Estudo de hemoglobinas variantes com mobilidadeseletroforética semelhante à da hemoglobina S emcrianças do programa de triagem neonatal de MinasGerais (PTN-MG)

Abstract

Some hemoglobin (Hb) variants have similar electrophoretic mobility to that of Hb S (S-like), which can lead to false diagnosis of sickle cell trait or disease. Most individuals with Hb variants are asymptomatic, but in several cases the Hb is associated with alpha-thalassemia and may result in hematological changes. The association of a Hb variantwith the Hb S or Hb C may result in complex clinical and/or laboratory features. Objective and Methods: The aim of this study was to determine the frequency of S-like Hb variants detected in the Newborn Screening Program in Minas Gerais and to assess the clinical relevance of these Hbs. The DNA from 118 children was tested for the bS allele. Alphathalassemia types 3.7 or 4.2 were tested in 112 children in 56 cities in Minas Gerais by mutiplex gap-PCR. The products of this PCR were then digested with restriction endonucleases to detect the mutations corresponding to Hb Stanleyville-II and Hb Hasharon, both already reported in Brazil. In the remaining cases sequencing of HBB or HBA genes was necessary to identify the Hb variant. Clinical data were obtained from children who attended medical consultations at Hemominas Foundation. Results: Of the 118 children, only six werepositive for the bS allele without any other variant. Among the remaining 112 children there were two children (1.8%) with homozygous Hb Stanleyville-II (-3.7; Stanleyville/-3.7; Stanleyville), 94 (84%) with heterozygous Stanleyville-II (genotypes: 84 / - a3.7; Stanleyville, five - a3.7 / - a3.7; Stanleyville and five aStanleyvillea/aa), and six heterozygous Hb Hasharon (all aa / -a 3.7; Hasharon). Five cases with Hb Ottawa, one with Hb St. Luke's and one with Hb Etobicoke, all - chain variants in heterozygosis and genotype /, were also identified. Hb G-Ferrara was identified in two children (one /-3.7 and the other aa/aa) and Maputo Hb ( / -a3.7) in one, both b variants in heterozygosis. Among the above mentioned Hbs, there were five associations with the bS allele: three children with Hb Stanleyville-II, one with Hb Ottawa and one with Hb St Luke's. Hb Etobicoke was found in a child who has a hybrid 212 gene. There was no clinical change due to these associations. In several children with Hb Stanleyville-II, all from African ancestry, microcytosis and hypochromia were observed because of the association with a 3.7 alpha thalassemia gene. PCR-RFLP (Restriction Fragment LengthPolymorphism) with specific endonucleases confirmed all mutations underlying the variant Hbs, with the exception of Hb G-Ferrara, for which no diagnostic endonuclease was found. Conclusions: 1. Hemoglobin Stanleyville-II is relatively common in Minas Gerais (incidenceof 1:11,500); 2. Hbs Stanleyville-II, Ottawa, Maputo, G-Ferrara, Hasharon, Etobicoke and St. Luke's can lead to false diagnosis of sickle cell trait or disease in newborn screening, unless additional tests for a correct diagnosis are used; 3. These Hbs seem to have no clinical relevance, even when found in combination with the bS allele, as happened in this study with the Hb St Luke's, Stanleyville-II, and Ottawa; 4. Hypochromia and microcytosis detected in several children are due to the co-inheritance of -thalassemia and not to the presence of HbStanleyville-II; 5. Asymptomatic cases of heterozygous Hb Maputo and Hb G-Ferrara were identified. Genetic counseling about the possibility of having children or grandchildren with sickle cell disease if the bS allele is co-inherited was given to these families; 6. Specific PCRRFLPmay be used as a diagnostic tool for all variant Hbs identified in the present study, except for Hb G-Ferrara.