Clinical Features for Heterozygosity of the Common β Globin Gene Frameshift 41/42 Mutation: Identification and Properties of Hb Salouël (β[Phe41Leu;Val59X])

Coding region amino acid substitutions within the globin genes can cause thalassemia, any of a wide variety of pathologic variants or entirely harmless variant haemoglobin. Which occurs depends upon the nature and location of the mutation. Both structural and thalassemia-producing mutations can occur for a unique mutation depending on the mutant sequence produced. Homozygotes for frameshift β 41/42 and compound β 41/42 and beta0-thalassemia produced heterozygotes for frameshift severe symptoms and have a thalassemia major phenotype. Severe phenotype has not been reported in the β 41/42 frameshift heterozygosity and the resulting abnormal hemoglobin has never been studied. We report the β c.126–129del CTTT frameshift mutation (exon1) resulting in a truncated β chain (58 AA) in a heterozygous Vietnamese adult patient who was referred to the hospital for anemia and renal failure The haematological indices were as follows: Hb 9.0g/dL, RBC 4.38×10¹²L, MCH 19.8 pg, MCV 62 fL, and reticulocytes 100 000/mm³ (4.4%). Serum creatinine was 221 mmol/L and creatinine clearance was low (22 ml/mn/1.73m²). Both kidneys were small (L=64mm;l=22mm). Renal biopsy was not performed for this reason. Hemoglobin electrophoresis at alkaline pH did not show any abnormal band while electrophoresis at acidic pH on citrate agar plate demonstrated an abnormal band less cathodal than Hb F. Hb A2 was 5.7%. The elution profile obtained from VARIANT ^™ HPLC system was normal when using the β-thalassemia short program (Bio-Rad Laboratories, Hercules, CA, USA), while an abnormal peak representing 10% of the total amount of Hb was identified in the D/E area when using the HbA1c elution program. Both the isopropanol precipitation and heat stability tests showed a relevant molecular instability of the variant Hb. Heinz body test was positive. The Hb variant was isolated from the unstable component. The seven common α-thalassemia deletion defects as well as triplication were excluded using gap-PCR. Except for Hb Indianapolis, no clear cases of renal failure have ever been reported in association with the heterozygous state for unstable β chain. We have studied the truncated β globin chain in order to understand the instability of the abnormal hemoglobin. We have built a 3D model of the abnormal variant. Using biochemistry techniques and molecular modelling we show that:

• the synthetic 58 amino acids truncated protein was unable to fold in physiological conditions,

• sequence from AA 41 to AA 53 is structured as a primary helix but fully buried and unable to be exposed to the solvent,

• the Phe42Ser substitution is responsible for the major structural defect,

• key residues in position β 42, 44, 45, 48, 49, 51, 54, 55 and 57 are very sensitive for stability of hemoglobin as most of the substitutions for these residues disturb subunit packing in flexible joint probably leading to dissociation at β globin gene with this 4 bp deletion encodes the α1-β1 interface.

The a truncated translational product which is unstable in the erythrocyte β–globin chains. Our data leading to the unbalancing of the α and suggest that this thalassemic hemoglobinopathy may have a variable phenotypic pattern in heterozygotes and may sometimes be associated with haemolytic crisis responsible for late renal failure.