Molecular defect of truncated beta-spectrin associated with hereditary elliptocytosis. Beta-spectrin Gottingen

Hereditary elliptocytosis: spectrin and protein 4.1R

Hereditary elliptocytosis (HE) is a common disorder of erythrocyte shape, occurring especially in individuals of African and Mediterranean ancestry, presumably because elliptocytes confer some resistance to malaria. The principle lesion in HE is mechanical weakness or fragility of the erythrocyte membrane skeleton due to defects in alpha-spectrin, beta-spectrin, or protein 4.1. Numerous mutations have been described in the genes encoding these proteins, including point mutations, gene deletions and insertions, and mRNA processing defects. Several mutations have been identified in a number of individuals on the same genetic background, suggesting a "founder effect." The majority of HE patients are asymptomatic, but some may experience hemolytic anemia, splenomegaly, and intermittent jaundice.

Regulation of alternative pre-mRNA splicing during erythroid differentiation

Although the mature enucleated erythrocyte is no longer active in nuclear processes such as pre-mRNA splicing, the function of many of its major structural proteins is dependent on alternative splicing choices made during the earlier stages of erythropoiesis. These splicing decisions fundamentally regulate many aspects of protein structure and function by governing the inclusion or exclusion of exons that encode protein interaction domains, regulatory signals, or translation initiation or termination sites. Alternative splicing events may be partially or entirely erythroid-specific, ie, distinct from the splicing patterns imposed on the same transcripts in nonerythroid cells. Moreover, differentiation stage-specific splicing "switches" may alter the structure and function of erythroid proteins in physiologically important ways as the cell is morphologically and functionally remodeled during normal differentiation. Derangements in the splicing of individual mutated pre-mRNAs can produce synthesis of truncated or unstable proteins that are responsible for numerous erythrocyte disorders. This review will summarize the salient features of regulated alternative splicing in general, review existing information concerning the widespread extent of alternative splicing among erythroid genes, and describe recent studies that are beginning to uncover the mechanisms that regulate an erythroid splicing switch in the protein 4.1R gene.

Spectrin mutations in hereditary elliptocytosis and hereditary spherocytosis

Hereditary elliptocytosis (HE), its aggravated form hereditary pyropoikilocytosis (HPP), and hereditary spherocytosis (HS) designate a set of congenital hemolytic syndromes. The responsible mutations lie in several genes encoding proteins of the red cell membrane. In particular, they involve the SPTA1 and SPTB genes that encode erythroid spectrin alpha- and beta-chains, respectively. In situ, spectrin is a alpha 2 beta 2 fibrillar tetramer resulting from the head-to-head self-association of two alpha beta dimers. In HE, the 24 known alpha-chain mutations lie in the self-association site or its vicinity, whereas the 17 beta-chain mutations occur in the self-association site itself (record of November 30, 1995). Allele alpha LELY (LELY: Low Expression LYon) is found in ethnic groups remote from one another with a uniform frequency (20-30% of all alpha-alleles). It allows an expanded expression of any HE alpha-allele located in trans and results in severe HE or in HPP. In HS, a number of spectrin mutations have been recorded recently. Allele alpha LEPRA (LEPRA: Low Expression PRAgue) would occur in a recurrent fashion.

Beta spectrin PRAGUE: a truncated beta spectrin producing spectrin deficiency, defective spectrin heterodimer self-association and a phenotype of spherocytic elliptocytosis

Spherocytic elliptocytosis is a phenotypic hybrid between hereditary spherocytosis (HS) and hereditary elliptocytosis (HE) characterized by the presence of spheroovalocytes and spherocytes which exhibit increased osmotic fragility, indicating a deficiency of surface area. Both the spherocytic red cell morphology and the increased osmotic fragility distinguish this clinical entity from common HE. In contrast to common HE, the molecular basis of spherocytic elliptocytosis is unknown. Here we describe two members of a family who both have the characteristic features of spherocytic HE. We show that the underlying defect involves a G to C transversion at the -1 position of the acceptor splice site upstream of exon X of beta spectrin leading to skipping of exon X from the mutant beta spectrin mRNA allele. The mutant mRNA is present in reticulocytes in similar amounts as the normal mRNA. Pulse-labelling of erythroblasts prepared from peripheral blood in a two-phase liquid-culture system reveals a decreased synthesis of the truncated beta spectrin, a finding which is likely to underlie the moderately severe spectrin deficiency in the two patients. In addition, this mutant spectrin, similar to the previously reported spectrins, is defective in spectrin heterodimer self-association. The spectrin deficiency, which represents a common finding in the majority of patients with HS, together with weakened spectrin heterodimer self-association, as found in the majority of patients with common HE, provides a molecular explanation for the phenotype of spherocytic elliptocytosis in this kindred and, most likely, in other patients carrying similar beta spectrin mutations.

Hereditary elliptocytosis associated with spectrin Le Puy in a Japanese family: ultrastructural aspect of the red cell skeleton

A dominantly-inherited hereditary elliptocytosis of intermediate severity was recorded in a Japanese family from Yamagata. The condition was associated with a spectrin truncated beta-chain (MW: 214 kD; 31% of total beta-spectrin), and a defect of mutant spectrin as regards tetramerization and phosphorylation. cDNA analysis revealed skipping of exon X, the third-to-last exon of the spectrin beta-gene. At the gene level, a one-base substitution (A-->G) changed position +4 of the 5' donor splice site consensus sequence of intron X. This mutation has been described before in a French kindred, defining spectrin Le Puy. Electron micrographs following quick-freeze deep-etching showed that the skeletal network was disorganized.

Severe poikilocytosis associated with a de novo alpha 28 Arg-->Cys mutation in spectrin

Severe poikilocytosis was observed in an Italian child. The mutation responsible was a de novo alpha 28 Arg-->Cys substitution (CGT-->TGT) in spectrin, a mutation known to cause hereditary elliptocytosis or hereditary pyropoikilocytosis. In this particular case the severity of the manifestations were accounted for by the occurrence, in trans to the alpha 28 mutation, of the alpha V/41 polymorphism. The latter has been shown previously to be associated with structural abnormalities at the alpha IV-alpha V domain junction and with a low expression level. The pronounced alteration of the dimer self association process was also explained by the location of the alpha 28 mutation. This mutation occurs in helix 3 of repeating segment alpha 1, e.g. precisely in the head-to-head contact between the spectrin alpha and beta chains. The present phenotype was compared to that yielded by another alpha 28 mutation (Arg-->His) also combined, in trans, with the alpha V/41 polymorphism. The pictures were very much alike, stressing the functional importance of residue alpha 28. The de novo character of the present mutation strengthens the view that codon alpha 28 is a 'hot spot' for mutations.