Genome Analysis of a Mantle Cell Lymphoma Case with t(5;18)(p15;q21) Lacking the Translocation Involving Cyclin D1

Background: Mantle cell lymphoma (MCL) is a moderately aggressive lymphoma and its pathogenesis is not fully understood. The genetic hallmark of MCL is the chromosomal translocation t(11;14)(q13;q32), causing upregulation of cyclin D1. However, approximately 10% of MCL does not harbor this typical alteration. Besides, the chromosome 5p15 region is recurrently targeted by chromosomal translocations in B-cell malignancies including MCL. The pathogenesis of the MCL cases as described above were not clear, and next generation sequencing analysis would be some help for understanding them. We experienced a MCL case which lacks t(11;14) and has a translocation involving 5p15 locus, and performed whole genome sequencing (WGS) analysis of this patient to elucidate its pathological mechanisms.

Case: A 62 years old woman with long history of MCL. The karyotype of this patient was 47, XX, +5, t(5;18)(p15;q21), and did not have t(11;14). The pathological findings of resected spleen and bone marrow biopsy showed medium size B cell proliferation with overexpressed cyclin D1 and SOX11, and the diagnosis as MCL was re-confirmed.

Methods and Results: We confirmed a chromosome translocation t(5;18) by WGS. Its breakpoint was localized in the intergenic region, so this translocation did not create a fusion gene. This region on chromosome 5 contains the telomerase reverse transcriptase (TERT) and cleft lip and palate transmembrane protein 1-like (CLPTM1L) genes, and WGS analysis also identified SET-binding protein 1 (SETBP1) as the gene located near the breakpoint on chromosome 18. The expression of TERT and SETBP1 of this case was assessed by RT-PCR, and found that both genes were significantly increased compared to EBV-immortalized human B-lymphocyte cell lines. In addition, WGS analysis found the BRAF V600E mutation, which is quite rare among hematologic malignancies excluding hairy cell leukemias. Next, we investigated the role of these aberrations in the overexpression of cyclin D1. In this study, we used some non-hematologic cancer cell lines for transfection assays. The forced expression of TERT did not show any effect on the cyclin D1 expression in several cancer cell lines that we examined. However, BRAF wild type increased the cyclin D1 level in the breast cancer cell lines, MCF-7 and MDA-MB-231. Moreover, the BRAF V600E transfectant showed more upregulated level of cyclin D1 compared to BRAF wild type. These data suggested the possibility of the association of BRAF in the regulation of cyclin D1 expression.

Conclusion: Our data suggested that the genes located around the breakpoint would not be related to the regulation of cyclin D1 expression in the MCL case with t(5;18), and that BRAF might contribute to the upregulation of cyclin D1.