The Histone Methytransferase MMSET Regulates Class-Switch Recombination
Authors: Wu Xiaosheng , Pei Huadong , Liu Tongzheng , Yu Kefei , Jelinek Diane F. , Lou Zhenkun 
Published: October 2019

Abstract
Abstract 691
Malignant plasma cells in multiple myeloma (MM) patients display a variety of recurrent genetic abnormalities. In this regard, tumor cells in approximately 15% of all MM patients will exhibit a translocation involving the immunoglobulin (Ig) heavy chain locus at 14q32 and the short arm of chromosome 4. The breakpoint on chromosome 4 (4p16) frequently results in overexpression of FGFR3 and/or full-length or truncated versions of the multiple myeloma SET domain protein, MMSET. MM patients with t(4;14) translocations are considered to have high-risk disease. MMSET has been shown to have histone methyltransferase activity and we have recently shown that this protein plays a pivotal role in DNA repair and maintenance of genetic stability (1). Thus, dysregulation of MMSET may result in aberrant responses to DNA damage, which may be related to the poor prognosis of MM patients with t(4;14) translocations. The MMSET gene is also known as the Wolf-Hirschhorn syndrome candidate (WHSC1) gene. Expression of the WHSC1 gene is uniformly misregulated due to haploinsufficiency in patients with Wolf-Hirschhorn syndrome (WHS) resulting in characteristic facial features and developmental disorders. Of great interest, WHS patients also display significant antibody deficiencies and IgG and IgA deficiencies are particularly frequent. Currently, the underlying cause of antibody deficiencies in WHS patients remains unknown. However, our recent studies have shown that a robust DNA repair process in germinal center B cells is required for fertile antibody maturation processes (2). This observation, taken together with our recent discover that MMSET regulates the recruitment of 53BP1 to sites of DNA damage through its histone methyltransferase activity during DNA damage repair (1), suggested to us the hypothesis that MMSET may also be critically involved in Ig gene maturation, particularly as it concerns class switch recombination, a process known to result from double strand DNA breaks and subsequent effective DNA repair. By using shRNA knockdown technology in murine lymphoma cell line CH12F3 cells, which can be specifically induced to switch from IgM to IgA expression ex vivo by CD40 ligand stimulation in the presence of IL-4 and TGFb, we clearly demonstrate that downregulation of MMSET expression by shRNA significantly impaired class switch recombination from IgM to IgA. While it plays no detectable roles in cell viability, proliferation, or apoptosis, we found that MMSET is important for histone methylation at H3K36 and H4K20 sites of the Igh loci, which in turn modulate the recruitment of 53BP1 to the Igh loci as well as the transcription of the Igh switch regions, leading to defective class switch recombination. Further DNA sequence analysis of post-switched Sm-Sa junctions from MMSET compromised cells showed a significant increase in microhomology suggesting that homologous recombination (HR) repair is alternatively used as a compensatory mechanism during DNA repair of the switch region when MMSET is absent. Our results suggest that defective CSR caused by MMSET deficiency may underpin the antibody deficiency phenotype in WHS patients. Furthermore, our results showing that MMSET expression dose dictates the usage choice between two competing DNA repair pathways, i.e., error-prone non-homologous end joining (NHEJ) and error-free HR, may also suggest that MMSET overexpression in MM may favor usage of the NHEJ pathway therefore leading to more error-prone DNA repair and possibly additional genetic damage.

Disclosures:
No relevant conflicts of interest to declare.