The spectrum of mutations in Btk that cause X-linked agammaglobulinemia

Autoimmunity and inflammation in X-linked agammaglobulinemia

PURPOSE

In the past, XLA was described as associated with several inflammatory conditions, but with adequate immune globulin treatment, these are presumed to have diminished. The actual prevalence is not known.

METHODS

A web-based patient survey was conducted December 2011- February 2012. Respondents were recruited from the Immune Deficiency Foundation (IDF) patient database, online patient discussion forums and physician recruitment of patients. The questionnaire was developed jointly by IDF and by members of the USIDNET-XLA Disease Specific Working Group. Information regarding inflammatory conditions in patients with XLA was also obtained from the United States Immune Deficiency Network (USIDNET) Registry.

RESULTS

Based on 128 unique patient survey responses, the majority of respondents (69%) reported having at least one inflammatory symptom, with 53% reporting multiple symptoms. However, only 28% had actually been formally diagnosed with an inflammatory condition. Although 20% reported painful joints and 11% reported swelling of the joints, only 7% were given a diagnosis of arthritis. Similarly, 21% reported symptoms of chronic diarrhea and 17% reported abdominal pain, however only 4% had been diagnosed with Crohn's disease. Data from the USIDNET Registry on 149 patients with XLA, revealed that 12% had pain, swelling or arthralgias, while 18% had been diagnosed with arthritis. Similarly, 7% of these patients had abdominal pain and 9% chronic diarrhea.

CONCLUSIONS

Although patients with XLA are generally considered to have a low risk of autoimmune or inflammatory disease compared to other PIDD cohorts, data from this patient survey and a national registry indicate that a significant proportion of patients with XLA have symptoms that are consistent with a diagnosis of arthritis, inflammatory bowel disease or other inflammatory condition. Documented diagnoses of inflammatory diseases were less common but still increased over the general population. Additional data is required to begin implementation of careful monitoring of patients with XLA for these conditions. Early diagnosis and proper treatment may optimize clinical outcomes for these patients.

Biology and novel treatment options for XLA, the most common monogenetic immunodeficiency in man

INTRODUCTION

X-linked agammaglobulinemia (XLA) is the most common primary immunodeficiency in man, and is caused by a single genetic defect. Inactivating mutations in the Bruton's tyrosine kinase (BTK) gene are invariably the cause of XLA,. XLA is characterized by a differentiation arrest at the pre-B cell stage, the absence of immunoglobulins and recurrent bacterial infections, making it an insidious disease that gradually disables the patient, and can result in death due to chronic lung disease. Current treatment involves prophylactic antibiotics and immunoglobulin infusions, which are non-curative. This disease is a good candidate for curative hematopoietic stem cell (HSC)-based gene therapy, which could correct the B cell and myeloid deficiencies.

AREAS COVERED

This paper reviews the basic biology of BTK in B cell development, the clinical features of XLA, and the possibilities of gene therapy for XLA, covering the literature from 1995 to 2010.

EXPERT OPINION

Work from various laboratories demonstrates the feasibility of using gene-corrected HSCs to complement the immune defects of Btk-deficiency in mice. We propose that it is timely to start clinical programs to develop stem cell based therapy for XLA, using gene-corrected autologous HSC.

XLA patients with BTK splice-site mutations produce low levels of wild-type BTK transcripts

X-linked agammaglobulinemia is caused by mutations in the BTK gene, which result in a precursor B-cell differentiation arrest in the bone marrow and the absence of or strongly reduced B lymphocytes in blood. We identified a patient with a mild clinical phenotype, low numbers of B lymphocytes, and a splice-site mutation in the BTK gene. The precursor B-cell compartment in the bone marrow of this patient was almost identical to that in healthy children. Using real-time quantitative polymerase chain reaction, we were able to detect low levels of wild-type BTK transcripts in his granulocytes. Therefore, we speculated that wild-type BTK transcripts might be responsible for a milder clinical and immunological phenotype, as has been shown in several other diseases. Consequently, we quantified the expression of wild-type BTK transcripts in granulocytes of eight additional patients with splice-site mutations and compared their phenotypes with 17 patients with other types of BTK mutations. In these eight patients, the presence of low levels of wild-type BTK transcripts did not show a clear correlation with the percentage, absolute number, or immunophenotype of B lymphocytes nor with age or serum immunoglobulin levels at diagnosis. Nevertheless, we postulate that the presence of wild-type BTK transcripts can be one of the many factors that influence the clinical and immunological phenotype in X-linked agammaglobulinemia.

