Hereditary Spherocytosis via the SLC4A1 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). Type 4 HS makes up 15-20% of cases with patients presenting with mainly with mild to moderate anemia. A few cases have been associated with severe anemia in type 4 HS (Jarolim et al 1996; Dhermy et al. 1997). Mushroomed-shaped or “pincered” red cells may also appear in addition to spherocytes in peripheral blood smears in type 4 HS (An and Mohandas 2008).

Mutations in the SLCA41 gene have been shown to cause both ovalocytosis and distal renal tubular acidosis. Ovalocytosis is predominant in southeastern-asian populations and is a form of hereditary elliptocytosis that is often asymptomatic. A few affected individuals have been reported to have varying degrees of hemolysis (Reardon et al. 1993; Coetzer et al. 1996). Distal renal tubular acidosis occurs when kidneys fail to remove acids from blood in the urine. Common symptoms include fatigue, kidney stones, nephrocalcinosis, impaired growth, rickets, and muscle weakness (Fry and Karet 2007).

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).

HS type 4 is inherited through an autosomal dominant manner through mutations in the SLC4A1 gene. Missense changes represent over half of the causative mutations in the SLC4A1 gene and can disrupt binding to membrane skeleton proteins, or prevent incorporation of Band 3 protein into the plasma membrane. Nonsense, splice site mutations and small indels leading to loss of Band 3 protein have also been described (Tse and Lux 1999). To date, no gross deletions have been reported in the SLC4A1 gene. Southeast Asian ovalocytosis is due to a 27 bp inframe deletion defined as c.1198_1224 (p.Ala400_Ala408) residing at the cytoplasmic/transmembrane domain of Band 3 (Jarolim et al. 1999). Distal renal tubular acidosis is similarly caused by missense mutation in the SLC4A1 gene non-overlapping with known HS missense mutations (Bruce et al. 1997; Tanphaichitr et al. 1998). The SLC4A1 gene encodes the Band 3 protein, an anion exchanger mediating exchange of chloride for bicarbonate across the plasma membrane. Band 3 protein also links the plasma membrane to the spectrin based cytoskeleton through cytoplasmic interaction with ankryin. This interaction network is essential in regulation cohesion between the plasma membrane and underlying cytoskeletion providing red blood cell flexibility when navigating narrow vasculature (An and Mohandas 2008).

Mutations in the SLC4A1 gene are responsible for 15-20% of HS cases. SLC4A1 is the only known gene involved with ovalocytosis and autosomal dominant distal renal tubular acidosis (An and Mohandas 2008; Fry and Karet 2007). Analytical sensitivity is high as all mutations reported to date are detectable by sequencing.

This test provides full coverage of all coding exons of the SLC4A1 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).

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).