Diversity of immunoglobulin kappa light chain gene rearrangements and evidence for somatic mutation in V kappa IV family gene segments in X-linked agammaglobulinemia

Mortality and morbidity in patients with X-linked agammaglobulinaemia

BACKGROUND

X-linked agammaglobulinaemia (XLA) is a genetic disorder characterised by a defect in the generation of mature B cells, lack of antibodies production, and susceptibility to recurrent bacterial infections. Understanding of the risk factors responsible for morbidity and mortality in these patients can help in a better management of this disorder. However, there is a lack of specific studies in the literature regarding the morbidity and mortality of XLA patients. This study is designed to evaluate morbidities and mortality and survival rates in Iranian patients with XLA diagnosis during the past 20 years.

METHODS

We have registered the clinical data of the XLA patients and followed them up until 2010. At the time of diagnosis, a four-page questionnaire including complete medical information was filled out for all patients. Follow-up information was gathered either by reviewing the patients' hospital records or regularly visiting the patients.

RESULTS

Among 41 patients, 26.8% died during the follow up period. All of the complications before the initiation of treatment such as pneumonia, otitis media and diarrhoea were reduced after immunoglobulin replacement, except sinusitis and conjunctivitis. There were significant associations between some immunological and clinical characteristics such as lymphocyte subsets, consanguinity marriage and mortality.

CONCLUSION

Despite recent advances in the treatment of XLA, these patients still suffer from severe complications. Associations between poor prognosis and clinical and some immunological characteristics of the patients may help physicians to select poor prognoses patients at higher risk of mortality to develop prevention strategies for them.

Bruton's tyrosine kinase is essential for human B cell tolerance

Most polyreactive and antinuclear antibodies are removed from the human antibody repertoire during B cell development. To elucidate how B cell receptor (BCR) signaling may regulate human B cell tolerance, we tested the specificity of recombinant antibodies from single peripheral B cells isolated from patients suffering from X-linked agammaglobulinemia (XLA). These patients carry mutations in the Bruton's tyrosine kinase (BTK) gene that encode an essential BCR signaling component. We find that in the absence of Btk, peripheral B cells show a distinct antibody repertoire consistent with extensive secondary V(D)J recombination. Nevertheless, XLA B cells are enriched in autoreactive clones. Our results demonstrate that Btk is essential in regulating thresholds for human B cell tolerance.

Bruton's tyrosine kinase regulates the activation of gene rearrangements at the lambda light chain locus in precursor B cells in the mouse

Bruton's tyrosine kinase (Btk) is a nonreceptor tyrosine kinase involved in precursor B (pre-B) cell receptor signaling. Here we demonstrate that Btk-deficient mice have an ∼50% reduction in the frequency of immunoglobulin (Ig) λ light chain expression, already at the immature B cell stage in the bone marrow. Conversely, transgenic mice expressing the activated mutant Btk^E41K showed increased λ usage. As the κ/λ ratio is dependent on (a) the level and kinetics of κ and λ locus activation, (b) the life span of pre-B cells, and (c) the extent of receptor editing, we analyzed the role of Btk in these processes. Enforced expression of the Bcl-2 apoptosis inhibitor did not alter the Btk dependence of λ usage. Crossing 3-83μδ autoantibody transgenic mice into Btk-deficient mice showed that Btk is not essential for receptor editing. Also, Btk-deficient surface Ig⁺ B cells that were generated in vitro in interleukin 7-driven bone marrow cultures manifested reduced λ usage. An intrinsic defect in λ locus recombination was further supported by the finding in Btk-deficient mice of reduced λ usage in the fraction of pre-B cells that express light chains in their cytoplasm. These results implicate Btk in the regulation of the activation of the λ locus for V(D)J recombination in pre-B cells.

A directory of human germ-line V kappa segments reveals a strong bias in their usage

From the genomic DNA of a single individual, we have amplified, cloned and sequenced 37 human germ-line V kappa segments. Four of these segments were new. We then compiled a comprehensive directory of all germ-line V kappa segments and identified 50 different sequences with open reading frames. Comparison with 236 rearranged sequences revealed that no more than 24 of these germ-line sequences could be assigned rearranged counterparts, that some of these were rarely used, and that only about 11 sequences are used frequently. This suggests that the expressed V kappa repertoire is mainly derived from a limited number of segments. Most surprisingly, the J kappa-distal region of the locus appears to be rarely used: we could unambiguously assign 162 rearranged sequences to V kappa segments of the J kappa-proximal region, but only 5 to segments of the J kappa-distal region.

The Bruton's tyrosine kinase gene is expressed throughout B cell differentiation, from early precursor B cell stages preceding immunoglobulin gene rearrangement up to mature B cell stages

X-linked agammaglobulinemia (XLA) is an immunodeficiency disease in man, resulting from an arrest in early B cell differentiation. The gene defective in XLA has recently been identified and encodes a cytoplasmic protein tyrosine kinase, named Bruton's tyrosine kinase (btk), essential for cell differentiation and proliferation at the transition from pre-B to later B cell stages. In this study we investigated btk expression by Northern blotting experiments in a series of human (precursor-) B cell lines, acute lymphoblastic leukemias and plasmacytomas. btk was found to be already expressed in very early stages of B cell differentiation, even prior to immunoglobulin (Ig) heavy (H) or light (L) chain gene rearrangements. Transcripts were also detected at the pre-B cell stage and in mature B cells, irrespective of the Ig H chain class expressed. Approximately at the transition from mature B cells to plasma cells, expression of the btk gene is down-regulated. In addition, the btk gene was found to be expressed in myeloid cell lines and acute myeloid leukemias. btk expression in myeloid cells is probably not a prerequisite for myeloid differentiation, since myeloid cells in XLA patients seem not to be affected. No btk expression was found in T-lineage cells. The btk expression profile, i.e. from early precursor-B cell stages preceding Ig rearrangement up to mature B cells, supports the hypothesis that the XLA defect resides in a critical step of B cell development which is independent of the Ig gene recombination machinery.

Comparison of human germ-line kappa gene sequences to sequence data from the literature

The question of which germ-line V kappa genes are expressed was studied by sequencing 70 different cDNA clones from a human spleen library and one clone from a fetal liver library. The sequences were compared to a data base containing all germ-line V kappa gene and pseudogene sequences. In addition, 51 rearranged genomic V kappa genes, 170 cDNA and 74 kappa proteins from the literature were assigned to specific germ-line V kappa genes and included in the comparisons. Not all the known, potentially functional V kappa genes were found to be expressed, while some genes with minor defects are. The total number of expressed genes is smaller than expected: so far 21 germ-line genes and 5 pairs of duplicated identical genes are known to be transcribed. The corresponding numbers for rearranged genomic V kappa genes and kappa proteins are 17 plus 4 and 7 plus 7, respectively. A second aim of the study was to find out whether the expressed repertoire contains derivatives of germ-line V kappa genes still missing in our data base; no evidence for the existence of such genes was found. Several cDNA clones contained additional nucleotides between the V kappa and J kappa gene segments, which may be germ-line derived, inserted by terminal deoxynucleotidyl transferase or introduced by other mechanisms. Somatic gene conversion seems not to play a major role in creating the human kappa gene diversity. Various aspects of the hypermutation of kappa genes are discussed and the formation of block mutations, i.e. the alterations of two or more adjacent nucleotides is stressed as a remarkable feature of the process.