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DNA methylation variation of human-specific Alu repeats

ABSTRACT: DNA methylation is the major repression mechanism for human retrotransposons, such as the Alu family. Here, we have derived methylation levels regarding 5238 loci belonging to two Alu subfamilies, AluYa5 and AluYb8, using High-Throughput Targeted Repeat Element Bisulfite Sequencing (HT-TREBS). The results indicate that ~90% of loci are repressed by high methylation levels. Of the remaining loci, many of these hypomethylated elements are found near gene promoters and show high levels of DNA methylation variation. We have characterized this variation in the context of tumorigenesis and inter-individual differences. Comparison of a primary breast tumor and its matched normal tissue revealed early DNA methylation changes in ~1% of AluYb8 elements in response to tumorigenesis. At the same time, AluYa5/Yb8 elements proximal to promoters also showed differences in methylation of up to one order of magnitude even between normal individuals. Overall, the current study demonstrates that early loss of methylation occurs during tumorigenesis in a subset of young Alu elements, suggesting their potential clinical relevance. However, techniques such as deep-bisulfite-sequencing of individual loci using HT-TREBS are required to distinguish clinically relevant loci from the background observed for AluYa5/Yb8 elements in general with regard to high levels of inter-individual variation in DNA methylation.

ORGANISM(S): Homo sapiens

PROVIDER: GSE74420 | GEO | 2016/10/27

SECONDARY ACCESSION(S): PRJNA300357

REPOSITORIES: GEO

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Project description:Purpose: Irinotecan (SN38) and oxaliplatin are chemotherapeutic agents used in the treatment of colorectal cancer. However, the frequent development of resistance to these drugs represents a considerable challenge in the clinic. Alus as retrotransposons comprise 11% of the human genome. Genomic toxicity induced by carcinogens or drugs can reactivate Alus by altering DNA methylation. Whether or not reactivation of Alus occurs in SN38 and oxaliplatin resistance remains unknown. Methods: We applied reduced representation bisulfite sequencing (RRBS) to investigate the DNA methylome in SN38- or oxaliplatin-resistant colorectal cancer cell line models. Moreover, we extended the RRBS analysis to tumor tissue from 14 patients with colorectal cancer who either did or did not benefit from capecitabine + oxaliplatin treatment. For the clinical samples, we applied a concept of DNA methylation entropy to estimate the diversity of DNA methylation states of the identified resistance phenotype-associated methylation loci observed in the cell line models. Results: We identified different loci being characteristic for the different resistant cell lines. Interestingly, 53% of the identified loci were Alu sequences -- especially the Alu Y subfamily. Furthermore, we identified an enrichment of Alu Y sequences that likely results from increased integration of new copies of Alu Y sequence in the drug-resistant cell lines. In the clinical samples, SOX1 and other SOX gene family members were shown to display variable DNA methylation states in their gene regions. The Alu Y sequences showed remarkable variation in DNA methylation states across the clinical samples. Our findings imply a crucial role of Alu Y in colorectal cancer drug resistance. Our study underscores the complexity of colorectal cancer aggravated by mobility of Alu elements and stresses the importance of personalized strategies, using a systematic and dynamic view, for effective cancer therapy. Investigation of the representive methylome of well-established SN38 and Oxaliplatin resistant cell line models and 14 clinical colorectal metastatic samples that have developed resistance to XELOX to review the epigenetic mechnism of the drug resistance.

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DNA methylation variation of human-specific Alu repeats

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