Abstract 6067: Characterizing the landscape of rare structural variant events in newly diagnosed multiple myeloma
Authors: Chojnacka Monika , Diamond Benjamin , Ziccheddu Bachisio , Rustad Even , Maclachlan Kylee , Papadimitriou Marios , Boyle Eileen , Blaney Patrick , Usmani Saad , Morgan Gareth , Landgren Ola , Maura Francesco 
Published: April 2023

Background: Structural variants (SV) are known to play a critical role in the pathogenesis of multiple cancer types. Using whole genome sequencing (WGS), we recently characterized the SV landscape of 752 newly diagnosed multiple myeloma (MM) patients, identifying 68 SV hotspots and 152 recurrent copy number aberrations (CNA; Rustad et al. Blood Cancer Discovery 2020). Despite comprehensive annotation, more than half of SVs were not linked to any known MM genomic driver. The biological impact of these SV events, here defined as rare SV, occurring in 93% (702/752) of patients, is unknown.

Methods: To study the biological impact of rare SVs, we interrogated WGS (n=752) and RNAseq (n=591) in the CoMMpass trial. Recurrent SVs identified by involvement in canonical Ig translocations, recurrent CNAs, or SV hotspots were excluded. All SVs within an event must not involve a recurrent region to be defined rare. To determine SV class-specific gene relationships, breakpoint enrichment was compared against a permuted background model for each SV class and gene expression direction, up to 1 Mb. Genes were considered affected if expression was above a gene specific outlier Z-score of +/- 2. Lastly, we modeled breakpoint density to the nearest MM superenhancer up to 10 MB, and compared to permuted background rates.

Results: Of the total 8,942 SVs, 4,959 (55%) were identified as rare. 201 (34%) patients had at least 1 enriched rare SV event associated with gene expression outliers. Amongst over-expressed gene outliers, rare templated insertions and duplications were enriched within the gene body and up to 1 MB away. Rare inversions were enriched in genes 100kb and 1MB away, and rare translocations were associated with outliers 1 MB away. Amongst under-expressed gene outliers, rare complex SVs were enriched within the gene body, while deletion SVs were enriched in the gene body and up to 1 MB away. Rare duplications, translocations and templated insertions were enriched up to 1 MB of superenhancers. Rare templated insertions were significantly enriched against the background model (p &lt; 0.001, Fisher Exact). Overall, 82% (104/126) of gene outliers affected by rare templated insertions were associated with superenhancers, (95 over-expressed, e.g. IRF6 and 9 under-expressed), 54% (130/237) by rare translocations, (105 overexpressed, e.g. FAM46A and 25 under-expressed), and 55% (96/172) by rare duplication events, (93 overexpressed, e.g. MAPK13, and 7 under-expressed). In addition, among the 853 outlier genes affected by enriched rare SVs, at least 15 are involved with B-cell development, suggesting a potential driver role in myeloma pathogenesis.

Conclusion: In summary, leveraging WGS and RNA-seq of clinical samples, we demonstrate that rare SVs are frequently associated with aberrant gene expression, expanding our understanding of their potential role in heterogenous clinical response in patients diagnosed with MM.

Citation Format: Monika Chojnacka, Benjamin Diamond, Bachisio Ziccheddu, Even Rustad, Kylee Maclachlan, Marios Papadimitriou, Eileen Boyle, Patrick Blaney, Saad Usmani, Gareth Morgan, Ola Landgren, Francesco Maura. Characterizing the landscape of rare structural variant events in newly diagnosed multiple myeloma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6067.