Identification of a Novel Hematopoietic Cell Translocation in a Patient with Paroxysmal Nocturnal Hemoglobinuria (PNH).

Abstract

PNH is an acquired disorder of clonal hematopoiesis, in which the affected hematopoietic stem cells (HSC’s) have a mutation of the X-linked PIG-A gene. PIG-A is required for synthesis of the glycosylphosphatidyl inositol (GPI) moiety that serves as an anchor for a functionally diverse group of membrane proteins. Consequently all GPI-anchored proteins are deficient on progeny of affected HSC’s. GPI-anchor protein deficiency clearly accounts for some of the clinical manifestations of PNH but does not explain either clonal selection or clonal expansion. Further, it is not clear if clonal selection and clonal expansion are independent processes. Observations reported herein suggest that clonal selection and clonal expansion are part of the same process and that PNH is an example of non-malignant clonal evolution. A 31 y/o female presented with complaints of fatigue, fever, numbness in the lower extremity and dark urine. CBC revealed a WBC of 4100/μl, hemoglobin 3.8 gram%, hematocrit of 12.9% and platelet count of 171,000/μl. Laboratory studies were indicative of intravascular hemolysis, and subsequent flow cytometric analysis of the peripheral blood showed that 88% of the PMN were GPI-anchor protein deficient. Bone marrow biopsy showed a normocellular marrow with a relative erythrocytosis but with no evidence of dysplasia. Cytogenetic studies of the bone marrow demonstrated 46XX, ins (12) (p12~13q13q12) in all 20 metaphase cells analyzed. The findings were that of a female with an apparently balanced intrachromosomal inverted insertion at the 400 band level. FISH probes supported the conventional cytogenetics and indicated that the insertion split the TEL locus at 12p13. Subsequent analysis of mitogen stimulated peripheral blood lymphocytes revealed a normal karyotype. These finding show that the rearrangement of chromosome 12 identified in the bone marrow cells is an acquired cytogenetic abnormality and suggest a causal relationship with PNH in this case. TEL, a member of the ETS family of transcription factors is a fusion partner in a number of translocation that result in hematopoietic cell malignancies (e. g., ALL, AML, MDS). While the rearrangement described herein appear to be novel, deletions of 12p12 have been observed in other patients with PNH, and a structural rearrangement of 12q13 (one of the three breakpoints in the insertion in this case) has also been reported in a patient with PNH. Together, these findings suggest the following: (1) non-random cytogenetic abnormalities may be associated with PNH; (2) PNH may be an example of non-malignant clonal evolution in which the PIG-A mutation contributes directly to clonal expansion (in this specific case by acting in concert with a non-transforming proliferative advantage arising from the rearrangement involving TEL); (3) clonal selection and clonal dominance may not be independent process in PNH. Studies designed to characterize the consequences of the structural rearrangement of chromosome 12 in this case are in progress.