Regulation of chromatin is highly dynamic and critically important in developmental processes, including cell fate determination and maintenance of cell identity. Several multi-subunit protein complexes including Polycomb Repressor Complexes PRC1, PRC2 and non-canonical PRC1 (nc-PRC1), are central to this dynamic modification of chromatin and co-operate to compact chromatin and silence genes through post-translational modifications (PTMs) at specific histone tail residues. There has been much recent evidence of mutations in genes encoding subunits of these and other chromatin-modifying complexes in human cancers. BCOR is a subunit of the nc-PRC1 with crucial roles in development. Somatic internal tandem duplication (ITD) of BCOR appears to form the oncogenic basis to the development of a majority of Clear Cell Sarcoma of Kidney (CCSK), the second commonest paediatric renal malignancy, but also of high-grade endometrial stromal sarcomas and undifferentiated round-cell sarcomas of infancy. The ITD is invariably within exon 15, encoding the so-called PUFD domain of BCOR, a region critical for interaction with the RAWUL domain of PCGF1, a further nc-PRC1 subunit. The effect of BCOR-ITD on nc-PRC1 complex formation and function in chromatin remodeling is unknown, and forms the basis of this investigation. We already have preliminary data from mass spectrometry and size-exclusion chromatography as well as co-immunoprecipitation, to suggest altered interactions of BCOR-ITD with other subunits of nc-PRC1. We will investigate this further in cell line models generated to mimic various of the BCOR-ITDs compared with wild-type BCOR. The chromatin binding and activity of several polycomb proteins will be evaluated in these cell line models by ChIP sequencing along with the profile of resultant histone PTMs, validated in tumour samples and then correlated with transcriptomic profiling. Our preliminary observations suggest a novel mechanism of oncogenesis and with the emergence of ‘epigenetic’ therapies for human cancers, this becomes a compelling investigation of utmost importance.
