Page 27 - 2019_06-Haematologica-web

Sickle cell trait is not completely harmless
tain conditions can be irreversible, leading to distortion of the erythrocyte and loss of deformability, ultimately causing vaso-occlusion in the microvasculature and hemolysis. The tendency for HbS to polymerize is highly dependent on the hemoglobin composition in the ery- throcyte – mainly the concentration of intracellular HbS, as well as the concentration and type of non S hemoglo- bin. HbA and HbF present in the cell reduce the concen- tration of HbS, but HbF additionally has an inhibitory effect on HbS polymerization. Individuals with SCT, who have HbA in excess of HbS (30-40% of the total hemoglo- bin), do not typically suffer from the effects of sickling and are asymptomatic. Apart from the intracellular hemo- globin composition, other factors that influence the like- lihood of HbS polymerization include oxygen saturation, intracellular pH and 2,3-diphosphoglycerate levels. Environmental and co-inherited genetic factors can change these parameters, modulating the kinetics of HbS polymerization in individuals with SCT.
Clinically symptomatic HbAS individuals could be con- sidered to fall under four genetic categories: (i) co-inheri- tance of HbS with a genetic modifier; (ii) dominant forms of HbS alleles; (iii) apparent heterozygosity for HbS; and (iv) non-Mendelian inheritance of HbS.
Co-inheritance of HbS with a genetic modifier
Compound heterozygosity for HbS and genetic vari- ants causing non-hemoglobin red cell (i.e. membrane and enzyme) disorders, while uncommon, can have impor- tant modifying effects on the clinical outcome of SCT. Two cases of HbS carriers experiencing typical SCD com- plications of chronic hemolytic anemia, recurrent acute pain, recalcitrant leg ulceration and end-stage renal dis- ease, have been reported.46,47 Each of the probands had co- inherited mutations in the PKLR gene, causing a deficien- cy of pyruvate kinase protein. Pyruvate kinase is a key enzyme in the final step of glycolysis; it converts phos- phoenolpyruvate to pyruvate, generating 50% of the total red cell ATP that is essential for metabolism of the red blood cell. Of particular relevance to SCD, a reduction in pyruvate kinase activity also leads to accumulation of the upstream enzyme substrates, including 2,3-diphospho- glycerate which decreases oxygen affinity, favoring poly- merization of deoxy-HbS. In one case, fresh blood was available for in-vitro functional assays that supported the role of 2,3-diphosphoglycerate as a key factor in HbS polymerization.46
Another example is the co-inheritance of hereditary spherocytosis, implicated as the cause of splenic infarc- tion in a handful of SCT cases.48 The presence of heredi- tary spherocytosis is believed to increase mean corpuscu- lar hemoglobin concentration and intracellular HbS con- centration, thereby increasing the propensity to HbS polymerization. Along the same lines, glucose-6-phos- phate dehydrogenase deficiency, a common red cell enzyme disorder in individuals of African ancestry, might be expected to increase hemolysis and modify the clinical phenotype of SCD and SCT. However, there is a lack of evidence addressing co-inheritance of SCT and glucose-6- phosphate dehydrogenase deficiency, and even the evi- dence for SCD is mixed, with no consistent differences in red cell indices, degree of anemia, hemolysis, frequency of vaso-occlusive complications, or stroke risk in subjects who have co-inherited glucose-6-phosphate dehydroge- nase deficiency.49-51 These conflicting reports could be
related to limitations of the methodologies used for the enzyme assays, or the panel of glucose-6-phosphate dehydrogenase variants genotyped. These reports serve as an important caveat when examining past anecdotal evidence of complications in SCT, as in-depth genetic investigation is often lacking in reported cases of sympto- matic SCT.
Dominant forms of HbS alleles
Rare HbS alleles that behave dominantly have been reported. One example is that found in a 19-month old girl, identified as having SCT through newborn screening; she developed a vaso-occlusive crisis with splenic seques- tration during a flight and required splenectomy.52 Subsequent DNA sequencing analysis confirmed that the baby had inherited a maternal normal HbA allele but had acquired a new mutation, bLeu68Phe, cis to the paternal HbS allele. bLeu68Phe has previously been identified as Hb Rockford, a known Hb variant with reduced affinity for oxygen. The double mutant, named Hb Jamaica Plain (JP, bGlu6Val,Leu68Phe), has similar electrophoretic motility as HbS, hence it was missed at the newborn screening. The bLeu68Phe mutation in Hb JP causes it to desaturate easily at lower oxygen tension, thus polymerizing more readily than typ- ical HbS, converting it to a dominant mutation.52
Two other Hb variants with double mutations, HbS Antilles (bGlu6Val, Val23Ile) and HbS-Sao Paolo (bGlu6Val, Lys65Glu), also promote polymerization through reduced oxygen affinity in heterozygotes.53,54 In addition, HbS Antilles has lower solubility and HbS-Sao Paolo forms more stable polymers than HbS, potentially further enhancing irreversible poly- merization and red blood cell sickling. In the case of HbS Antilles, family studies led to the identification of 24 HbA/S Antilles individuals in the proband’s family, many of whom had recurrent sickle pain crises, chronic hemolytic anemia, and splenomegaly,54 a phenotype sim- ilar to that of HbSC disease.
Apparent heterozygosity for HbS
Compound heterozygotes of bS and very mild b-tha-
lassemia mutations (bS/b++-thalassemia) can appear as
having SCT, but the giveaway here is an excess of HbS
over HbA. A 38-year old, previously healthy man, pre-
sented with a 6-month history of worsening pruritis,
jaundice and ascites. Extensive work-up for causes of liver
disease was negative, but hemoglobin electrophoresis
showed 49.6% HbS and 41.3% HbA. The patient had not
received any blood transfusion. While this result could
easily be misinterpreted as HbS trait, given the slight
increase of HbS over HbA, DNA and family studies were
pursued. He was revealed as a compound heterozygote S
for b and a novel, very mild b-thalassemia mutation (b IVS2-844 C→A) that was transmitted to both of his sons, and the liver pathology was ascribed as sickle-related.55
Another diagnostic conundrum is the discrepant find- ings of HbSS on hemoglobin electrophoresis but HbAS on genetic testing. Two such cases have been reported, where the individuals presented with typical hematolog- ic and clinical phenotypes of SCD.56 In both instances, DNA testing showed that the individual possessed one HbA and one HbS allele, but expression of the HbA allele was abolished by a deletion of the upstream b locus con- trol region, resulting in sole expression of the HbS allele and, thereby, a functionally homozygous HbS pheno- type.56
haematologica | 2019; 104(6)
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