Clonal dominance of PNH‐ another piece to the jigsaw

Paroxysmal nocturnal haemoglobinuria (PNH) is an acquired disorder of haematopoietic stem cells (HSCs) due to somatic mutations of phosphatidylinositol glycan A gene (PIGA) localised on chromosome Xp22.1. The hallmark is selection and subsequent expansion of HSCs and their progeny, which lack glycosyl phosphatidylinositol-anchored protein (GPIAP ) on their cell surface. (Parker, 2016) In view of its close association with aplastic anaemia (AA), the ‘immune escape hypothesis’, providing relative growth advantage, has been ‘accepted’ cause for the selective advantage for the GPI-AP population (Gargiulo et al, 2013). Is it also responsible for the clonal dominance and expansion of PIGA mutated cells? Are there any other alternative mechanisms? If so, are these cell intrinsic or extrinsic? If the mechanism is intrinsic to the PIGA HSCs, is this related to absent GPI moiety or anchor protein or is there an accessory genetic event? Collaborating mutations, either additional PIGA or others (HMGA2, JAK2, BCOR), might be necessary to give growth advantage, akin to the oncogenic events needed for transformation in pre-leukaemic states, such as in myelodysplastic syndromes (MDS). (Kulasekararaj et al, 2014; Shen et al, 2014) Additionally, the pathways relevant for stem cell selfrenewal, proliferation, differentiation or stromal microenvironment might be dysregulated, leading to clonal dominance. Hosokawa et al (2017) explore the mechanism responsible for the clonal expansion of GPI-AP in patients with bone marrow failure with moderate/large PNH clone. RNA sequencing, performed on GPI-AP and GPI-AP fractions (from PNH patients and healthy donors), revealed differential expression of genes involved in neutrophil adhesion, granulocyte homeostasis and interleukin 8 (IL8) signalling. Flow cytometric expression of cytokine/chemokine receptors (CXCR2, CXCR1, CSF3RB and TNFRSF10C) was performed to validate the RNA sequencing data. The authors show upregulation of CXCR2 (IL8 receptor) in GPI-AP granulocytes compared to GPI-AP from PNH patients and healthy donors. Additionally, the plasma levels of CXCR2 ligands/cytokines (Macrophage migratory inhibitory factor, SDF-1a [CXCL12] and b-NGF) were markedly elevated in PNH patients. The functional consequence of an overactive CXCR2 pathway is upregulation of NFkB phosphorylation via AKT/ERK-1/2 (AKT/MAPK1/MAPK3) leading to dominance/expansion of GPI-AP granulocytes. The upregulation of CXCR2 and its downstream targets was more specific to the PIGA mutated granulocytic series, as this differential expression was less appreciable in GPI-AP monocytes, and even less so in the stem/progenitor fraction. The variability in CXCR2 expression between different GPI-AP cell lineages from primitive HSCs to well differentiated cells, such as neutrophils, monocytes and lymphocytes, indicate that this is not directly linked to the absence of GPI moiety on cell surface. The differential basal levels of CXCR2 in neutrophils (high), monocytes (intermediate) compared to HSCs (low) could also have contributed to upregulated expression in GPI-AP population compared with GPI-AP. Recent data from Sinclair et al (2016) indicate that CXCR2 regulates the survival and self-renewal of haematopoietic stem/progenitor cells (HSPCs) and stem cells with overexpression of CXCR2/ CXCL4 have relative survival advantage akin to PIGA mutated HSPCs. Perhaps this means that CXCR2 overexpressing GPI-AP cells have increased self-renewal and, as such, a survival advantage relative to normal neutrophils (GPI-AP) in PNH patients. Additionally, complement activation, which is rampant in PNH, augments neutrophil adhesion and possibly upregulates CXCR2 preferentially on GPI-AP granulocytes. Hosokawa et al (2017) also show the differential expression of CXCR2 was unrelated to anti-complement therapy and, probably, cell intrinsic to the PIGA mutated population, especially granulocytes. Although the data do not support any evidence for growth advantage of PIGA mutant (GPI-AP ) HSPCs, it postulates and proves the role of CXCR2 pathway in expansion of GPIAP mature myeloid populations in PNH relative to both GPI-AP neutrophils and GPI-AP non-myeloid cells, especially lymphocytes. The pathways, either cell intrinsic or extrinsic, leading to clonal dominance of GPI-AP HSPCs is not only critical for understanding the pathophysiology of the disease but also helps devise targeted non-complement based therapeutic strategies to cure PNH or prevents its evolution in patients with aplastic anaemia. A rare but intriguing Correspondence: Dr A G Kulasekararaj, Department of Haematology, Kings College Hospital, Denmark Hill, London SE5 9RS, UK E-mail: austin.kulasekararaj@nhs.net editorial comment