Pathogenetic mechanisms of hematological abnormalities of patients with MYH9 mutations.

Mutations of MYH9, the gene for non-muscle myosin heavy chain IIA (NMMHC-IIA), cause a complex clinicalphenotype characterized by macrothrombocytopenia and granulocyte inclusion bodies, often associatedwith deafness, cataracts and/or glomerulonephritis. The pathogenetic mechanisms of these defects areeither completely unknown or controversial. In particular, it is a matter of debate whether haploinsufficiencyor a dominant-negative effect of mutant allele is responsible for hematological abnormalities. We investigated11 patients from six pedigrees with different MYH9 mutations. We evaluated NMMHC-IIA levels in plateletsand granulocytes isolated from peripheral blood and in megakaryocytes (Mks) cultured from circulatingprogenitors. NMMHC-IIA distribution in Mks and granulocytes was also assessed. We demonstrated that allthe investigated patients had a 50% reduction of NMMHC-IIA expression in platelets and that a similar defectwas present also in Mks. In subjects with R1933X and E1945X mutations, the whole NMMHC-IIA of plateletsand Mks was wild-type. No NMMHC-IIA inclusions were observed at any time of Mk maturation. In granulocytes,the extent of NMMHC-IIA reduction in patients with respect to control cells was significantly greaterthan that measured in platelets and Mks, and we found that wild-type protein was sequestered within mostof the NMMHC-IIA inclusions. Altogether these results indicate that haploinsufficiency of NMMHC-IIA inmegakaryocytic lineage is the mechanism of macrothrombocytopenia consequent to MYH9 mutations,whereas in granulocytes a dominant-negative effect of mutant allele is involved in the formation of inclusionbodies. The finding that the same mutations act through different mechanisms in different cells is surprisingand requires further investigation.