Using a Next Generation Sequencing Panel to Discover the Obscure Causes of Hereditary Hemolytic Anemias

Hereditary hemolytic anemias are a heterogeneous group of disorders with consequences ranging from non-anemic hemolysis to severe life-threatening anemia. However, the late morbidity in patients without transfusions is often underappreciated because of erythropoietic compensatory stimulation inducing hematopoiesis by erythroferrone/hepcidin axis. Principal causes of hereditary hemolytic anemias are germline mutations of red cell cytoskeleton (e.g. hereditary spherocytosis and elliptocytosis/pyropoikilocytosis) or enzyme deficiencies (e.g. Glucose 6 phosphate dehydrogenase deficiency and pyruvate kinase deficiency). Routine morphological and biochemical analysis may be inconclusive and misleading particularly in transfusion-dependent infants and children. Molecular studies have not been extensively used to diagnose these disorders due to the complex genetic nature of these disorders, and multi-gene disorders. In these cases, patients may undergo multiple rounds of single gene testing, which can be very costly and time consuming. The advent of next generation sequencing (NGS) methods in the clinical laboratory has made diagnosing complex genetic disorders feasible. Our diagnostic panel includes 28 genes encoding cytoskeletal proteins and enzymes, and covers the complete coding region, splice site junctions, and, where appropriate, deep intronic or regulatory regions. Targeted gene capture and library construction for next-generation sequencing (NGS) was performed using Sure Select kit (Agilent Technologies, Santa Clara, USA). Prior to sequencing on the Illumina Next Seq, (Illumina Inc) instrument, indexed samples are quantified using qPCR and then pooled. Samples were sequenced using 2x150 paired end sequencing. We now report the first 68 patients evaluated using our NGS panel. The age of the patients ranged from newborn to 62 years. These patients presented with symptoms ranging from mild lifelong anemia to severe hemolytic anemia with extreme hyperbilirubinemia. Genetic variants were classified using the American College of Medical Genetics (ACMG) guidelines. We identified pathogenic variants in 11 patients and likely pathogenic variants in 12 others, the majority of these were novel. Many variants with unknown significance were also identified that could potentially contribute to disease. The most commonly mutated genes were SPTB and SPTA1, encoding spectrin subunits. Some complex interactions were uncovered i.e. SPTA1 mutations along with alpha LELY leading to hereditary pyropoikilocytosis; Spectrin variants along with Gilbert syndrome causing severe hyperbilirubinemia in neonates; and Spectrin variants in combination with PKLR and G6PD variants. Our results demonstrate that many patients with hemolytic anemia harbor complex combinations of known and novel mutations in RBC cytoskeleton/enzyme genes, but their clinical significance is further augmented by polymorphisms of UGT1A1 gene contributing to severe neonatal hyperbilirubinemia and its consequences. To conclude, next-generation sequencing provides a cost-effective and relatively rapid approach to molecular diagnosis, especially in instances where traditional testing failed. We have used this technology successfully to determine the molecular causes of hemolytic anemia in many cases with no prior family history.