Assessing DNA Damage Responses Using B Lymphocyte Cultures

MYSM1 attenuates DNA damage signals triggered by physiologic and genotoxic DNA breaks

BACKGROUND

Patients with deleterious variants in MYSM1 have an immune deficiency characterized by B-cell lymphopenia, hypogammaglobulinemia, and increased radiosensitivity. MYSM1 is a histone deubiquitinase with established activity in regulating gene expression. MYSM1 also localizes to sites of DNA injury but its function in cellular responses to DNA breaks has not been elucidated.

OBJECTIVES

This study sought to determine the activity of MYSM1 in regulating DNA damage responses (DDRs) to DNA double-stranded breaks (DSBs) generated during immunoglobulin receptor gene (Ig) recombination and by ionizing radiation.

METHODS

MYSM1-deficient pre- and non-B cells were used to determine the role of MYSM1 in DSB generation, DSB repair, and termination of DDRs.

RESULTS

Genetic testing in a newborn with abnormal screen for severe combined immune deficiency, T-cell lymphopenia, and near absence of B cells identified a novel splice variant in MYSM1 that results in nearly absent protein expression. Radiosensitivity testing in patient's peripheral blood lymphocytes showed constitutive γH2AX, a marker of DNA damage, in B cells in the absence of irradiation, suggesting a role for MYSM1 in response to DSBs generated during Ig recombination. Suppression of MYSM1 in pre-B cells did not alter generation or repair of Ig DSBs. Rather, loss of MYSM1 resulted in persistent DNA damage foci and prolonged DDR signaling. Loss of MYSM1 also led to protracted DDRs in U2OS cells with irradiation induced DSBs.

CONCLUSIONS

MYSM1 regulates termination of DNA damage responses but does not function in DNA break generation and repair.

Nuclease-independent functions of RAG1 direct distinct DNA damage responses in B cells

Developing B cells generate DNA double-stranded breaks (DSBs) to assemble immunoglobulin receptor (Ig) genes necessary for the expression of a mature B cell receptor. These physiologic DSBs are made by the RAG endonuclease, which is comprised of the RAG1 and RAG2 proteins. In pre-B cells, RAG-mediated DSBs activate the ATM kinase to coordinate canonical and non-canonical DNA damage responses (DDR) that trigger DSB repair and B cell developmental signals, respectively. Whether this broad cellular response is distinctive to RAG DSBs is poorly understood. To delineate the factors that direct DDR signaling in B cells, we express a tetracycline-inducible Cas9 nuclease in Rag1-deficient pre-B cells. Both RAG- and Cas9-mediated DSBs at Ig genes activate canonical DDR. In contrast, RAG DSBs, but not Cas9 DSBs, induce the non-canonical DDR-dependent developmental program. This unique response to RAG DSBs is, in part, regulated by non-core regions of RAG1. Thus, B cells trigger distinct cellular responses to RAG DSBs through unique properties of the RAG endonuclease that promotes activation of B cell developmental programs.