Paroxysmal Nocturnal Hemoglobinuria Caused By Pigt Mutations; Atypical PNH

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired GPI-anchor deficiency caused by the somatic mutation of PIGA gene in a hematopoietic stem cell (PIGA-PNH). Loss of function of PIGA causes loss of GPI anchored proteins including complement regulatory proteins. Main symptoms are hemolytic anemia and venous thrombosis caused by the uncontrolled complement activation, and bone marrow failure. For X-linked PIGA, one hit of somatic mutation causes GPI-deficiency. All other genes involved in GPI-anchor biosynthetic pathway are autosomal and hits to two alleles are required to cause GPI-deficiency. In 2013, a PNH patient caused by the PIGT mutations was reported. This patient had a splice site mutation in one germ line allele and a somatic 8Mb deletion including entire PIGT. Recently, we found a Japanese patient who has a nonsense mutation in a germ line allele and a somatic 18Mb deletion in chromosome 20q including PIGT. Here we compare two patients and report that these cases of PNH are atypical in two points. One is that PIGT-PNH cases had unusual symptoms similar to auto inflammatory syndrome. Both had long lasting chronic urticaria and joint pain. Additionally, the Japanese case had recurrent aseptic meningitis and the German patient had ulcerative colitis. It should be noted that these symptoms had lasted over ten years before hemolytic attacks occurred and diagnosis as PNH was made. Urticaria, joint pain and recurrent meningitis were controllable by eculizumab, anti-C5 antibody, showing involvement of complement activation. These inflammatory symptoms are specific to PIGT-PNH and are not seen in PIGA-PNH. PIGT is involved in transfer of GPI to proteins and its defect causes accumulation of free GPI, non-protein linked GPI. In fact, FACS analysis using the antibody which recognizes free GPI revealed that GPI negative granulocytes, monocytes, and B cells from the PIGT-PNH patient, but not from PIGA-PNH patients, highly expressed free GPI. We speculate that this free GPI together with complement activation cause inflammasome activation and are establishing assay system. The other atypical nature is a mechanism of clonal expansion. In typical PNH, autoimmunity against hematopoietic stem cells causing bone marrow failure is thought to positively select GPI-deficient cells. In contrast, the somatically deleted 18Mb included a region commonly deleted in some patients with myeloproliferative noeplasms. This region contains several maternally imprinted genes, defective expression of which is likely causally related to clonal expansion in atypical PNH cases. Thus, the symptoms and mechanism of clonal expansion of PIGT-PNH are quite different from those of PIGA-PNH, so that PIGT-PNH should be categorized as an atypical PNH. Screening of PIGT-PNH among the PNH patients by detecting free GPI using FACS analysis should be feasible.