PIGA – Paroxysmal Nocturnal Hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) is a rare, acquired clonal hematopoietic stem cell disorder caused by somatic mutations in the X-linked PIGA gene. PIGA encodes the catalytic subunit of the glycosylphosphatidylinositol (GPI)-N-acetylglucosaminyltransferase, the first enzyme in GPI anchor biosynthesis. Loss of function in PIGA leads to absence of GPI-anchored complement regulators CD55 and CD59 on blood cells, resulting in chronic intravascular hemolysis, hemoglobinuria, thrombosis and cytopenias.

Extensive mutational screening in PNH cohorts has revealed a broad spectrum of PIGA variants including frameshifts, nonsense, missense and splice-site changes dispersed throughout the coding sequence. For example, the recurrent nonsense variant c.1234C>T (p.Arg412Ter) has been identified in multiple unrelated PNH patients (PMID:22305531). Next-generation sequencing of 85 PNH patients uncovered 124 distinct PIGA mutations and a mutational hotspot in exon 2, confirming high allelic heterogeneity (PMID:32359022).

PNH arises through somatic clonal expansion rather than germline inheritance, and affected individuals display mosaicism with multiple independent PIGA mutations in hematopoietic lineages. Familial segregation is not observed, and the disease manifests sporadically without transmission to offspring.

Functional studies have defined the mechanism of pathogenicity and confirmed causality. Retroviral delivery of a transmembrane form of CD59 into GPI-deficient PNH B cells restored surface CD59 expression and conferred resistance to complement-mediated lysis (PMID:7522635). Site-directed mutagenesis of Pig-A protein residues (His128, Ser129, Ser155) demonstrated critical regions required for GPI biosynthesis (PMID:9398536). Flow cytometry assays of GPI-anchored proteins remain the diagnostic gold standard.

Animal and in vitro Pig-a mutation assays further validate PIGA as an in vivo reporter gene. Mutant frequencies in mouse and rat erythroid cells mirror bone marrow mutation burdens, and sequencing of sorted GPI-deficient cells confirms Pig-a mutations with agent-specific spectra. These models support the use of Pig-a assays for mutagenicity testing and mechanistic studies of PIGA deficiency.

Integration of genetic, functional and clinical data establishes a Definitive gene–disease association between PIGA and Paroxysmal Nocturnal Hemoglobinuria. Key take-home: early identification of somatic PIGA mutations via flow cytometry and molecular testing is critical for guiding complement inhibitor therapy and prognostic stratification in PNH.

References

• Blood • 1994 • Mutations within the Piga gene in patients with paroxysmal nocturnal hemoglobinuria. PMID:8167330
• American Journal of Human Genetics • 2012 • The phenotype of a germline mutation in PIGA: the gene somatically mutated in paroxysmal nocturnal hemoglobinuria. PMID:22305531
• International Journal of Laboratory Hematology • 2020 • Identification of acquired PIGA mutations and additional variants by next-generation sequencing in paroxysmal nocturnal hemoglobinuria. PMID:32359022
• Blood • 1994 • Expression of recombinant transmembrane CD59 in paroxysmal nocturnal hemoglobinuria B cells confers resistance to human complement. PMID:7522635
• Blood Cells, Molecules & Diseases • 1997 • Structural and functional analysis of the Pig-a protein that is mutated in paroxysmal nocturnal hemoglobinuria. PMID:9398536

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