Disrupted cell signalling, hijacked stem cells and the power of ‘mini-guts’ – we hear from Dr Vivian Li on the challenges and opportunities of developing colorectal cancer treatments.
Colorectal cancer is the second most common cause of cancer death in the UK.
However, cancer survival statistics suggest it can be curable if diagnosed early. In England between 2013 to 2017, the 5-year survival of colorectal cancer (CRC) diagnosed at early stage is around 80-90%. Sadly, this drops drastically to 10% when diagnosed at late stage.
So, clearly early diagnosis is vital. However, development of advanced therapy is equally important to improve survival rate. Research over the last few years has suggested that a good strategy for finding tumour-specific drug targets revolves around cell-growth signalling molecules. Indeed, in 2017 my lab published work which used various bowel cancer models, including newly-developed organoids, showing a new putative drug target that selectively kills cancer cells.
Signalling, stem cells and colorectal cancer
Intestinal epithelium covers the inner lining of the gut tube and is responsible for essential functions such as food digestion, absorption and creating a barrier against the external environment.
The surface area of the adult human intestine is approximately the size of a tennis court. Damage caused by processing food means epithelial cells undergo constant self-renewal – replenished by intestinal stem cells every five days or so. These stem cells are located within the inner most projections, or basal crypts, of the intestine. Stem cells at the bottom continuously divide and generate new daughter cells that migrate upwards and mature into different functional cell types.
A stable number of stem cells must be maintained in a healthy gut. The intestinal epithelium will fail to regenerate and function if there are too few stem cells; on the other hand, too many stem cells will cause tissue overgrowth and eventually tumour development. It is therefore important to understand how stem cell numbers are precisely controlled.
We have found that many signalling molecules involved in normal stem cell control are often hijacked by cancer cells, resulting in expansion of the stem cell population. This suggests some conserved mechanisms may exist regulating both healthy stem cells and cancer. For example, when healthy intestinal stem cells are damaged (by mutagens or chemicals), it has been shown that neighbouring epithelial cells have the flexibility to de-differentiate and regenerate stem cells for tissue repair. Interestingly, cancer cells possess similar regenerative properties, meaning cancer stem cells killed by chemotherapy or radiotherapy can be regenerated and cause disease relapse.
More studies on how epithelial cell flexibility is achieved will be crucial to the understanding of cancer recurrence and will provide critical insight into cancer treatment.
Targeting WNT in colorectal cancer
One of the key signalling pathways regulating intestinal stem cell numbers is WNT signalling – a network of proteins functioning together to promote cell growth and maintain stem cell identity.
Unsurprisingly, cancer cells often hijack WNT signalling resulting in excess tissue growth. In particular, the mutation of the APC gene – one of the WNT signalling components – is found in over 80% of bowel cancers. The mutation locks the cells in a constant WNT-active state, promoting cell growth. This is often the first step of tumour initiation in bowel cancer.
Although WNT signalling is an obvious drug target, the risk of toxicity means that currently there is no approved drug in the clinic targeting the pathway, or indeed APC mutations. Inhibition of WNT signalling kills not only cancer cells but also the surrounding normal stem cells in the healthy intestine. This causes severe gastrointestinal inflammation by disrupting the regeneration of intestinal epithelium. To address the toxicity challenge, work in my lab explores whether WNT signalling is differentially activated in cancer cells and healthy stem cells, which may potentially offer unique treatment opportunity.
Using CRISPR gene-editing tools to model different APC mutations in human cells, we have discovered a unique aspect of WNT activation in cancer cells involving a protein known as USP7 – ordinarily involved in protein destruction machinery. Inhibition of USP7 selectively suppresses WNT activity and cell growth in APC-mutated cancer cells, but not normal cells. Excitingly, this finding represents a new tumour-specific target for bowel cancer patients carrying an APC mutation. I am hopeful that in time, this may overcome the toxicity issue in WNT inhibitor development.
We also managed to gain evidence for the tumour-specific role of the USP7 inhibitor by using another exciting aspect of current CRC research – organoids.
Organoid technology has proven to be a major biomedical research breakthrough in recent years. They are essentially stem cells growing three-dimensionally in a dish to form balls of cells resembling the structure and function of the native organ.
Organoids, or “mini-organs”, expand and differentiate into various cell types able to carry out different functions. Organoids also have the added benefit of no real limits on growth and expansion in the lab – making them an ideal cell source for regenerative medicine.
My lab has pioneered the use of mini-gut organoids to build functional small bowel constructs in a dish, with the intention to treat patients with intestinal failure – a condition where the gut fails to absorb sufficient water, nutrients and electrolytes to sustain life.
In collaboration with the Great Ormond Street Hospital, we managed to generate small bowel grafts in a dish using patient-derived cells and scaffolds. The engineered grafts were able to digest complex sugar, absorb amino acids, and exhibit some degree of barrier function. These lab-grown intestinal grafts could offer a safe alternative to traditional donor transplants in the future, overcoming organ shortage and graft-host rejection issues.
Alongside their regenerative power, organoids can also be used for disease modelling. Cancer organoids can be established from tumour biopsies or resected tumours from patients, which can then be used for drug screening and drug safety tests.
Tumour organoids have been shown to faithfully represent the morphology, histopathology and mutational landscape of the primary tumours. They can also be applied for personalised medicine, whereby tumour organoids established from patients will be expanded in the lab and used for drug screening to identify good responders before the treatment. Stratifying patients in this way can, of course, improve cancer treatment efficiency and reduce unnecessary side effects for patients who may not respond to the treatment.
Although the technology is still in pre-clinical stage, organoids offer an exciting research tool to bridge the gap between basic science and clinical translation to advance our understanding of cancer biology and improve treatment efficacy. With additional safety, quality control and standardisation, I am convinced organoid technology will be able to apply to a wide range of human diseases in the near future.
Vivian Li is a group leader at the Francis Crick Institute.