Hijacked in cancer: the KMT2 (MLL) family of methyltransferases

Key Points The histone–lysine N -methyltransferase (KMT2) family comprises a set of lysine methyltransferases that methylate the lysine 4 residue on histone H3 (H3K4). KMT2 family members demonstrate different substrate specificity in vitro and their methyltransferase activities are dependent, to varying degrees, on association with three core subunits (WD repeat protein 5, retinoblastoma binding protein 5 and ASH2L). KMT2 family members have intrinsically different biochemical properties and are recruited to different genomic regions owing to their distinct domain structures and distinct interacting proteins. KMT2 family members have important roles in transcription regulation. Among them, KMT2C and KMT2D are crucial for monomethylation of H3K4 at distal regulatory enhancers, whereas KMT2F and KMT2G are responsible for the majority of H3K4 trimethylation at transcription start sites. There is extensive interplay between KMT2-dependent H3K4 methylation and DNA methylation, underlying the potential epigenetic stability of this histone methylation. Mutations in the KMT2 family are among the most common genetic aberrations in human cancer — including haematological malignancies as well as solid tumours, such as large intestine, lung, endometrial, breast, bladder and brain cancers. Mutations in the KMT2 family frequently involve the SET domain and the plant homeotic domains. Of somatic mutations in cancers with known zygosity, heterozygous mutations predominate. These features suggest that the wild-type KMT2 allele may be required for tumour survival, similar to KMT2A -rearranged mixed lineage leukaemia. KMT2 family members may have distinct roles in cancer. Although it remains unclear whether cancer-derived KMT2 mutations are 'drivers' or 'passengers', mechanistic studies in animal models suggest that KMT2C may be a tumour suppressor and KMT2A and KMT2D may be proteins derived from proto-oncogenes. The targeting of the fusion protein and wild-type KMT2A, as well as their interacting proteins, has emerged as a promising strategy to treat mixed lineage leukaemia, and may apply more broadly to a variety of cancers. Histone–lysine N -methyltransferase 2 (KMT2) family proteins, initially named the mixed lineage leukaemia (MLL) family, are altered in many types of cancers beyond MLL. Inhibitors of KMT2 function are being developed and could work as therapeutics in a variety of cancer types. Histone–lysine N -methyltransferase 2 (KMT2) family proteins methylate lysi