Two Mouse Genetic Models of Thrombocytopenia.

Thrombocytopenia is a disorder in which there is a low level of circulating platelets. There are two main underlying causes; a hematopoietic defect in which there is a decreased production of platelet cells or an autoimmune disorder in which platelets are actively destroyed. Two mutant mouse strains have been obtained from the mutagenesis program at The Heart, Lung, Blood and Sleep Disorders Center at The Jackson Laboratory that each exhibit a dramatic decrease in the level of platelets in comparison to the C57BL/6J wild-type strain making them potentially valuable models for thrombocytopenia. The mutations were crossed to BALB/cByJ and to 129Sv/ImJ and an F2 intercross strategy was used to map the mutation in each strain. Candidate gene sequencing of strain HLB219 revealed a point mutation in the Mpl gene on chromosome 4, which encodes the thrombopoietin (TPO) receptor. This mutation causes an amino acid change from a conserved cysteine to an arginine at codon 40. This ligand-receptor complex plays a pivotal role in megakaryocyte proliferation and maturation and platelet production. The mutation in HLB219 is consistent with other recessive loss-of-function mutations in Mpl that cause hematopoietic defects and thrombocytopenia. HLB219 homozygotes have an elevated circulating TPO level and decreased numbers of hematopoietic progenitors in a colony formation unit (CFU) assay. A competitive repopulation assay showed that HLB219 bone marrow is unable to contribute to hematopoiesis in a lethally-irradiated mouse recipient. Interestingly, HLB219/C57BL/6J heterozygotes show a unique overdominance effect in which platelet levels are elevated compared to wild-type C57BL/6J mice despite having normal TPO levels and reduced megakaryocyte levels in the CFU assay. The mutation in the second mouse model of thrombocytopenia, strain HLB381, has been mapped using a genome-wide linkage scan of 50 F2 intercross progeny. The candidate interval has been narrowed by screening >240 F2 progeny with additional microsatellite and SNP markers. The candidate interval spans 4.2 megabases at the centromeric end of chromosome 8. In contrast to results obtained with HLB219, a competitive repopulation assay demonstrated that HLB381 has normal hematopoietic capability. However, chimeric mice from these assays have low platelet levels, indicating that an autoimmune defect may be responsible for the phenotype. Furthermore, HLB381 mice have normal levels of circulating TPO. Current studies are focused on determining whether HLB381 may serve as a heritable model for idiopathic thrombocytopenic purpura (ITP), an autoimmune disorder.