Epithelial Expression of Human ABO Blood Group Genes Is Dependent upon a Downstream Regulatory Element Functioning through an Epithelial Cell-specific Transcription Factor, Elf5

Abstract

The human ABO blood group system is of great importance in blood transfusion and organ transplantation. The ABO system is composed of complex carbohydrate structures that are biosynthesized by A-and B-transferases encoded by the ABO gene. However, the mechanisms regulating ABO gene expression in epithelial cells remain obscure. On the basis of DNase I-hypersensitive sites in and around ABO in epithelial cells, we prepared reporter plasmid constructs including these sites. Subsequent luciferase assays and histone modifications indicated a novel positive regulatory element, designated the ؉22.6-kb site, downstream from ABO, and this was shown to enhance ABO promoter activity in an epithelial cell-specific manner. Expression of ABO and B-antigen was reduced in gastric cancer KATOIII cells by biallelic deletion of the ؉22.6-kb site using the CRISPR/Cas9 system. Electrophoretic mobility shift assay and chromatin immunoprecipitation assay demonstrated that the site bound to an epithelial cell-specific transcription factor, Elf5. Mutation of the Ets binding motifs to abrogate binding of this factor reduced the regulatory activity of the ؉22.6-kb site. Furthermore, ELF5 knockdown with shRNA reduced both endogenous transcription from ABO and B-antigen expression in KATOIII cells. Thus, Elf5 appeared to be involved in the enhancer potential of the ؉22.6-kb site. These results support the contention that ABO expression is dependent upon a downstream positive regulatory element functioning through a tissue-restricted transcription factor, Elf5, in epithelial cells.The human ABO blood group system is of great importance in blood transfusion and organ transplantation. The system comprises complex carbohydrate structures that are biosynthesized by the A-and B-transferases encoded by the A and B genes, respectively (1). The ABO genes consist of seven exons spanning more than 20 kb of genomic DNA, and two critical single-base substitutions in the last coding exon result in amino acid substitutions responsible for the difference in donor nucleotide sugar substrate specificity between A-and B-transferases (2). A single base deletion in exon 6 has been ascribed to a shift in the reading frame of codons and abolition of A-transferase activity in most O alleles. On the other hand, the distribution of the A-and B-antigens is cell type-specific; for example, the antigens are expressed on red blood cells and epithelial cells, as well as in salivary glands, although they are absent from the central nervous system, muscle, and connective tissue. Moreover, ABH antigens are known to be expressed during the maturation of erythroid and epithelial cells; for example, when erythroid cells differentiate in vitro, ABO is expressed at an undetectable level in the early phase, increases subsequently, and then decreases later (3, 4). In addition to the normal cell differentiation process, changes in ABH antigen expression have also been documented in abnormal processes such as tumorigenesis (1). Reduction or complete deletion of A/B antigen expressi...

Human ABO gene transcriptional regulation

“…Subsequent plasmid‐based reporter assays indicated a distal regulatory region between +22563 and +22781 relative to the translation start site of ABO , termed the +22.6‐kb site, in KATOIII cells (Fig. A), and the regulatory activity of the region was specific to epithelial cells . Subsequently, we validated the significance of the +22.6‐kb site with use of KATOIII cells with homozygote deletion of the site constructed by the CRISPR/Cas9 system.…”

“…As shown above, ABO transcription is regulated by the proximal promoter and cell‐specific regulatory regions. However, ABO transcription was not completely lost in the cells with biallelic deletions of the +22.6‐kb site . Also, reporter assays demonstrated transcriptional activity in the DHS region +36.0 (Fig.…”

“…In addition, variants in the GATA motif were found in B m and A m , in which B‐ or A‐antigen expression is reduced on RBCs, while a large amount of B or A substance is present in the saliva of secretors. Similarly, plasmid‐based reporter assays demonstrated that the epithelial cell–specific regulatory activity of the +22.6‐kb site was dependent upon binding of the epithelial cell–specific TF Elf5 . In fibroblasts not expressing GATA‐1 or ‐2, or Elf5, it is plausible that abrogation of the siteʼs cell‐specific regulatory activity contributes to lack of ABO expression .…”

“…1A), and the regulatory activity of the region was specific to epithelial cells. 48 Subsequently, we validated the significance of the +22.6-kb site with use of KATOIII cells with homozygote deletion of the site constructed by the CRISPR/Cas9 system. EMSAs and ChIP assays demonstrated that the region bound an epithelial cell-specific TF, Elf5 (Fig.…”

A novel mutation +5904 C>T of RUNX1 site in the erythroid cell‐specific regulatory element decreases the ABO antigen expression in Chinese population

“…It was found that most of the individuals with Bm phenotype were due to the deletion of the whole 5·8‐kb region in the Japanese . Furthermore, some mutations in the 5·8‐kb site were found in the individuals with ABO subtypes, including 23 bp deletion, 5890T>G, 5893G>A, and 5909A>G. These mutations could reduce the erythroid cell‐specific enhancer activity of the 5·8‐kb site and downregulate the transcription of A or B allele .…”