Composition of precursor B-cell compartment in bone marrow from patients with X-linked agammaglobulinemia compared with healthy children

X-linked agammaglobulinemia (XLA) is characterized by a severe B-cell deficiency, resulting from a differentiation arrest in the bone marrow (BM). Because XLA is clinically and immunologically heterogeneous, we investigated whether the B-cell differentiation arrest in BM of XLA patients is heterogeneous as well. First, we analyzed BM samples from 19 healthy children by flow cytometry. This resulted in a normal B-cell differentiation model with eight consecutive stages. Subsequently, we analyzed BM samples from nine XLA patients. Eight patients had amino acid substitutions in the Bruton's tyrosine kinase (BTK) domain or premature stop codons, resulting in the absence of functional BTK proteins. In seven of these eight patients a major differentiation arrest was observed at the transition between cytoplasmic Ig(mu-) pre-B-I cells and cytoplasmic Ig(mu+) pre-B-II cells, consistent with a role for BTK in pre-B-cell receptor signaling. However, one patient exhibited a very early arrest at the transition between pro-B cells and pre-B-I cells, which could not be explained by a different nature of the BTK mutation. We conclude that the absence of functional BTK proteins generally leads to an almost complete arrest of B-cell development at the pre-B-I to pre-B-II transition. The ninth XLA patient had a splice site mutation associated with the presence of low levels of wild-type BTK mRNA. His BM showed an almost normal composition of the precursor B-cell compartment, suggesting that low levels of BTK can rescue the pre-B-cell receptor signaling defect, but do not lead to sufficient numbers of mature B lymphocytes in the peripheral blood.

Itk/Emt/Tsk activation in response to CD3 cross-linking in Jurkat T cells requires ZAP-70 and Lat and is independent of membrane recruitment

The Tec family tyrosine kinase, Itk has been implicated in T cell antigen receptor (TCR) signaling, yet little is known about Itk regulation. Here, we investigate the role of the tyrosine kinase ZAP-70 in regulating Itk. Whereas Itk was activated in Jurkat T cells in response to CD3 cross-linking, Itk activation was defective in the ZAP-70-deficient P116 Jurkat T cell line. Itk responsiveness to TCR engagement was restored in P116 cells stably transfected with ZAP-70 cDNA. ZAP-70 itself could not directly phosphorylate the Itk kinase domain, indicating an indirect regulation of Itk activity. No role was found for ZAP-70 in regulating Itk recruitment to the plasma membrane, an event that has been suggested to be rate-limiting for the activation of Tec family kinases. Indeed, Itk was found to be constitutively targeted to the membrane fraction in both Jurkat and P116 cells. Lat, a prominent in vivo substrate of ZAP-70 that mediates assembly of multimolecular signaling complexes at the plasma membrane of T cells was also found to be required for TCR-stimulated Itk activation. Itk could not be activated by CD3 cross-linking in a Lat-negative cell line, unless Lat expression was restored. Lat and Itk were observed to co-associate in response to CD3 cross-linking in Jurkat T cells, but not in P116 T cells. The Lat-Itk association correlated with Lat tyrosine phosphorylation, which was deficient in the P116 T cells. These data suggest that ZAP-70 and Lat play important, probably sequential, roles in regulating the activation of Itk following TCR engagement.

Mutations in btk in patients with presumed X-linked agammaglobulinemia

In 1993, two groups showed that X-linked agammaglobulinemia (XLA) was due to mutations in a tyrosine kinase now called Btk. Most laboratories have been able to detect mutations in Btk in 80%-90% of males with presumed XLA. The remaining patients may have mutations in Btk that are difficult to identify, or they may have defects that are phenotypically similar to XLA but genotypically different. We analyzed 101 families in which affected males were diagnosed as having XLA. Mutations in Btk were identified in 38 of 40 families with more than one affected family member and in 56 of 61 families with sporadic disease. Excluding the patients in whom the marked decrease in B cell numbers characteristic of XLA could not be confirmed by immunofluorescence studies, mutations in Btk were identified in 43 of 46 patients with presumed sporadic XLA. Two of the three remaining patients had defects in other genes required for normal B cell development, and the third patient was unlikely to have XLA, on the basis of results of extensive Btk analysis. Our techniques were unable to identify a mutation in Btk in one male with both a family history and laboratory findings suggestive of XLA. DNA samples from 41 of 49 of the mothers of males with sporadic disease and proven mutations in Btk were positive for the mutation found in their son. In the other 8 families, the mutation appeared to arise in the maternal germ line. In 20 families, haplotype analysis showed that the new mutation originated in the maternal grandfather or great-grandfather. These studies indicate that 90%-95% of males with presumed XLA have mutations in Btk. The other patients are likely to have defects in other genes.

Phosphatidylinositol-3,4,5-trisphosphate (PtdIns-3,4,5-P3)/Tec kinase-dependent calcium signaling pathway: a target for SHIP-mediated inhibitory signals

Tec family non-receptor tyrosine kinases have been implicated in signal transduction events initiated by cell surface receptors from a broad range of cell types, including an essential role in B-cell development. A unique feature of several Tec members among known tyrosine kinases is the presence of an N-terminal pleckstrin homology (PH) domain. We directly demonstrate that phosphatidylinositol-3,4,5-trisphosphate (PtdIns-3,4,5-P3) interacting with the PH domain acts as an upstream activation signal for Tec kinases, resulting in Tec kinase-dependent phospholipase Cgamma (PLCgamma) tyrosine phosphorylation and inositol trisphosphate production. In addition, we show that this pathway is blocked when an SH2-containing inositol phosphatase (SHIP)-dependent inhibitory receptor is engaged. Together, our results suggest a general mechanism whereby PtdIns-3,4,5-P3 regulates receptor-dependent calcium signals through the function of Tec kinases.

