High-resolution Nanopore methylome-maps reveal random hyper-methylation at CpG-poor regions as driver of chemoresistance in leukemias.[RNA-Seq]

Aberrant DNA-methylation at CpG dinucleotides is a hallmark of cancer and is associated with the emergence of resistance to anti-cancer treatment, though molecular mechanisms and biological signifi- cance remain elusive. Genome-scale methylation maps by currently used methods are based on chemical modification of DNA and are best suited for analyses of methylation at CpG-rich regions (CpG islands). We report the first high-coverage whole-genome map in cancer using the long-read nanopore technol- ogy, which allows simultaneous DNA-sequence and -methylation analyses on native DNA. We analyzed clonal epigenomic/genomic evolution in Acute Myeloid Leukemias (AMLs) at diagnosis and relapse, af- ter chemotherapy. Long-read sequencing coupled to a novel computational method allowed definition of differential methylation at unprecedented resolution (> 99% CpGs), extending analyses of CpG is- lands to sparse CpGs, which represent half of all differentially-methylated regions. We showed that the relapse methylome is characterized by hypermethylation at both CpG islands and sparse CpGs. Hyper- methylated genes, however, only accounted for < 5% of all differentially-expressed genes (DEGs) in the relapsed AMLs and were not enriched for chemoresistance genes. A few under-expressed transcription- factors (1 to 6 in the different AMLs) hyper-methylated at sparse CpGs support ∼ 40% DEGs and are highly-enriched in chemoresistance genes. Hypermethylated regions at sparse CpGs were poorly con- served in the relapsed AMLs, under-represented at their genomic positions and showed high methylation- entropy, as compared to CpG islands. Relapsed AMLs carried a few patient-specific structural-variants and DNA-mutations, apparently not involved in drug-resistance. Thus, drug-resistance in AMLs is due to the selection of random epigenetic alterations at sparse CpGs of a few transcription factors, which then induce reprogramming of the relapsing phenotype, independently of clonal genomic-evolution.