PNH and complement gene variants

In this issue of Blood, Prata et al report on rare genetic variants of the complement factor H (CFH) gene, which are overrepresented in patients diagnosed with paroxysmal nocturnal hemoglobinuria (PNH) and also seem to affect hematologic response to standard anti-C5 treatment with eculizumab.¹ 

Germline variants in genes coding for different complement components (eg, C3, complement factor B, etc) or complement regulators (eg, CFH, complement factor I [CFI], etc) have been associated with different complement-mediated diseases, such as atypical hemolytic-uremic syndrome, C3 glomerulopathy, age-related macular degeneration, and transplant-associated microangiopathies. In PNH, the impairment of complement regulation is due to the lack of the 2 surface complement regulators CD55 and CD59.² Systematic studies on germline variants of complement genes in PNH are rare. The only well-documented variants are the C5 polymorphism p.Arg885His, which prevents eculizumab binding, thereby conferring intrinsic resistance to this anti-C5 agent,³ and the hypomorphic variant of CR1, which is associated with increased surface C3 opsonization and poor response to eculizumab due to extravascular hemolysis.⁴ 

In their work, Prata et al systematically screened a sizeable population of 84 PNH patients by next-generation sequencing, looking for germline variants in the complement genes CFH, CFI, membrane cofactor protein, and C3. Both common and rare variants were found. Although common variants were found at expected frequencies, rare variants of CFH were found at significantly higher frequencies than in healthy individuals, suggesting that they may play a role in PNH pathophysiology. In the second part of their study, Prata et al investigated the possible impact of these germline variants on disease outcomes, including pretreatment presentation and response to eculizumab. Whereas the most common variants CFH p.His402Tyr and CR1 p.His1208Arg were not associated with a different disease presentation or response to eculizumab, rare CFH germline variants were associated with statistically significant worse event-free and failure-free survival. Indeed, all patients carrying rare CFH variants had red blood cell transfusion, thrombosis, or increase in eculizumab dose within 15 months from treatment initiation, suggesting a functional role for these gene variants. Development of aplastic anemia or evolution to a myeloid malignancy was not associated with either common or rare complement gene variants.

It is not easy to reconcile the finding of an increased frequency of rare germline CFH variants in PNH patients. Indeed, the well-established dual pathophysiology of PNH implies that (1) a phosphatidylinositol N-acetylglucosaminyltransferase subunit A (PIGA) mutation must occur in somatic hematopoietic stem cells (HSCs) and that (2) these PIGA-mutated HSCs expand over normal hematopoiesis owing to an immune privilege.⁵ This then suggests that 1 of these 2 independent events may be more frequent in patients carrying these CFH variants. It does not seem possible that the inherited CFH variant confers a higher somatic mutation rate to affected cells, and, in any case, genetic instability of the PIGA gene does not contribute to PNH pathogenesis.⁶ Thus, the only alternate possibility, albeit still unlikely, is that these CFH variants might increase the likelihood of the T cell–mediated aplastic anemia underlying the expansion of PIGA-mutated HSCs. The observation that rare CFH variants may impact the response to anticomplement treatment is extremely important, and one may try to interpret their findings based on our knowledge of complement regulation. Endogenous complement regulation is largely individual owing to broad inherited heterogeneity; this interindividual variability may serve as the permissive environment to develop some complement-mediated diseases. But this inherited variability may also shape the clinical phenotype of diseases with an independent pathophysiology, such as PNH, in which complement derangement is the consequence of the disease but not its actual cause.

Endogenous, physiologic regulation of the complement cascade is based on both fluid-phase proteins (CFH and CR1) and membrane-bound proteins (monocyte chemoattractant protein, CD55, and CD59). The pleiotropic role of CFH in complement regulation becomes even more crucial when key players are missing, such as CD55 and CD59 in PNH (see figure). In vitro data suggest that CFH may cooperate with CD55 and CD59 in protecting erythrocytes from complement-mediated lysis, and it may partially protect PNH erythrocytes from immediate lysis.⁷ Recently, impaired CFH recruitment on PNH erythrocytes has been implicated in the pathophysiology of PNH.⁸ However, the physiologic concentrations of CFH are unable to rescue PNH erythrocytes from lysis,⁷ and prevention of MAC-mediated lysis can be observed only with supraphysiologic concentrations (approximately 10-fold higher) or with engineered proteins merging CFH regulatory domains with C3-binding structures, which may increase its affinity for host surface (eg, TT30 and mini-FH).⁹ These observations seem in agreement with the finding of this study, given that CFH variants with hypothetical impaired function may not necessarily impact the disease phenotype, owing to the dominant role of CD55 and CD59 deficiency. In contrast, functional differences in complement regulation may emerge once the lack of CD59 is overcome by eculizumab. In this scenario, even subtle differences in the regulation of the alternative pathway may lead to different extent of surface C3 activation and C3d deposition,¹⁰ eventually accounting for different degrees of C3-mediated extravascular hemolysis (as demonstrated for CR1⁴ but still not proven for these CFH variants). Interestingly, as novel strategies of complement therapeutics are in their advanced development,⁹ the next question is how CFH variant may affect the efficacy of proximal complement inhibitors. Because all proximal inhibitors target key components of the alternative pathway, aiming to intercept the same crucial step regulated by CFH (ie, the C3 convertase activity), one would anticipate that functional differences in CFH (and CR1) activity may remain neutral. However, functional differences in CFH (and CR1) activity may result in pharmacodynamic differences leading to breakthrough hemolysis (the most feared complication when proximal inhibitors are used as monotherapy).⁹ 

In conclusion, it seems conceivable that modest differences in CFH activity may affect the clinical presentation of PNH and eventually shape the hematologic response to anticomplement treatment. Further functional data are needed, especially to anticipate how inherited variants of CFH (and of other complement regulator genes) may impact the response to the novel complement proximal inhibitors.

Conflict-of-interest disclosure: The author declares no competing financial interests.