Frequent detection of T cells with mutations of the hypoxanthine-guanine phosphoribosyl transferase gene in patients with paroxysmal nocturnal hemoglobinuria.

Acquired mutations of the PIG-A gene result in the hemolysis characteristic of paroxysmal nocturnal hemoglobinuria (PNH). Although the etiology of the mutation(s) is unclear, mutable conditions have been suggested by the coexistence of multiple clones with different mutations of PIG-A and by the appearance of leukemic clones in patients with PNH. This study sought to test this hypothesis by examining the frequency of hypoxanthine-guanine phosphoribosyl transferase (HPRT) gene mutations, identified by both resistance to 6-thioguanine (6-TG) and gene analysis. T-cell colonies resistant to 6-TG formed in methylcellulose culture were found in 8 (67%) of 12 PNH patients and 3 (18%) of 17 age-matched healthy volunteers (P <.02, Fisher exact probability test). The incidence of resistant colonies ranged from 40 to 367 (mean 149, x 10(-7)) in the 8 patients and from 1 to 16 (mean 7, x 10(-7)) in the 3 healthy donors. Thus, the HRPT gene mutated more frequently in patients with PNH than in healthy controls (P <.02, Mann-Whitney test). Analysis of bone marrow cells supported these findings. Like the PIG-A mutations in PNH, the HPRT mutations were widely distributed in the coding regions and consisted primarily of base deletions. Unlike PNH cells, 6-TG-resistant cells expressed CD59, indicating that the HPRT mutations did not occur in PNH clones. No correlation was noted between HPRT mutation frequency and content of therapy received by the patients. It is concluded that in PNH patients, conditions exist that favor the occurrence of diverse somatic mutations in blood cells.