The function of gene body DNA methylation in alternative splicing, and its relation
to disease pathogenesis is not fully elucidated. The gene for familial Mediterranean
fever (MEFV) encodes the pyrin protein and contains a 998 bp CpG
island, covering the second exon, which is differentially methylated in FMF patients
compared to healthy controls. Our further observation of increased exon 2-spliced
MEFV transcript in leukocytes of FMF patients provoked us to test
the role of exon methylation in alternative splicing using inflammatory cell culture
models. First, in vitro exon methylation triggered an increased
level of exon 2 exclusion using a splicing cassette in a promyelocytic leukemia cell
line (HL-60). HL-60 cells subjected to methylating and demethylating agents, as well
as cells differentiated to neutrophil-like cells, exhibited different levels of
spliced/unspliced transcripts. We observed increased levels of spliced transcripts in
neutrophil-like (p = 0.0005), activated (p = 0.0034) and methylated cells (p <
0.0001), whereas decreased levels in demethylated cells (p = 0.0126) compared to
control untreated HL-60 cells. We also showed that the protein isoform of pyrin
lacking the exon 2 has an adverse subcellular localization in neutrophil-like cells.
Therefore, it remains in the cytoplasm rather than the nucleus. This may point to an
epigenetic involvement in an important inflammatory gene.