Silent carrier beta-thalassaemia due to a severe beta-globin mutation interacting with other genetic elements

Clinical Classification, Screening, and Diagnosis in Beta-Thalassemia and Hemoglobin E/Beta-Thalassemia

This article reviews the classification of beta-thalassemia syndromes, correlating clinical severity and genotype in the earlier classification, and broadening it recently based on clinical severity and transfusion status. The classification is dynamic, and individuals may progress from transfusion-independent to transfusion-dependent. Early and accurate diagnosis prevents delays in instituting treatment and comprehensive care, and precludes inappropriate and potentially harmful interventions. Screening can inform risk in an individual and subsequent generations when partners may be carriers as well. This article discusses the rationale for screening of the at-risk population. In the developed world, a more precise genetic diagnosis must be considered.

Diversity of alpha-globin mutations and clinical presentation of alpha-thalassemia in Israel

Alpha-thalassemia is among the world's most common single gene disorders, caused primarily by gene deletions. In Israel, where alpha(o)-trait thalassemia is uncommon, it is of particular importance because of its phenotypic interactions with beta-thalassemia in hetero- and homozygotes. In a study of 232 individuals referred for molecular evaluation of anemia, 303 chromosomes carried alpha-globin gene abnormalities; 6 gene rearrangements and 11 point mutations were identified. This unexpected heterogeneity is in part due to the many ethnic subgroups represented by these patients. Our findings include nine unique Israeli alleles, 3 of which are described here for the first time. An equal number of point mutations was found in the alpha2-globin gene as compared to alpha1. A threonine deletion in codon 39 of the alpha1-globin gene, found frequently in Arabs, is unique to Israel and probably represents one of several indigenous alleles. Among Arabs, point mutations were more frequent than large deletions. Surprisingly, in Ashkenazi Jews, who resided for many centuries in a nonmalarial environment, a single alpha-globin gene deletion -alpha(3.7) was found in many cases. The clinical presentation of individuals carrying two or more alpha-globin lesions was highly variable. In general, the severity correlated inversely with the number of functional alpha-globin genes. In some cases, impairment of two alpha-globin genes by point mutations led to a thalassemia-intermedia-like picture which could be misdiagnosed as beta-thalassemia. We conclude that alpha-thalassemia is phenotypically and genotypically more heterogeneous than previously recognized. DNA analysis is invaluable as it provides a specific diagnosis and enables reliable genetic counseling.

Patterns of variant polyadenylation signal usage in human genes

The formation of mature mRNAs in vertebrates involves the cleavage and polyadenylation of the pre-mRNA, 10-30 nt downstream of an AAUAAA or AUUAAA signal sequence. The extensive cDNA data now available shows that these hexamers are not strictly conserved. In order to identify variant polyadenylation signals on a large scale, we compared over 8700 human 3' untranslated sequences to 157,775 polyadenylated expressed sequence tags (ESTs), used as markers of actual mRNA 3' ends. About 5600 EST-supported putative mRNA 3' ends were collected and analyzed for significant hexameric sequences. Known polyadenylation signals were found in only 73% of the 3' fragments. Ten single-base variants of the AAUAAA sequence were identified with a highly significant occurrence rate, potentially representing 14.9% of the actual polyadenylation signals. Of the mRNAs, 28.6% displayed two or more polyadenylation sites. In these mRNAs, the poly(A) sites proximal to the coding sequence tend to use variant signals more often, while the 3'-most site tends to use a canonical signal. The average number of ESTs associated with each signal type suggests that variant signals (including the common AUUAAA) are processed less efficiently than the canonical signal and could therefore be selected for regulatory purposes. However, the position of the site in the untranslated region may also play a role in polyadenylation rate.