Analysis of SWAP-70 as a candidate gene for non-X-linked hyper IgM syndrome and common variable immunodeficiency.

SWAP-70 is a recently identified protein that functions as the only B cell-specific component of an isotype switch recombination complex called SWAP. The SWAP complex has specificity for the switch regions upstream of the constant region immunoglobulin genes and it facilitates the transfer of DNA between switch regions. These features suggested that mutations in the gene encoding SWAP-70 might result in humoral immunodeficiency. To test this hypothesis we determined the genomic structure of this gene and used single-stranded conformational polymorphism (SSCP) analysis to screen DNA from 38 patients with either non-X-linked hyper IgM syndrome or common variable immunodeficiency. The results demonstrated that SWAP-70 consists of 12 exons spread over 89 kb at chromosome 11p15.2. SSCP analysis of the patient population revealed five polymorphic variants in the gene, one of which (Q505E) is an amino acid substitution in the putative nuclear export signal of SWAP-70. However, none of the alterations appeared to be associated with disease in the patients screened.

Defects in early B-cell development: comparing the consequences of abnormalities in pre-BCR signaling in the human and the mouse.

Patients with genetic defects in B-cell development provide an unusual opportunity to dissect the requirements for normal B-cell maturation. It is striking that all of the known genetic defects that result in a failure of B-cell development involve signaling through the pre-B-cell receptor (pre-BCR). Approximately 85% of affected patients are males with mutations in the X chromosome-encoded cytoplasmic tyrosine kinase Btk. Preliminary experiments using stem cell transplants and retroviral-mediated gene therapy in Btk-deficient mice suggest that it may be relatively easy to correct serum immunoglobulins but harder to correct antibody production to T-cell-independent antigens in this disorder. About 3-6% of patients with defects in B-cell development have deletions or critical base pair substitutions in the mu constant region gene. Patients with defects in Igalpha, lambda5 and B-cell linker protein (BLNK) have also been described. All of these patients have a block at the pro-B to pre-B-cell transition. Defects in Btk, lambda5 and BLNK result in a more severe phenotype in the human compared to the mouse. These findings suggest that requirements for signaling through the pre-BCR are more stringent in the human compared to the mouse. Possible explanations for this observation are discussed.

Gene conversion events contribute to the polymorphic variation of the surrogate light chain gene lambda 5/14.1.

Normally occurring and experimentally induced models of immunodeficiency indicate that B cell development and antibody production are influenced by genetic factors. It is highly likely that polymorphic variants in genes that encode receptors for growth and differentiation factors, signal transduction molecules, and components of the B cell and pre-B-cell receptor complex contribute to this genetic control. We have identified a surprisingly large number of polymorphic variants in lambda5/14.1. Together with VpreB, lambda5/14.1 forms the surrogate light chain in the pre-B-cell receptor complex. Thirteen variant alleles of lambda5/14.1 were found in 134 unrelated individuals. Nine of these variants result in changes in the amino acid sequence of this small protein. The majority of the single base pair substitutions in lambda5/14.1 could be attributed to gene conversion events in which donor sequences from the lambda5 pseudogenes, 16.1, 16.2, and Glambda1, replace the wild-type sequence in the lambda5/14.1 functional gene. These findings indicate that gene conversion events play a major role in generating diversity that could affect stability or expression of the pre-B-cell receptor complex.

Mutations in Igalpha (CD79a) result in a complete block in B-cell development.

Mutations in Btk, mu heavy chain, or the surrogate light chain account for 85-90% of patients with early onset hypogammaglobulinemia and absent B cells. The nature of the defect in the remaining patients is unknown. We screened 25 such patients for mutations in genes encoding components of the pre-B-cell receptor (pre-BCR) complex. A 2-year-old girl was found to have a homozygous splice defect in Igalpha, a transmembrane protein that forms part of the Igalpha/Igbeta signal-transduction module of the pre-BCR. Studies in mice suggest that the Igbeta component of the pre-BCR influences V-DJ rearrangement before cell-surface expression of mu heavy chain. To determine whether Igalpha plays a similar role, we compared B-cell development in an Igalpha-deficient patient with that seen in a mu heavy chain-deficient patient. By immunofluorescence, both patients had a complete block in B-cell development at the pro-B to pre-B transition; both patients also had an equivalent number and diversity of rearranged V-DJ sequences. These results indicate that mutations in Igalpha can be a cause of agammaglobulinemia. Furthermore, they suggest that Igalpha does not play a critical role in B-cell development until it is expressed, along with mu heavy chain, as part of the pre-BCR.