Agammaglobulinaemia with absent B cells comprises diseases due to mutations in BTK, molecules signalling downstream of the BCR, and components of the BCR itself; in about 5% of cases the genetic cause remains unclear. Here we describe a novel human immunodeficiency, characterised by agammaglobulinaemia and B-lymphopenia, due to hypomorphic mutations in ZIP7, a ubiquitously expressed zinc transporter. whose absence leads to cell death through ER stress.
By whole exome sequencing, we identified biallelic hypomorphic mutations of ZIP7 in 6 patients affected by agammaglobulinaemia and B-lymphopenia. We introduced one of the identified mutations in mice using CRISPR/Cas9, obtaining a series of alleles around the targeted position, and performed detailed cellular and immunologic analyses.
Mice bearing homozygous null mutation of ZIP7 could not be obtained, implying embryonic lethality. In contrast, animals with hypomorphic ZIP7 function were viable and lacked B cells, phenocopying affected patients. We found reduced levels of cytoplasmic zinc in primary B cell lines derived from mutant mice. Late-pre and immature B cells lacked signatures of ER stress, but showed increased phosphatase activity, reduced signalling downstream of the BCR and failure to progress through the transitional stages to fully mature B cells.
We identify a novel and indispensable role of ZIP7 in B cell development. Intact ZIP7 is necessary to regulate subcellular Zn distribution, reducing phosphatase activity and allowing signalling downstream of the nascent BCR. B cell development is highly dependent on the integrity of BCR signalling; this is the first study describing the role of zinc in this process.