MLL1 is required for maintenance of intestinal stem cells

Abstract

Epigenetic mechanisms are gatekeepers for the gene expression patterns that establish and maintain cellular identity in mammalian development, stem cells and adult homeostasis. Amongst many epigenetic marks, methylation of histone 3 lysine 4 (H3K4) is one of the most widely conserved and occupies a central position in gene expression. Mixed lineage leukemia 1 (MLL1/KMT2A) is the founding mammalian H3K4 methyltransferase. It was discovered as the causative mutation in early onset leukemia and subsequently found to be required for the establishment of definitive hematopoiesis and the maintenance of adult hematopoietic stem cells. Despite wide expression, the roles of MLL1 in non-hematopoietic tissues remain largely unexplored. To bypass hematopoietic lethality, we used bone marrow transplantation and conditional mutagenesis to discover that the most overt phenotype in adult Mll1-mutant mice is intestinal failure. MLL1 is expressed in intestinal stem cells (ISCs) and transit amplifying (TA) cells but not in the villus. Loss of MLL1 is accompanied by loss of ISCs and a differentiation bias towards the secretory lineage with increased numbers and enlargement of goblet cells. Expression profiling of sorted ISCs revealed that MLL1 is required to promote expression of several definitive intestinal transcription factors including Pitx1, Pitx2, Foxa1, Gata4, Zfp503 and Onecut2, as well as the H3K27me3 binder, Bahcc1. These results were recapitulated using conditional mutagenesis in intestinal organoids. The stem cell niche in the crypt includes ISCs in close association with Paneth cells. Loss of MLL1 from ISCs promoted transcriptional changes in Paneth cells involving metabolic and stress responses. Here we add ISCs to the MLL1 repertoire and observe that all known functions of MLL1 relate to the properties of somatic stem cells, thereby highlighting the suggestion that MLL1 is a master somatic stem cell regulator.

Epigenetic modifier balances Mapk and Wnt signalling in differentiation of goblet and Paneth cells

“…At the base of Mll1 −/− crypts, Lyz-positive Paneth cells acquired Alcian blue–positive staining (compare Fig 1A with Figs 1B and S1F ). In agreement with our previous characterisation of Mll1-deficient intestines ( Goveas et al, 2021 ), the loss of Mll1 did not alter the number of chromogranin A (ChroA)-positive enteroendocrine cells per crypt-villus axis ( Fig S1G , quantification on the right).…”

“…Mll1 −/− crypts, however, exhibited an increased number of cells with goblet cell-like features ( Fig S1D , lower right panel, quantification below). By Alcian blue staining we identified these cells as mucus-containing goblet-like cells ( Fig 1A , quantification on the right), as also previously described ( Goveas et al, 2021 ). To exclude transient effects, we analysed mutant intestinal epithelia at 30–50 d after mutagenesis.…”

“…In the light of our data, this highlights the importance of a proper balance of Mll1 and PcG activities for establishing and maintaining cell identity in Paneth/goblet cell progenitors. Our data suggest that Mll1—like PRC2—exerts a dual role in intestinal homeostasis: it sustains intestinal progenitor cells and needs to be down-regulated to enable terminal differentiation ( Grinat et al, 2020 ; Goveas et al, 2021 ), but also plays a role in controlling cell fate specification of secretory Paneth and goblet cells.…”

“…Besides the role of transcription factors, epigenetic regulation of gene expression has emerged as a powerful determinant of cell type identity as well as stem cell maintenance ( Jadhav et al, 2016 ; Piunti & Shilatifard, 2016 ; Brand et al, 2019 ; Grinat et al, 2020 ; Goveas et al, 2021 ). We have recently reported a crucial role of the histone methyltransferase Mll1 in development of colorectal cancer ( Grinat et al, 2020 ; Heuberger et al, 2021 ).…”

“…Ablation of Mll1 caused differentiation of Wnt-activated cancer stem cells by increasing a secretory gene expression profile ( Grinat et al, 2020 ). Further work showed that Mll1 is required for intestinal stem cell maintenance at homeostasis ( Goveas et al, 2021 ).…”