Mutations in the human λ5/14.1 gene result in B cell deficiency and agammaglobulinemia

B cell precursors transiently express a pre-B cell receptor complex consisting of a rearranged mu heavy chain, a surrogate light chain composed of lambda 5/14.1 and VpreB, and the immunoglobulin (Ig)-associated signal transducing chains, Ig alpha and Ig beta. Mutations in the mu heavy chain are associated with a complete failure of B cell development in both humans and mice, whereas mutations in murine lambda 5 result in a leaky phenotype with detectable humoral responses. In evaluating patients with agammaglobulinemia and markedly reduced numbers of B cells, we identified a boy with mutations on both alleles of the gene for lambda 5/14.1. The maternal allele carried a premature stop codon in the first exon of lambda 5/14.1 and the paternal allele demonstrated three basepair substitutions in a 33-basepair sequence in exon 3. The three substitutions correspond to the sequence in the lambda 5/14.1 pseudogene 16.1 and result in an amino acid substitution at an invariant proline. When expressed in COS cells, the allele carrying the pseudogene sequence resulted in defective folding and secretion of mutant lambda 5/14.1. These findings indicate that expression of the functional lambda 5/14.1 is critical for B cell development in the human.