Imprinted genes are expressed from only one of the parental chromosomes and are marked epigenetically by DNA methylation and histone modifications. The imprinting center 2 (IC2) on mouse distal chromosome 7 is flanked by several paternally repressed genes, with the more distant ones imprinted exclusively in the placenta. We found that most of these genes lack parent-specific DNA methylation, and genetic ablation of methylation does not lead to loss of their imprinting in the trophoblast (placenta). The silent paternal alleles of the genes are marked in the trophoblast by repressive histone modifications (dimethylation at Lys9 of histone H3 and trimethylation at Lys27 of histone H3), which are disrupted when IC2 is deleted, leading to reactivation of the paternal alleles. Thus, repressive histone methylation is recruited by IC2 (potentially through a noncoding antisense RNA) to the paternal chromosome in a region of at least 700 kb and maintains imprinting in this cluster in the placenta, independently of DNA methylation. We propose that an evolutionarily older imprinting mechanism limited to extraembryonic tissues was based on histone modifications, and that this mechanism was subsequently made more stable for use in embryonic lineages by the recruitment of DNA methylation.

Hereditary mixed polyposis syndrome (HMPS) is characterised by the development of mixed morphology colorectal tumours and is caused by a 40 kb duplication that results in aberrant epithelial expression of the mesenchymal Bone Morphogenetic Protein antagonist, GREM1. Here we use HMPS tissue and a mouse model of the disease to show that epithelial GREM1 disrupts homeostatic intestinal morphogen gradients, altering cell-fate, that is normally determined by position along the vertical epithelial axis. This promotes the persistence and/or reacquisition of stem-cell properties in Lgr5 negative (non-expressing) progenitor cells that have exited the stem-cell niche. These cells form ectopic crypts, proliferate, accumulate somatic mutations and can initiate intestinal neoplasia, indicating that the crypt base stem-cell is not the sole cell-of-origin of colorectal cancer. Furthermore, we show that epithelial expression of GREM1 also occurs in traditional serrated adenomas, sporadic pre-malignant lesions with a hitherto unknown pathogenesis and these lesions can be considered the sporadic equivalents of HMPS polyps.

The hereditary mixed polyposis syndrome (HMPS) was first described about 50 years ago in a large Ashkenazi Jewish family from St Mark’s Hospital, London. The family showed apparent autosomal dominant inheritance of multiple types of colorectal polyp, with colorectal carcinoma in a high proportion of individuals. In the last 15 years, we have mapped the HMPS gene to chromosome 15q13.3 and identified an ancestral haplotype common to all the known HMPS families. Here, we have used genetic mapping, copy number analysis, exclusion of mutations by high-throughput sequencing, gene expression analysis and functional assays to show that HMPS is caused by a large duplication spanning the 3′ end of the SCG5 gene and a region upstream of the GREM1 locus. This mutation has no effect on SCG5 expression, but is associated with greatly increased, allele-specific GREM1 expression. Whilst GREM1 is expressed in intestinal sub-epithelial myofibroblasts in controls, HMPS patients predominantly express GREM1 in the epithelium of the large bowel. The HMPS duplication contains predicted transcriptional enhancer elements; we have shown that some of these interact with the GREM1 promoter and are capable of driving gene expression in vitro. Increased GREM1 expression is predicted to lead to reduced bone morphogenetic protein pathway activity, a mechanism that also underlies tumorigenesis in juvenile polyposis of the large bowel. The pathogenic mechanism in HMPS is extremely unusual in Mendelian cancer syndromes and highlights ectopic gene expression as a mechanism of tumorigenesis.