Clinically Significant Aberrant HAS1Vb Splicing of the Hyaluronan Synthase 1 Gene (HAS1) Requires a Combination of Mutations and Deletions in Introns 3 and 4 of HAS1 Minigene,
Authors: Kriangkum Jitra , Belch Andrew R , Pilarski Linda M. 
Published: October 2019

Abstract
Abstract 3931
Clinically important aberrant splicing of the hyaluronan synthase 1 gene (HAS1) occurs in malignant B cells from multiple myeloma (MM) and Waldenstrom macroglobulimenia (WM) patients but is undetectable in B cells from healthy donors (HD)1. HAS1 splice variants are generated by aberrant alternative splicing (AS) within exons 3, 4 and 5, involving exon 4 skipping (Va), partial intron 4 retention (Vc, Vd) or a combination of both (Vb). Aberrant splicing of HAS1Vb has previously shown to correlate with reduced survival in a cohort of MM patients1. In patients, frequent mutations have been identified in introns 3 and 4, suggesting their roles in AS1. To identify genetic regions involved in splice site selection leading to HAS1Vb expression, we performed in vitro sequence manipulation of introns 3 and 4. We established a mammalian expression system to analyse HAS1 splicing by fusing a minigene extending from exon 3 to exon 5 with the upstream cDNA sequence. In an unaltered sequence of the HAS1 minigene, splicing of HAS1 full length predominated, with trace amount of variants, including the novel HAS1Vd, identified for the first time in transfectants. HAS1Vd shared an alternative acceptor site with HAS1Vb (retained 59 bp of downstream intron 4) but unlike HAS1Vb, retained exon 4. Analysis of peripheral blood mononuclear cells (PBMC) revealed that 9% of healthy donors (n=102) and MM patients (n=93) expressed HAS1Vd. About 2% coexpressed HAS1Vb and Vd. In this cohort of patients, HAS1Vb was found in 20% of unfractionated MM PBMC compared to 5% in HD PBMC, supporting our previous finding that HAS1Vb was restricted to sorted MM B cells1and suggesting that in HD PBMC, non-B cells are responsible for this splicing. While MM PBMCs expressed HAS1Vb>Vd, HD PBMCs and transfectants expressed HAS1Vd>Vb, indicating that splicing directed by the minigene construct was substantively different from that occurring in MM patients. In transfectants, partial deletion of HAS1 intron 4 increased HAS1Vd expression but did not affect HAS1Vb despite the fact that both variants use the same 3' splice site in intron 4. Proper joining of exons 3, 4 and 5 required a minimum of 172 bp of downstream intron 4 and acceptor site selection may be regulated by sequence between 172 and 84 bp upstream of exon 5. Thus, changes in intron 4 alone were insufficient to promote the splicing pattern observed in patients (i.e., elevated HAS1Vb). We then employed site directed mutagenesis to alter multiple G-rich regions in downstream intron 3, a strategy which enhanced exon 4 skipping as shown by increased HAS1Va expression, but did not lead to HAS1Vb splicing. Minimal manipulation to promote exon 4 skipping included base changes within the 2 tandem G-repeats (8 bp) located 73 bp upstream of exon 4, where splicing enhancer/silencer sequences were also identified. Thus, changes in intron 3 also affected the HAS1 splicing profile but like deletions in intron 4, were insufficient on their own to promote HAS1Vb expression. We next developed expression constructs that combined deletion in intron 4 with mutations in intron 3. We are able to demonstrate that when both introns were co-modified, the splicing pattern shifted towards increased HAS1Vb expression, similar to that observed in malignant cells from MM patients. Our previous work showed that deletions or mutations in the 3' end of intron 4 are frequent in MM; in silico analysis predicts splicing to form HAS1Vb2. To determine whether intron 3 genetic changes occur in patients, we sequenced intron 3 from genomic DNA of 50 MM PBMC. In 22/50 patients, 18 recurrent mutations unique to MM were identified in intron 3; many were also recurrent in our previous study2. Individual mutations recurred in 2–7 patients. Among these, 17/18 recurrent mutations increased the G-C content of intron 3 and 6/18 created or disrupted G runs in intron 3. This supports the idea that in MM patients, cumulative variations in introns 3 and 4 alter splice site selection, operationally resulting in loss of HAS1Vd and enhanced expression of the clinically relevant HAS1Vb variant. We speculate that individuals who accumulate genetic variations in introns 3 and 4 of HAS1 are predisposed to aberrant splicing of HAS1 which may contribute to development of malignancy in MM and WM.

Disclosures:
Belch: Celgene: Research Funding; Onyx: Research Funding.