Novel Genetic Variants in Complement-Mediated Thrombotic Microangiopath

BACKGROUND: Complement-mediated thrombotic microangiopathies (TMAs) are a subset of TMAs in which the thrombocytopenia, microangiopathic hemolytic anemia, and end organ damage are caused by mutations in genes encoding the alternative complement pathway. Numerous complement genes have been implicated in complement-mediated TMAs including complement factor H (CFH), CD46 (MCP), complement factor I (CFI), complement component 3 (C3), complement factor B (CFB), factor H related 5 (CFHR5), and thrombomodulin (THBD) among others.  Genetic analysis typically yields a mutation in 60% of patients whose TMA is presumed to be mediated by complement dysregulation.  However, the description of novel disease-associated variants may increase this proportion.

METHODS:  A retrospective study of patients with TMAs diagnosed between 2000 and 2014 was performed.  TMA diagnosis was made based on thrombocytopenia and evidence of microangiopathic hemolytic anemia.  Analysis was performed with Alamut¨ software with additional in silico prediction tools (SIFT, MutationTaster, and Polyphen) for classification of gene variants.  Variants of unknown significance (VUS) and likely pathogenic variants were further assessed using several mutation databases, including HGMD, ClinVar, and Factor H database

RESULTS:  Of patients diagnosed with a TMA, genetic analysis was performed in only a 10% of patients.  Of the 29 patients with genetic studies performed, mutations were identified in 18 patients (62%).  The majority of the mutations had been described previously in the literature, but four novel variants were identified: three missense and one splice-site.  The table below summarizes these variants as well as laboratory findings on presentation.  These were two variants of CFH, one variant of CFHR5, and one variant of CFI.  In silico modeling of these variants revealed two polymorphisms likely to be pathogenic, one polymorphism likely benign given the lack of predicted splicing changes, and one VUS.

Protein	Mutation	Classification	Age	Sex	Hemoglobin (g/dL)	Platelets (thousands)	Creatinine (mg/dL)
CFH	c.245-10_245-9dup	Likely benign	61	F	11.6	51	6.7
CFH	c.476G>A, p.Ser159Asn	Likely pathogenic	43	F	9.1	101	9.1
CFHR5	c.1412G>A p.Gly471Glu	Likely pathogenic	30	F	9.8	129	4.9
CFI	c.1190T>A p.Val397Glu	Unknown significance	51	M	9.4	125	6.1

DISCUSSION:  With therapy available to target the alternative complement pathway, genetic analysis to identify genetic variants capable of causing complement mediated TMAs is an essential part of the evaluation.  This genetic data must be interpreted and correlated with functional analysis and clinical phenotype.  The reporting of novel variants in clinical databases, with inclusion of relevant clinical findings, is necessary to accurately classify and verify variants as pathologic mutations or benign polymorphisms.  The full understanding of this diverse disease requires a more complete understanding of its genetics.  While complement pathway-directed therapies are available, their rational use requires thorough interpretation of laboratory data, including genetic analysis.