Hereditary Spherocytosis/Elliptocytosis via the SPTA1 Gene

Hereditary Spherocytosis (HS), also known as Minkowski-Chauffard disease, affects one in 2,000 individuals. HS is a condition where red blood cells lose their typical biconcave disc shape and appear spherical. The spherical appearance impairs membrane flexibility making it hard for red blood cells to transverse narrow capillaries, especially in the spleen. This impairment causes anemia due to chronic extravascular hemolysis, jaundice, formation of bilirubin gallstones, reticulocytosis and splenomegaly characteristic of HS disease (Aster et al. 2013; An and Mohandas 2008). Disease severity can range with 20-30% having mild form, 60-70% having moderate form, and 10-20% having severe form of HS. People with mild forms may be asymptomatic whereas severe forms of the disease present in newborns with life threatening anemia and require blood transfusions. There are five types of HS defined by the gene mutation causative for disease: Type I-ANK1, type 2-SPTB, type 3-SPTA1, type 4-SLC4A1, and type 5-EPB42 (Bolton-Maggs et al. 2004; Delaunay 2007). Patients with mutations in the SPTA1 gene tend to have severe HS (Wichterle et al 1996).

Hereditary Elliptocytosis (HE) is a milder red blood cell membrane disorder affecting one in 5,000 individuals. Red blood cells in these patients are elongated into cigar or oval shape with flexibility being impaired less than individuals with HS. The majority of patients are asymptomatic with ~10% having moderate to severe anemia and intermittent episodes of hemolysis, jaundice, and splenomegaly. Symptoms may present at 4-6 months in severe cases but usually resolve by 6-12 months. Severe HE may present with hereditary pyropoikilocytosis with newborns presenting with hemolytic anemia and requiring frequent blood transfusions. Elliptocytosis may be prominent in other disorders including iron deficiency, leukemia, megaloblastic anemia, myelofibrosis, sickle cell disease, thalassemia, and polycythemia. Therefore, genetic testing is helpful in differential diagnosis of these diseases (Gallagher 2004).

HS in inherited in an autosomal dominant manner in 75% of cases through mutations in the ANK1, SPTB, and SLC4A1 genes. Autosomal recessive forms are inherited through mutations in the ANK1, SPTA1, and EPB42 genes (Bolton-Maggs et al. 2004). Most mutations reported to date are private with de novo dominant mutations being six times more common than recessive mutations (Miraglia del Giudice et al. 2001). Mutations in the ANK1, SPTB, SLC4A1, SPTA1, and EPB42 genes account for 60%, 10%, 15%, 10%, and 5% cases of HS respectively (An and Mohandas 2008).

HE is inherited in an autosomal dominant manner through mutation in the SPTA1 (65% of cases), SPTB (30% of cases), or EPB41 (5% of cases) genes. HE with hereditary pyropoikilocytosis is caused by homozygous recessive or compound heterozygous mutations in SPTA1, SPTB, and EPB41 genes. De novo mutations are rarely reported (Gallagher 2004).

HS type 3 is inherited in an autosomal recessive manner and HE in an autosomal dominant manner through mutations in the SPTA1 gene. Together the SPTA1 and SPTB genes encode alpha and beta spectrin proteins, respectively. These proteins form head to tail helical dimers and link red blood cell membrane proteins to actin to maintain membrane elasticity (Gimm et al 2002). Missense mutations at the N-terminus of α-spectrin are most common for both HS and HE and disrupt interactions with β-spectrin (Gaetani et al. 2008). Splicing mutations, most notably the c.4339-99C>T known as α-spectrin LEPRA, are the second most common group of causative mutations and result in premature protein termination (Wichterle et al. 1998). Nonsense mutations and large deletions have each only been reported in one case (Iolascon et al. 2011; Tolpinrud et al. 2008).

Mutations in the SPTA1 gene are causative for ~5% of HS and 65% of HE cases (Ah and Mohandas 2008; Gallagher 2004). NextGen sequencing analytical sensitivity is >95% for detection of causative mutations. Gross deletions have been reported in only a single case (Iolascon et al. 2011).

This test provides full coverage of all coding exons of the SPTA1 gene plus 10 bases of flanking noncoding DNA in all available transcripts along with other non-coding regions in which pathogenic variants have been identified at PreventionGenetics or reported elsewhere. We define full coverage as >20X NGS reads or Sanger sequencing. PGnome panels typically provide slightly increased coverage over the PGxome equivalent. PGnome sequencing panels have the added benefit of additional analysis and reporting of deep intronic regions (where applicable).

Testing includes detection of the c.4339-99C>T, α-spectrin LEPRA variant.

Dependent on the sequencing backbone selected for this testing, discounted reflex testing to any other similar backbone-based test is available (i.e., PGxome panel to whole PGxome; PGnome panel to whole PGnome).