A Large Deletion Within the Protein 4.1 Gene Associated With a Stable Truncated mRNA and an Unaltered Tissue-Specific Alternative Splicing

Cytoskeletal Protein 4.1R in Health and Diseases

The protein 4.1R is an essential component of the erythrocyte membrane skeleton, serving as a key structural element and contributing to the regulation of the membrane's physical properties, including mechanical stability and deformability, through its interaction with spectrin-actin. Recent research has uncovered additional roles of 4.1R beyond its function as a linker between the plasma membrane and the membrane skeleton. It has been found to play a crucial role in various biological processes, such as cell fate determination, cell cycle regulation, cell proliferation, and cell motility. Additionally, 4.1R has been implicated in cancer, with numerous studies demonstrating its potential as a diagnostic and prognostic biomarker for tumors. In this review, we provide an updated overview of the gene and protein structure of 4.1R, as well as its cellular functions in both physiological and pathological contexts.

Alternative polyadenylation in a family of paralogous EPB41 genes generates protein 4.1 diversity

Alternative polyadenylation (APA) is a step in mRNA 3'-end processing that contributes to the complexity of the transcriptome by generating isoforms that differ in either their coding sequence or their 3'-untranslated regions (UTRs). The EPB41 genes, EPB41, EPB41L2, EPB41L3 and EPB41L1, encode an impressively complex array of structural adaptor proteins (designated 4.1R, 4.1G, 4.1B and 4.1N, respectively) by using alternative transcriptional promoters and tissue-specific alternative pre-mRNA splicing. The great variety of 4.1 proteins mainly results from 5'-end and internal processing of the EPB41 pre-mRNAs. Thus, 4.1 proteins can vary in their N-terminal extensions but all contain a highly homologous C-terminal domain (CTD). Here we study a new group of EPB41-related mRNAs that originate by APA and lack the exons encoding the CTD characteristic of prototypical 4.1 proteins, thereby encoding a new type of 4.1 protein. For the EPB41 gene, this type of processing was observed in all 11 human tissues analyzed. Comparative genomic analysis of EPB41 indicates that APA is conserved in various mammals. In addition, we show that APA also functions for the EPB41L2, EPB41L3 and EPB41L1 genes, but in a more restricted manner in the case of the latter 2 than it does for the EPB41 and EPB41L2 genes. Our study shows alternative polyadenylation to be an additional mechanism for the generation of 4.1 protein diversity in the already complex EPB41-related genes. Understanding the diversity of EPB41 RNA processing is essential for a full appreciation of the many 4.1 proteins expressed in normal and pathological tissues.

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.

A premature termination codon within an alternative exon affecting only the metabolism of transcripts that retain this exon

Protein 4.1 pre-mRNA splicing is regulated in tissue- and development-specific manners. Exon 16, which encodes the N-terminal region of the spectrin/actin-binding domain, is one of the alternatively spliced sequence motifs. It is present in late differentiated erythroid cells but absent from early erythroblasts and from lymphoid cells. We describe a single nucleotide deletion of the erythroid protein 4.1 gene associated with hereditary elliptocytosis. The deletion located in exon 16 leads to a frameshift and a premature termination codon within the same exon. In an effort to examine the premature stop codon effect in relationship with exon 16 alternative splicing, we analyzed erythroid and lymphoid protein 4.1 mRNAs using the mutation and a linked downstream polymorphism as markers. We found that the premature stop codon does not affect the tissue-specific alternative splicing among the two cell types analyzed and that the resulting alteration of mRNA metabolism correlates with the retention of exon 16 in reticulocytes. Conversely, skipping of exon 16 in lymphoid cells converts the mutant mRNA to a normal lymphoid-specific mRNA isoform, hence bypassing the nonsense codon. Consistent with data obtained on constitutive nonsense exons, our observations argue in favor of a stop codon recognition mechanism that occurs after the regulated splicing status of the nonsense exon has been achieved.

Red blood cell membrane disorders

The recent discovery of the specific molecular defects in many patients with hereditary spherocytosis and hereditary elliptocytosis/pyropoikilocytosis partially clarifies the molecular pathology of these diseases. HE and HPP are caused by defects in the horizontal interactions that hold the membrane skeleton together, particularly the critical spectrin self-association reaction. Single gene defects cause red cells to elongate as they circulate, by a unknown mechanism, and are clinically harmless. The combination of two defective genes or one severe alpha spectrin defect and a thalassaemia-like defect in the opposite allele (alphaLELY) results in fragile cells that fragment into bizarre shapes in the circulation, with haemolysis and sometimes life-threatening anaemia. A few of the alpha spectrin defects are common, suggesting they provide an advantage against malaria or some other threat. HS, in contrast, is nearly always caused by family-specific private mutations. These involve the five proteins that link the membrane skeleton to the overlying lipid bilayer: alpha and beta spectrin, ankyrin, band 3 and protein 4.2. Somehow, perhaps through loss of the anchorage band 3 provides its lipid neighbours (Peters et al, 1996), microvesiculation of the membrane surface ensues, leading to spherocytosis, splenic sequestration and haemolysis. Future research will need to focus on how each type of defect causes its associated disease, how the spleen aggravates membrane skeleton defects (a process termed 'conditioning'), how defective red, cells are recognized and removed in the spleen, and why patients with similar or even identical defects can have different clinical severity. Emphasis also needs to be given to improving diagnostic tests, particularly for HS, and exploring new options for therapy, like partial splenectomy, which can ameliorate symptoms while better protecting patients from bacterial sepsis and red cell parasites, and perhaps from atherosclerosis (Robinette & Franmeni, 1977) and venous thrombosis (Stewart et al, 1996).