Defining critical roles for RNA helicases in B cells undergoing antibody gene diversification
B cells are crucial to immune defence by producing large quantities of antibodies with pathogen neutralizing capacities. Antibody responses can be made to target a limitless range of antigens with exquisite specificity and affinity, while eliciting different effector functions. This remarkable ability of the adaptive immune system relies on DNA mutation and recombination occurring at immunoglobulin (Ig) loci, that encode for antibody heavy and light chains.
Antibody gene diversification occurs in a step-wise manner during B cell development and differentiation into antibody secreting plasma cells. Early stages of B cell development in the bone marrow are characterized by V(D)J recombination at Ig loci, resulting in the generation of a vast repertoire of antibodies with different antigen specificities. Once a mature B cell encounters its cognate antigen, antibody responses can be further refined through processes known as class switch recombination (CSR) and somatic hypermutation (SHM). As a result, mature B cells differentiating into plasma cells change antibody isotype (or class, which determines different effector functions) and increase antibody affinity towards antigen.
RNA helicases control every step of mRNA metabolism and also play important roles in determining non-coding RNA function. Therefore, these RBPs are ideally suited to coordinate developmental stage transitions and the induction of DNA mutation/recombination at Ig loci, while controlling cell proliferation and the maintenance of genome stability. Our studies on the role of DDX1 in CSR constitute a paradigm of how RNA helicase activity controls non-coding RNA function through modulation of RNA secondary structure. We showed DDX1 is required for sequence-specific targeting of AID to the IgH locus during CSR.
CSR is directed to a particular CH exon by cytokine-induced, non-coding RNA transcription initiated upstream of that exon. These long non-coding RNAs are termed germline transcripts (GLTs) or switch RNA. Transcription of switch RNA promotes the formation of RNA:DNA hybrid structures (or R-loops) behind the elongating RNA polymerase II. G-quadruplex (G4) structures present in switch RNA also target AID to the IgH locus. DDX1 binds to G4 RNA and utilizes ATP to convert them into RNA:DNA-hybrids over complementary S-region DNA. DDX1 depletion in B cells results in decreased AID targeting to S-regions and impaired CSR. Notably R-loop levels over S-regions are diminished by chemical stabilization of G4 RNA or by the expression of a DDX1 ATPase deficient mutant that acts as a dominant-negative protein to reduce CSR efficiency
(Ribeiro de Almeida et al, Mol. Cell 2018).
Our research is focused on understanding how DDX1 and other RNA helicases control the molecular and cellular changes underpinning antibody gene diversification in B cells. Age-related defects in humoral immunity often result from the inability to generate a diverse repertoire of antibodies, and pathological outcomes associated with antibody gene diversification, such as autoimmunity and B cell lymphoma, show increased prevalence in older individuals. In the future, we envisage RNA helicases to constitute ideal therapeutic targets to modulate B cell function in the elderly, and this will require a better understanding of their fundamental roles in B cell biology.