Hemoglobin SC disease and alpha-thalassemia. Prolonged survival and mild clinical course

Outcome in hemoglobin SC disease: a four-decade observational study of clinical, hematologic, and genetic factors

Over the past 40 years, we observed 284 subjects with hemoglobin SC disease (Hb SC) for 2,837 person-years. We examined the association of the course of clinical events with hematologic and genetic factors. The mean entry age was 21 years, although 15% entered before one year of age. The mean Hb concentration was 11.3 g/dL, the mean fetal hemoglobin was 2.5%, and the mean MCV was 84.4 fL. Twenty-five subjects died at a median age of 37 years. Chronic organ-specific complications occurred in 112 subjects (39.4%), with advanced retinopathy in 65 subjects (22.9%) and osteonecrosis (avascular necrosis) in 42 subjects (14.8%). We identified the beta-globin haplotypes in 82 subjects and the alpha-gene status in 79. Twenty-nine percent had alpha-thalassemia-2. The beta(CI) haplotype was present in 85.4%. We found a decreased incidence of retinopathy in the beta(CI) subjects compared to the non-beta(CI) subjects (33% vs. 67%; P = 0.049) with a later mean onset age (29 years vs. 21 years; log-rank test, P= 0.026). We also found a consistent pattern of decreased morbidity in subjects who had alpha-thalassemia-2 in comparison to those who did not. We found a reduced risk of chronic organ-specific complications (log-rank test, P= 0.003), lower incidence of sickle crisis (48% vs. 80%, P= 0.001), later onset of gallbladder disease (age of onset: 55 years vs. 34 years; P= 0.055), and lower risk of osteonecrosis (log-rank test, P= 0.024). Our findings suggest that Hb SC subjects who have not inherited alpha-thalassemia-2 might benefit from erythrocyte rehydration therapy.

Rapid differentiation of five common alpha-thalassemia genotypes by polymerase chain reaction

The alpha-thalassemias are common genetic disorders that arise from reduced synthesis of the alpha-globin chains. At present, large-scale carrier screening and clinically valuable antenatal detection programs have not been established for the congenital disorder alpha-thalassemia (alpha-thal). We have developed a simple nonradioactive polymerase chain reaction (PCR) approach that can detect and differentiate several common alpha-globin gene deletional alpha-thals regardless of the break points. When three primer sets were used--two gene-specific sets for the alpha1- and alpha2-globin genes and one set for the beta-actin gene (serving as an internal control)--PCR products from genomic DNA were simultaneously amplified and analyzed after coamplification and gel electrophoresis. The number of alpha-globin genes present in the subjects was determined by the intensity of alpha1 and alpha2 bands normalized with that of beta-actin when using densitometry. Our results demonstrate that five common genotypes of deletional alpha-thal are differentiated by the ratios of alpha1/beta-actin and alpha2/beta-actin. We also examined the feasibility of coupling this allele-specific amplification to a color-complementary assay. This easy and reproducible PCR assay is suitable for identifying alpha-thal carriers in screenings of large populations and improving genetic counseling.

Genetic and hematological studies in a group of 114 adult patients with SC sickle cell disease

The clinical and biological heterogeneity of sickle cell hemoglobin (Hb) C disease (SC disease) is similar to sickle cell anemia, but has a much milder course. The effect of genetic factors such as alpha thalassemia or beta-globin gene haplotype has been analyzed in a limited number of cases. In this work, we report about 114 adult SC patients, aged 15 to 65 years (M/F = 0.93). The frequency of deletional alpha thalassemia (alpha(-3.7)) was found to be about 35%. The coinheritance of an alpha-thalassemia trait with SC disease had no effect on the hemoglobin level but hemolysis was significantly reduced. In these patients, as described for homozygous Hb S individuals, the Hb F level was higher in females than in males and in individuals carrying the beta(s)-Senegal haplotype. This haplotype involves the presence of an Xmnl site 5' to Ggamma, which is considered responsible for an increased Ggamma/Agamma ratio. Our survey showed that some genetic factors may modulate hematological parameters in SC disease.