Band 3 Courcouronne: Homozygous Mutation Ser667Phe Causes Severe Hereditary Spherocytosis and Incomplete Distal Renal Tubular Acidosis.
Authors: Delaunay Jean , Toye Ashley , Williamson Rosalind , Khanfar Moudji , Bader-Meunier Brigitte , Cynober Thérèse , Thibault Madeleine , Tchernia Gilbert Jean , Déchaux Michèle , Bruce Lesley 
Published: October 2019

Abstract
We describe the second case of a homozygous mutation in band 3 (anion exchanger 1 (AE1), SLC4A1) causing both hereditary spherocytosis (HS) and distal renal tubular acidosis (dRTA). This new variant differed from the previous homozygous variant (Band 3 Coimbra, Ribeiro et al, Blood2000: 96, 1602) in that a significant amount of band 3 was present in the red cell membrane and the dRTA was incomplete. In the proband, an Algerian male baby, a severe hemolytic anemia rapidly developed following birth. Low hemoglobin (3.5 g Hb/dL) at D12 demanded a first transfusion. Hepato-splenomegaly, marked palor and jaundice were noted. Eight transfusions were administered in the following months. Subtotal splenectomy, performed at the age of 9 months, cancelled the transfusional needs. Veinous blood examined after splenectomy showed band 3 to be reduced to ~35% of normal, as shown by immunoblotting. The other known proteins of the band 3/Rh macrocomplex were also found to be reduced. The parents were first cousins. Both showed mild spherocytosis associated with a mild band 3 deficiency. DNA sequence analysis revealed a novel homozygous mutation: TCC to TTC at codon 667 in exon 16, leading to an amino acid substitution: Ser667Phe, located in proposed transmembrane helix 8. Both parents were heterozygous for the same mutation. Anion transport (sulphate uptake) in the patient’s red cells was ~40% normal, showing that transport specific activity of the mutant band 3 was not affected. The mutant red cell band 3 and kidney band 3 were expressed in Xenopus oocytes, with and without co-expression of glycophorin A (GPA). There was very little chloride transport detected in oocytes expressing either mutant red cell or kidney protein alone, but transport was partially rescued by co-expression of GPA. After birth the child showed a temporary acidosis which spontaneously receded. No nephrocalcinosis has been noted to date. At 2 years of age, an ammonium chloride challenge suggested that the child has incomplete dRTA; over the seven hours of the test the blood bicarbonates decreased down to 15.6 mmoles/L, but urinary pH remained above 5.90. Stable expression of mutant kidney band 3 in non-polarised Madin-Darby canine kidney (MDCK) cells showed that the mutant protein was retained in the endoplasmic reticulum. We are currently investigating the effects of this mutant in polarized MDCK cells. Overall our results suggest that the Ser667Phe does not affect the anion transport function of band 3 but causes a trafficking defect in both red blood cells and kidney cells. The trafficking defect may be less severe in red blood cells where it is probably attenuated by the chaperone-like effect of GPA, which is not expressed in kidney cells. The fact that the hematological manifestations are far more conspicuous than their nephrologic counterpart will be discussed.