Consequences of UV exposure on the histone methylation marks deposited by ASH1L

The global genome nucleotide excision repair (GG-NER) recognizes and removes bulky ultraviolet (UV) lesions in DNA, including cyclobutane pyrimidine dimers (CPDs). It is therefore essential in the preventing the accumulation of mutations and subsequent development of skin cancer. Over the past decades, the pathway has been studied in detail. However, little is known about how the GG-NER interacts with and accesses chromatin. A recent study revealed that the histone methyltransferase ASH1L facilitates recognition by the GG-NER pathway of CPDs associated with nucleosomes. To investigate how ASH1L is involved in the GG-NER pathway, we examined the distribution of the histone marks reportedly deposited by ASH1L, H3K4me3 and H3K36me2. Our Western blots show that upon UV, overall levels of H3K4me3 decrease in ASH1L-/- cells whereas H3K36me2 levels remain unchanged. Furthermore, we report the successful establishment and optimization of H3K4me3 ChIP-seq in UV-treated wildtype and ASH1L-/- cells. At regions of the genome that are normally occupied by H3K4me3, our ChIP-seq data show an increase of H3K4me3 signal upon UV that is not maintained in the absence of ASH1L. This suggests that after UV exposure, ASH1L is responsible for trimethylating H3K4 at sites where this histone mark is usually found. We anticipate our H3K4me3 ChIP-seq data will provide a starting point to fully characterize the role of ASH1L in GG-NER by integrating several other genome-wide datasets, including XPC ChIP-seq, CPD-seq and XR-seq