Genetisch bedingte Veränderungen im Wnt-Signalweg sind in der Tumorigenese des Darms von zentraler Bedeutung. Mutationen des Wnt-Effektormoleküls β-Catenin in den adulten Stammzellen des Darmepithels führen zu unkontrollierter Proliferation und Expansion der Darmstammzellen und initiieren die Tumorentstehung. Auch in fortgeschrittenen Darmtumoren unterstützt die Wnt-Signalgebung maßgeblich das Tumorwachstum und den Erhalt von Tumorstammzellen. Nach erfolgreicher chemotherapeutischer Behandlung treten oftmals Tumorrezidive auf, für deren Entstehung therapieresistente Tumorstammzellen verantwortlich gemacht werden. Trotz intensiver Forschung fehlen in der Darmkrebstherapie nach wie vor Behandlungsansätze zur gezielten Therapie der Tumorstammzellen. Ziel dieser Dissertation ist es, unser Verständnis der molekularen Regulationsmechanismen in Kolonkarzinomen zu erweitern und die Entwicklung rationaler Behandlungsstrategien zu fördern. Ich konnte die Histonmethyltransferase Mll1 als entscheidenden Faktor in der epigenetischen Regulation humaner und muriner Darmkrebsstammzellen und -tumore identifizieren. Humane Kolonkarzinome weisen eine erhöhte Mll1-Expression auf, die mit dem Level an nukleärem β-Catenin korreliert. Im adulten Darmepithel ist Mll1 insbesondere in den Lgr5+ Stammzellen exprimiert und maßgeblich an der Wnt/β-Catenin-induzierten Stammzellexpansion sowie der Tumorentstehung beteiligt. Der konditionelle Verlust von Mll1 im murinen Darmkrebsmodell verhindert die β-Catenin-induzierte Tumorigenese. Mll1 unterstützt die Selbsterneuerungsfähigkeit und Proliferation der Tumorstammzellen, indem es die Expression von essentiellen Stammzellgenen wie dem Wnt-abhängigen Stammzellmarker Lgr5 aufrechterhält. Eine Inhibition der Mll1-Funktion in der Darmkrebstherapie kann eine gezielte Eliminierung der Tumorstammzellen ermöglichen, wodurch das fortschreitende Tumorwachstum unterbunden und die Bildung von Rezidiven verhindert werden kann. / Genetic mutations inducing aberrant activity of Wnt signalling are causative for intestinal tumorigenesis. Mutations of the Wnt effector molecule β-catenin in adult stem cells of the intestinal epithelium drive uncontrolled proliferation, expand the stem cell pool and initiate tumor formation. In advanced tumors, aberrant Wnt signalling promotes tumor growth and maintains cancer stem cells. The cancer stem cells are highly resistant to conventional chemotherapy and frequently initiate tumor relapse after completion of treatment. Despite extensive research, we are still lacking efficient therapies for colon cancer that specifically eliminate the cancer stem cells. This dissertation aims to expand our knowledge on molecular gene regulatory mechanisms in colon cancer cells to promote the identification and future development of rational therapies for colon cancer patients. I identified the histone methyltransferase Mll1 as an epigenetic regulator in human and mouse intestinal cancer stem cells and tumors. Human colon carcinomas with nuclear β-catenin exhibit high levels of Mll1. In the adult intestinal epithelium of mice, Mll1 is highly expressed in the Lgr5+ stem cells and is a prerequisite for the oncogenic Wnt/β-catenin-mediated stem cell expansion and tumorigenesis. Conditional knockout of Mll1 in an intestinal mouse tumor model prevents the β-catenin-driven intestinal tumorigenesis. Knockdown of Mll1 impairs the self-renewal and proliferation of colon cancer sphere cultures and halts tumor growth in xenografts. Mechanistically, Mll1 sustains the expression of intestinal stem cell genes including the Wnt/β-catenin target gene Lgr5 by antagonizing gene silencing through polycomb repressive complex 2-mediated H3K27 tri-methylation. Interfering with Mll1 function can efficiently eliminate colon cancer stem cells, and has potential as a rational therapy for colon cancer.

Tumors exhibit complex organization and contain a variety of cell populations. The realization that the regenerative properties of a tumor may be largely confined to a cell subpopulation (cancer stem cell) is driving a new era of anti-cancer research. Cancer stem cells from Glioblastoma Multiforme tumors express markers that are also expressed in non-cancerous neural stem cells, including nestin and Sox2. We previously showed that the transcription factor Hes3 is a marker of neural stem cells, and that its expression is inhibited by JAK activity. Here we show that Hes3 is also expressed in cultures from glioblastoma multiforme which express neural stem cell markers, can differentiate into neurons and glia, and can recapitulate the tumor of origin when transplanted into immunocompromised mice. Similar to observations in neural stem cells, JAK inhibits Hes3 expression. Hes3 RNA interference reduces the number of cultured glioblastoma cells suggesting a novel therapeutic strategy.

Tumors exhibit complex organization and contain a variety of cell populations. The realization that the regenerative properties of a tumor may be largely confined to a cell subpopulation (cancer stem cell) is driving a new era of anti-cancer research. Cancer stem cells from Glioblastoma Multiforme tumors express markers that are also expressed in non-cancerous neural stem cells, including nestin and Sox2. We previously showed that the transcription factor Hes3 is a marker of neural stem cells, and that its expression is inhibited by JAK activity. Here we show that Hes3 is also expressed in cultures from glioblastoma multiforme which express neural stem cell markers, can differentiate into neurons and glia, and can recapitulate the tumor of origin when transplanted into immunocompromised mice. Similar to observations in neural stem cells, JAK inhibits Hes3 expression. Hes3 RNA interference reduces the number of cultured glioblastoma cells suggesting a novel therapeutic strategy.