Identification of four novel mutations in BTK from six Chinese families with X-linked agammaglobulinemia

Background: The defect of Bruton's tyrosine kinase (BTK) gene resulted in X-linked agammaglobulinemia (XLA), which is characterized by recurrent bacterial infections, immunodeficiency with low B-cell numbers and immunoglobulin. Diagnosis of XLA depends on clinical phenotype and genetic testing.Methods: Six unrelated Chinese families with high suspicion of XLA were enrolled in this study. Potential pathogenic variants were detected and validated by Whole Exome Sequencing (WES) and Sanger Sequencing. Western blot, Quantitative PCR (qPCR) analysis and immunofluorescence analysis were used to evaluate the preliminary function of candidate BTK variants.Results: A total of six variants were identified, four of which were not reported before. The novel missense mutation(c.1900 T > G) and deletion(c.897delG) were found that the mutant protein and mRNA expression levels have fallen by Western Blot and qPCR identification. We also constructed minigene expression vector to determine the deletion (c.1751-6_1755delttctagGGGTT) resulting a 35 bp skipping in exon 18. Meanwhile, the break point of gross deletion (Exon2-5) discovered based on WES was confirmed to be located at site ChX:101367539_101376531 through qPCR and Gap-PCR.Conclusion: This study makes definitive diagnosis for 6 families with suspected XLA and further expands the spectrum of BTK mutations, providing new information for the diagnosis of the disease.