酶，其主要底物为神经元细胞黏附分子1（neuronal cell adhesion molecule 1, NCAM1），参与神经元可塑性调控。既往研究表明，ST8SIA2的基因变异与双相情感障碍、精神分裂症及孤独症存在关联。此外，精神分裂症或双相情感障碍患者大脑中PSA-NCAM的表达异常，提示精神疾病中存在糖基化功能失调。为探究影响PSA-NCAM形成的序列变异的作用，本研究针对约100 kb的基因组区域（包含完整的ST8SIA2基因及其与NCAM1的互作区域），采用罗氏454（Roche 454）测序平台，对48例高加索裔双相情感障碍患者开展靶向重测序研究。本研究共鉴定出400余个DNA变异，其中包括47个未在dbSNP数据库中报道的潜在新型变异。通过Sequenom平台对部分变异进行验证，结果显示罗氏454的基因型分型结果具有较高可靠性：基因型一致性达97%，且80%的新型变异得到独立验证。本研究未发现会影响ST8SIA2功能或NCAM1糖基化能力、进而干扰PSA-NCAM形成的重大功能丧失型突变。但我们在ST8SIA2的非翻译区（untranslated region, UTR）鉴定出13个单核苷酸多态性（single nucleotide polymorphism, SNP），以及外显子5中的1个同义编码SNP（rs2305561, P207P），还有诸多可能影响ST8SIA2剪接或表达调控的其他非编码变异。我们针对特定单倍型计算了ST8SIA2基因区域的核苷酸多样性，发现与其他非疾病相关单倍型相比，特定“风险”及“保护”单倍型的多样性更低，提示潜在功能性变异可能起源于一系列单倍型谱系。我们鉴定出了存在于多种单倍型谱系中的常见变异及新型变异（rs11074064、rs722645、15∶92961050），这些变异可能通过多种增强子元件介导风险及保护单倍型的效应，是颇具说服力的候选变异。本研究使用的序列分析Galaxy工作流/管线可通过以下网址获取：https://main.g2.bx.psu.edu/u/a-shaw-neura/p/next-generation-resources。