biostudies-literatureThwaites DTBlood Cancer UK820-829https://www.ebi.ac.uk/biostudies/studies/S-EPMC6450058biostudies-literatureUnknown133(8)The Recombination Activating Genes, <i>RAG1</i> and <i>RAG2</i>, are essential for V(D)J recombination and adaptive immunity. Mutations in these genes often cause immunodeficiency, the severity of which reflects the importance of the altered residue or residues during recombination. Here, we describe a novel <i>RAG1</i> mutation that causes immunodeficiency in an unexpected way: The mutated protein severely disrupts binding of the accessory protein, HMGB1. Although HMGB1 enhances RAG cutting in vitro, its role in vivo was controversial. We show here that reduced HMGB1 binding by the mutant protein dramatically reduces RAG cutting in vitro and almost completely eliminates recombination in vivo. The RAG1 mutation, R401W, places a bulky tryptophan opposite the binding site for HMG Box A at boBloodA novel <i>RAG1</i> mutation reveals a critical in vivo role for HMGB1/2 during V(D)J recombination.PMC645005815042Carter CLawless DBoyes JMSavic SThwaites DTfalseA novel <i>RAG1</i> mutation reveals a critical in vivo role for HMGB1/2 during V(D)J recombination.The Recombination Activating Genes, <i>RAG1</i> and <i>RAG2</i>, are essential for V(D)J recombination and adaptive immunity. Mutations in these genes often cause immunodeficiency, the severity of which reflects the importance of the altered residue or residues during recombination. Here, we describe a novel <i>RAG1</i> mutation that causes immunodeficiency in an unexpected way: The mutated protein severely disrupts binding of the accessory protein, HMGB1. Although HMGB1 enhances RAG cutting in vitro, its role in vivo was controversial. We show here that reduced HMGB1 binding by the mutant protein dramatically reduces RAG cutting in vitro and almost completely eliminates recombination in vivo. The RAG1 mutation, R401W, places a bulky tryptophan opposite the binding site for HMG Box A at bo2019-01-01T00:00:00Z2019 Feb2026-04-29T02:03:44.14Z2026-04-07T14:47:49.142ZS-EPMC64500583053813610.1182/blood-2018-07-866939