53BP1 Supports Immunoglobulin Class Switch Recombination Independently of Its DNA Double-Strand Break End Protection Function

Abstract

Summary Class switch recombination (CSR) is a DNA recombination reaction that diversifies the effector functions of antibodies. CSR occurs via the formation and non-homologous end joining (NHEJ) repair of programmed DNA double-strand breaks (DSBs) at the immunoglobulin heavy chain locus. The DNA repair factors 53BP1 and Rif1 promote NHEJ and CSR by protecting DSBs against resection. However, to what extent repression of DNA end resection contributes to CSR is unknown. Here, we show that B lymphocytes devoid of 53BP1-Rif1-dependent DSB end protection activity undergo robust CSR. Inactivation of specific sets of phospho-sites within 53BP1 N-terminal SQ/TQ motifs abrogates Rif1 recruitment and inhibition of resection but only mildly reduces CSR. Furthermore, mutations within 53BP1 oligomerization domain abolish CSR without substantially affecting DNA end processing. Thus, inhibition of DNA end resection does not correlate with CSR efficiency, indicating that regulation of DSB processing is not a key determinant step in CSR.

A Paradigm Revolution or Just Better Resolution—Will Newly Emerging Superresolution Techniques Identify Chromatin Architecture as a Key Factor in Radiation-Induced DNA Damage and Repair Regulation?

“…Together, these findings might challenge the model, which states that HR can be activated only after unsuccessful NHEJ [55], provided that NHEJ failure does not occur very soon after irradiation. A current study [124] indicates that the activation of NHEJ vs. HR may not be directly dependent on resection. Therefore, the architecture of both DSB lesions and damaged domains may steer repair in a certain direction from the outset.…”

53BP1: a DSB escort

“…First, CSR requires the ability of 53BP1 to form stable oligomeric assemblies at sites of DNA damage. This attribute is dependent on the cooperation of the two LC8 motifs with the OD (Bothmer et al 2011;Lottersberger et al 2013;Becker et al 2018;Sundaravinayagam et al 2019). The LC8 motifs, through their interaction with DYNLL1, promote oligomerization in vitro and enhance the stability of 53BP1 at sites of DNA damage.…”

Uncoupling the DSB End-Protecting and CSR-Promoting Functions of 53BP1

“…One widely accepted role of 53BP1 has been its ability to limit excessive resection of DSBs to facilitate NHEJ for CSR (Bothmer et al, 2011). Sundaravinayagam et al (2019) now provide compelling evidence to challenge this prevalent notion by characterizing 53BP1 mutations that uncouple the end protection and CSR-promoting activities of 53BP1.…”

“…The study by Sundaravinayagam et al (2019) opens up new exciting platforms to elucidate the role of 53BP1 in effecting CSR beyond its commonly accepted role in limiting end resection. Further studies will be needed to ascertain whether B lymphocytes expressing oligomerizationdeficient 53BP1 possess an altered Igh locus architecture, both during resting and activated states, and how these chromatin changes influence CSR outcomes and potentially deleterious chromosomal translocations.…”