Leukemic Blasts with the Paroxysmal Nocturnal Hemoglobinuria Phenotype in Children with Acute Lymphoblastic Leukemia

It has been proposed that genomic instability is essential to account for the multiplicity of mutations often seen in malignancies. Using the X-linked PIG-A gene as a sentinel gene for spontaneous inactivating somatic mutations, we previously showed that healthy individuals harbor granulocytes with the PIG-A mutant (paroxysmal nocturnal hemoglobinuria) phenotype at a median frequency (f) of ∼12 × 10⁻⁶. Herein, we used a similar approach to determine f in blast cells derived from 19 individuals with acute lymphoblastic leukemia (ALL) and in immortalized Epstein-Barr virus–transformed B-cell cultures (human B-lymphoblastoid cell lines) from 19 healthy donors. The B-lymphoblastoid cell lines exhibited a unimodal distribution, with a median f value of 11 × 10⁻⁶. In contrast, analysis of the f values for the ALL samples revealed at least two distinct populations: one population, representing approximately half of the samples (n = 10), had a median f value of 13 × 10⁻⁶, and the remaining samples (n = 9) had a median f value of 566 × 10⁻⁶. We conclude that in ALL, there are two distinct phenotypes with respect to hypermutability, which we hypothesize will correlate with the number of pathogenic mutations required to produce the leukemia.