Abstract
Background and aims: Chylomicron retention disease (CRD) is caused by mutations in the SARA2 gene that encodes the SAR1B protein, involved in the vesicular coat protein complex II-dependent transport of proteins/lipoproteins from the endoplasmic reticulum to the Golgi apparatus. Given the large spectrum of CRD phenotypes, it appears important to search for SARA2 polymorphisms and probe functional cause-effects resulting from SAR1B knockdown. Methods: To examine the allelic frequencies of the SARA2 genetic variants, SNPs were assessed in the promoter (2kb, 3.5 and 6 kb regions upstream of the transcription start site), exon 4 and exon 8 in CRD and control subjects. In addition, SAR1B was suppressed in HepG2 and Caco-2/15 cell lines by specific siRNA, and lipid transport was determined. Results: No changes were noted in the polymorphisms in the promoter region (7 variants), exon 4 (2 variants) and exon 8 (1 variant), suggesting that they do not play an essential role in the several biochemical and clinical abnormalities characterizing CRD patients. On the other hand, SAR1B silencing resulted in multiple alterations in lipids, apolipoproteins (apo) and lipoproteins in Caco-2/15 and HepG2 cells. It reduced the output of triglycerides (TG) and TG-rich lipoproteins. Furthermore, SAR1B silencing limited the synthesis of apo B-48 and apo B-100 in Caco-2/15 cells and HepG2 cells, respectively. Conclusions: Our functional experiments show that the suppression of SAR1B has a negative impact on apo B synthesis and lipid/lipoprotein secretion. If previous studies have established only an association between SAR1B defects and lipid transport abnormalities, our findings constitute the first direct demonstration illustrating the SAR1B deficiency cause-effects in enterocytes and hepatocytes. Acknowledgment: This study was supported by the Canadian Institutes of Health Research and the J.A. DeSeve Research Chair in Nutrition
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