BTK mutations selectively regulate BTK expression and upregulate monocyte XBP1 mRNA in XLA patients

Mutations in the Bruton agammaglobulinemia tyrosine kinase (BTK) gene are responsible for X-linked agammaglobulinemia (XLA). Unfolded or misfolded proteins can trigger stress pathways in the endoplasmic reticulum (ER), known as unfolded protein response (UPR). The aim was to clarify the involvement of UPR in XLA pathophysiology. By reverse transcription-quantitative PCR, we evaluated the expression of BTK and 12 UPR-related genes in eight patients. Moreover, we assessed the BTK protein expression and pattern in the patients' monocytes by flow cytometry and fluorescence immunocytochemistry. We found a reduced BTK expression in patients with stop codon mutations (P < 0.02). However, missense mutations did not affect BTK expression. Flow cytometry showed a reduction of BTK in patients which was corroborated by an absent or nonfunctional protein synthesis revealed by immunocytochemistry. In contrast with the other UPR-related genes, X-box binding protein 1 (XBP1) was markedly upregulated in the patients (P < 0.01), suggesting Toll-like receptor (TLR) activation since BTK directly interacts with TLRs as a negative regulator and XBP1 can be activated in direct response to TLR ligation. Different BTK mutations can be identified by the BTK expression. Inasmuch as UPR-related genes were downregulated or unaltered in patients, we speculate the involvement of the TLRs-XBP1 axis in the XLA pathophysiology. Such data could be the basis for further studies of this novel pathomechanism concerning XLA.