CXCR4 Gene Dosage Is Critical for HSC Engraftment

Introduction: Warts, Hypogammaglobulinemia, Infections and Myelokathexis Syndrome (WHIMS) is an autosomal dominant immunodeficiency resulting from gain-of-function mutations in the chemokine receptor CXCR4.  We recently described a unique WHIMS patient who underwent spontaneous genetic and phenotypic reversion at approximately age 30 after being severely affected as a child.  Her reversion was due to a single catastrophic genetic event known as chromothripsis (chromosome shattering) resulting in the deletion of one copy of 163 genes in addition to her mutant copy of CXCR4 on chromosome 2.  This event was traced to a hematopoietic stem cell (HSC) that had spontaneously repopulated her bone marrow; however, which of the genes was responsible and the mechanism required further investigation.

Methods:    Mouse models of CXCR4 haploinsufficiency (Cxcr4^+/o) and WHIMS (Cxcr4^+/S338X) were used in competitive bone marrow repopulation experiments transplanting whole bone marrow cells or purified HSC.   Recipient mice were treated with / without lethal irradiation prior to transplant.  Genome editing with TALENs and CRISPR-Cas9 technology was used to target CXCR4 for deletion in human cell lines.

Results:  Cxcr4 haploinsufficiency markedly enhanced HSC engraftment potential in recipient WHIM mice whether the donor HSC were purified from whole bone marrow cells or not, and whether the recipient was conditioned by lethal irradiation or not.  Enhanced engraftment by Cxcr4 haploinsufficient donor HSC also occurred in wild-type mouse recipients, but to a lesser extent, and was also HSC intrinsic.  Genome editing experiments have been successful at deleting one or both copies of CXCR4 in human cell lines in up to 40% of treated cells, and in reducing CXCR4 surface expression.

Conclusion: While CXCR4 was already understood to be important in HSC biology, this patient and subsequent murine experiments have proven that the gene dosage of CXCR4 is a critical factor affecting HSC engraftment.  Genome editing is a promising technology for deleting one copy of CXCR4, ideally the WHIM allele, in autologous HSC as a strategy to cure WHIM syndrome.