Evidence of Common Mechanisms Underlying the Survival Advantage of the PIG-A mutant and 13q- Hematopoietic Stem Cell Clones In Patients with Immune-Mediated Bone Marrow Failure

Numerical karyotypic abnormalities of undetermined significance are occasionally detected in patients with bone marrow failure (BMF), including aplastic anemia (AA) and low-risk myelodysplastic syndrome (MDS). BMF patients with 13q deletions (13q-) are likely to respond to immunosuppressive therapy (IST) without a propensity to develop into leukemia (Ishiyama K et al, Br J Haematol; 117: 747. 2002), and they possess small populations of glycosylphosphatidyl-inositol anchored protein (GPI-AP)- deficient granulocytes and erythrocytes more frequently than patients with normal karyotypes%, 15 with AA and 8 with MDS) of 1705 patients. The proportion of 13q- clones in all metaphase cells ranged from 5% to 100% with a median of 27.5%. Fourteen of the 23 (61%) patients showed 13q- alone and 9 (39%) showed other karyotypic abnormalities in addition to 13q-. GPI-AP^- cells that accounted for 0.009% to 6.7% (median 0.148%) of granulocytes were detected in all (100%) of the 14 patients with 13q- alone, while the prevalence of increased GPI-AP^- cells in patients with 13q- plus other abnormalities and those with a normal karyotype was 44% and 43%, respectively. A FISH analysis of PB granulocytes in individual patients revealed apparently lower percentages of GPI-AP^- granulocytes than those of 13q- granulocytes, indicating that the GPI-AP^- granulocytes were derived from non 13q- HSCs. Ten patients with 13q- alone were treated with IST (ATG+cyclosporine in 4 and cyclosporine alone in 6) and all of them achieved either a partial remission or a complete remission. The SNP array analysis of 5 patients revealed common deletion of a 15 Mb (13.3 to 14.3) region containing the RB gene. Wild-type and PIG-A mutant K562 cells were incubated in the presence of TGF-beta, which can inhibit leukemic cell proliferation by down-regulating phosphorylated RB protein (pRB), to determine whether the RB gene loss in 13q- HSCs has any connection with a proliferative advantage of PIG-A mutant HSCs. PIG-A mutant K562 cells were less sensitive to TGF-beta inhibition of proliferation and down-regulation of pRB levels than wild-type K562 cells.

The markedly high prevalence of increased GPI-AP^- cells and good response to IST indicate that the presence of 13q- clones is highly associated with the immune pathophysiology of BMF. Decreased sensitivity to inhibitory signals by TGF-beta due to RB gene loss in 13q- HSCs and the loss of GPI-AP-type TGF-beta receptors in PIG-A mutant HSCs may therefore be a common mechanism that confers a survival advantage to these mutant HSCs over normal HSCs in patients with immune-mediated BMF.

No relevant conflicts of interest to declare.