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Molecular cloning of a human small intestinal apolipoprotein B mRNA editing protein.

ABSTRACT: Mammalian small intestinal apolipoprotein B (apo B) mRNA undergoes posttranscriptional cytidine deamination with the production of an in frame stop codon and the translation of apo B48. We have isolated a cDNA from human jejunum which mediates in vitro editing of a synthetic apo B RNA template upon complementation with chicken intestinal S100 extracts. The cDNA specifies a 236 residue protein which is 69% identical to the apo B mRNA editing protein (REPR) cloned from rat small intestine [Teng, B., Burant, C. F. and Davidson, N. O. (1993) Science 260, 1816-1819] and which, by analogy, is referred to as HEPR. HEPR does not contain the carboxyl-terminus leucine zipper motif identified in REPR but contains consensus phosphorylation sites as well as the conserved histidine and both cysteine residues identified as a Zn2+ binding motif in other cytidine deaminases. The distribution of HEPR mRNA was predominantly confined to the adult small intestine with lower levels detectable by reverse-transcription polymerase chain reaction amplification in the stomach, colon and testis. These differences in the structure and distribution of the human as compared to the rat apo B mRNA editing protein suggest an important evolutionary adaptation in the mechanisms restricting apo B48 production to the small intestine.

SUBMITTER: Hadjiagapiou C

PROVIDER: S-EPMC308087 | biostudies-other | 1994 May

REPOSITORIES: biostudies-other

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Molecular cloning of a human small intestinal apolipoprotein B mRNA editing protein.

Hadjiagapiou C C Giannoni F F Funahashi T T Skarosi S F SF Davidson N O NO

Nucleic acids research 19940501 10

Mammalian small intestinal apolipoprotein B (apo B) mRNA undergoes posttranscriptional cytidine deamination with the production of an in frame stop codon and the translation of apo B48. We have isolated a cDNA from human jejunum which mediates in vitro editing of a synthetic apo B RNA template upon complementation with chicken intestinal S100 extracts. The cDNA specifies a 236 residue protein which is 69% identical to the apo B mRNA editing protein (REPR) cloned from rat small intestine [Teng, B ...[more]

PMID: 8208612

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Project description:Hepatic apolipoprotein (apo) B-100 isolated from human plasma is known to contain N-linked oligosaccharides of high-mannose-type and complex-type structures. Sequencing data have revealed that apo B-48 of small-intestinal origin, which represents about 48% of apo B-100 polypeptide from the N-terminus, possesses six potential sites for N-linked oligosaccharides, of which five are likely to be glycosylated. The characterization of the carbohydrate moiety of apo B-48 is the focus of this study. Apo B-48 was labelled with L-[35S]methionine and D-[3H]glucosamine in organ culture of human small-intestinal explants. N-Glycanase treatment resulted in loss of radioactivity from D-[3H]glucosamine-labelled but not L-[35S]methionine-labelled apo B-48 secreted into the medium, and caused no distinct change in mobility of apo B-48 upon electrophoresis on 5% polyacrylamide gel. Analysis of monosaccharide content revealed the presence of 16.8, 17.8, 13.4, 3.4, 2.4 and 2.3 residues of N-acetylglucosamine, mannose, galactose, fucose, xylose and N-acetylgalactosamine respectively. Small-intestinal apo B-48 from human lymph chylomicrons bound to [14C]concanavalin A, and the binding could be inhibited with methyl alpha-D-mannoside. In addition, wheat-germ, peanut, Limulus, soya-bean and Ulex lectins bound apo B-48 specifically. To characterize the carbohydrate moiety further, N-linked oligosaccharides were released by N-Glycanase treatment and reduced with NaB3H4. Labelled oligosaccharides were separated on a concanavalin A-Sepharose column. The majority (78%) were biantennary complex-type structures, 16% were high-mannose type and 6% (not retained by the column) most probably represented higher-branched oligosaccharides. These results suggest the presence of one high-mannose-type and four biantennary complex-type oligosaccharides, as well as probable O-linked sugars in apo B-48. By the use of h.p.l.c., exoglycosidase treatments and ion-exchange chromatography, a mixture of high-mannose-type species with predominant Man8GlcNAc2 as well as monosialylated, desialylated and fucosylated forms of complex-type oligosaccharides were detected.

Project description:PurposeApolipoprotein B mRNA-Editing Catalytic Polypeptide-like (APOBEC) enzymes are mutagenic factors contributing to tumor progression and therapy resistance. However, the effects of APOBEC-induced protein changes have not been systematically assessed. Here, we describe the effects of APOBEC on the coding sequence in primary and metastatic estrogen receptor-positive (ER+)/human epidermal growth factor receptor 2-negative (HER2-) breast cancer (BC).MethodsWe determined the enrichment of amino acid (AA) changes resulting from APOBEC mutagenesis in 323 primary BC tumors and 424 metastatic breast cancer (mBC) lesions via comparison with a simulated mutational genomic landscape not under selection pressure. We subsequently explored genes with recurrent APOBEC-associated AA changes and investigated the clonality of individual APOBEC-associated mutations. Using public sequencing data from an independent primary BC and mBC cohort, we further confirm our findings by reporting genes having these enriched AA changes in an APOBEC context.ResultsOur analysis demonstrated that several APOBEC-derived AA changes are significantly enriched compared with a simulated AA change distribution drawn at random. Among the enriched AA changes, Glutamate(E)>Lysine(K) and Glutamate(E)>Glutamine(Q) were mostly found at hotspots in oncogenes, whereas termination codons (Glutamine[Q]>STOP[X] and Serine[S]>STOP[X]) occurred in tumor suppressor genes and, mostly, not at hotspot locations. These mutations are found in genes contributing to BC initiation, eg, introduction of termination codons in TP53, MAP3K1, and CDH1 (in lobular BC) and two oncogenic hotspots in PIK3CA (p.E542K and p.E545K). In endocrine-resistant BC, we observed APOBEC-induced termination codons at ER-modulating genes, KMT2C, ARID1A, NF1, and ZFHX3. Finally, in mBC, compared with single PIK3CA mutations, dual PIK3CA mutations occurred more frequently in an APOBEC context (Fisher's exact P < .001).ConclusionOur results show that APOBEC mutagenesis recurrently targets various known drivers of BC initiation, progression, and endocrine resistance.

Dataset Information

Molecular cloning of a human small intestinal apolipoprotein B mRNA editing protein.

Publications

Molecular cloning of a human small intestinal apolipoprotein B mRNA editing protein.

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