Bruton's tyrosine kinase expression and activity in X-linked agammaglobulinaemia (XLA): the use of protein analysis as a diagnostic indicator of XLA

Mutations in the Bruton's tyrosine kinase (BTK) gene result in XLA. Despite the large numbers of BTK mutations reported, no correlation can be made between the clinical phenotype and the gene defects. Analysis of Btk protein expression and activity in individuals with XLA was performed to characterize the relationship between a particular mutation, the resultant Btk protein and the clinical phenotype. In most patients studied, including those with atypical phenotypes, there was complete absence of protein expression and activity. Furthermore, in two undiagnosed individuals with a clinical phenotype suggestive of XLA, lack of protein expression was used to confirm an abnormality in Btk. These results underline the importance of protein analysis prior to speculating on protein structure and function based on the gene mutation. Lack of Btk expression in atypical phenotypes suggests that there is redundancy in B lymphocyte signalling such that alternative signalling molecules, or mechanisms, can compensate for the lack of Btk. We also suggest that analysis of Btk expression can be used as an indicator of XLA. These rapid assays may be used to screen a wider spectrum of individuals with humoral immunodeficiency in order to characterize fully the extent of Btk deficiency.

Transcriptional Regulatory Elements Within the First Intron of Bruton's Tyrosine Kinase

Defects in the gene for Bruton's tyrosine kinase (Btk) result in the disorder X-linked agammaglobulinemia (XLA). Whereas XLA is characterized by a profound defect in B-cell development, Btk is expressed in both the B lymphocyte and myeloid cell lineages. We evaluated a patient with XLA who had reduced amounts of Btk transcript but no abnormalities in his coding sequence. A single base-pair substitution in the first intron of Btk was identified in this patient, suggesting that this region may contain regulatory elements. Using reporter constructs we identified two transcriptional control elements in the first 500 bp of intron 1. A strong positive regulator, active in both pre-B cells and B cells, was identified within the first 43 bp of the intron. Gel-shift assays identified two Sp1 binding sites within this element. The patient's mutation results in an altered binding specificity of the proximal Sp1 binding site. A negative regulator, active in pre-B cells only, was located between base pairs 281 and 491 of the intron. These findings indicate that regulation of Btk transcription is complex and may involve several transcriptional regulatory factors at the different stages of B-cell differentiation.

Transcriptional Regulatory Elements Within the First Intron of Bruton's Tyrosine Kinase

Defects in the gene for Bruton's tyrosine kinase (Btk) result in the disorder X-linked agammaglobulinemia (XLA). Whereas XLA is characterized by a profound defect in B-cell development, Btk is expressed in both the B lymphocyte and myeloid cell lineages. We evaluated a patient with XLA who had reduced amounts of Btk transcript but no abnormalities in his coding sequence. A single base-pair substitution in the first intron of Btk was identified in this patient, suggesting that this region may contain regulatory elements. Using reporter constructs we identified two transcriptional control elements in the first 500 bp of intron 1. A strong positive regulator, active in both pre-B cells and B cells, was identified within the first 43 bp of the intron. Gel-shift assays identified two Sp1 binding sites within this element. The patient's mutation results in an altered binding specificity of the proximal Sp1 binding site. A negative regulator, active in pre-B cells only, was located between base pairs 281 and 491 of the intron. These findings indicate that regulation of Btk transcription is complex and may involve several transcriptional regulatory factors at the different stages of B-cell differentiation.

Xid and Xid-like immunodeficiencies from a signaling point of view

The analysis of Btk-associated molecules and ligand-induced Btk phosphorylation has suggested the existence of a complexed Btk-associated signaling network involved in the activation of B lymphocytes and mast cells. Recent gene targeting experiments have revealed protein kinase C betaI/II (PKCbetaI/II) as a critical component of the Btk-dependent signaling chain and have highlighted a potential role for the Btk-PKCbetaI/II interaction in the amplification of B cell receptor mediated signaling.

B-cell-specific demethylation of BTK, the defective gene in X-linked agammaglobulinemia

BTK, the gene that is defective in X-linked agammaglobulinemia, encodes a cytoplasmic tyrosine kinase that is critical for B-cell proliferation, or survival. To identify regulatory elements that control the expression of BTK we evaluated the methylation pattern of this gene in cell lines and in freshly isolated cells. An Hpa II site that was specifically demethylated in mature B cells but not in pre-B cells, T cells, neutrophils, or nonhematopoietic cells was identified in the tenth intron of BTK. In a 40 kilobase (kb) segment of DNA spanning the entire coding region of BTK plus 3 kb upstream of the first exon there were no other sites that demonstrated lineage-specific demethylation. The B-cell-specific demethylation site in intron 10, which falls within the SH2 domain, 26 kb distal to the first exon, occurs in a region rich in regulatory elements including two E2 boxes, two AP-2 sites, and a cAMP response element. It is likely that this site plays a role in maintaining BTK transcription in mature B cells.