10 January 2017
The transfer of mitochondrial DNA between cells in tumour models was first shown by Professors Mike Berridge (Malaghan Institute) and Jiri Neuzil, (Griffith University, Queensland and Institute of Biotechnology, Prague) and their research teams. The groundbreaking discovery was published in Cell Metabolism in 2015 and highlighted in Nature Reviews Cancer, both leading scientific journals. This previously unreported phenomenon is being investigated further thanks to New Zealand research grants from the Cancer Society, the Health Research Council and the Marsden Fund totalling $2.4 million. Other collaborative research funding through Dr Melanie McConnell at Victoria University of Wellington and Prof Neuzil in Prague is also supporting a wider range of projects concerned with intercellular mitochondrial transfer.
BONE MARROW TRANSPLANTATION STUDIES
One area of research is investigating mitochondrial DNA transfer following bone marrow transplantation in humans and in a mouse model. Recipients are given therapies to suppress the growth of their bone marrow or abnormally proliferating bone marrow cells, before receiving replacement bone marrow containing stem cells from a matched donor. After the transplant, recipients have a mixture of donor cells and their bone marrow cells, a small proportion of which may have imported mitochondrial DNA.
“We will sequence the mitochondrial DNA before and after transplantation to find out whether any donor DNA ends up in the patient’s own cells. We are also looking at erythroblasts (redcell precursors) in culture to see if they form the tiny tubular membrane connecting structures that were first observed more than 20 years ago, but are still not understood. These connections could provide a physical link for mitochondrial movement between cells,” says Professor Berridge. Mitochondrial DNA transfer is thought to happen more readily in damaged cells. In mouse studies, most of the recipient bone marrow will be damaged by irradiation before the donor marrow is given. “We are using every tool we have available to explore transplantation in mice, the result of which will shine light on what’s going on in humans. It’s more powerful than just using genetic tools because we are able to use fluorescent markers built into the mouse genome to see how the process occurs and how quickly it happens. Our experimental model system is highly relevant to human bone marrow transplantation.”
To date mouse chimeras (with mitochondrial DNA from donor and recipient) have been made and tagged with markers, so the movement from donor to recipient can be tracked. This is the first time that mitochondrial transfer between cells has been investigated in mouse bone marrow.
Following on from our research with breast cancer and melanoma cells lacking mitochondrial DNA, mitochondrial transfer is now being investigated in a highly treatment-resistant type of brain cancer, glioblastoma. The cancer cells’ mitochondrial DNA is removed chemically so their ability to import mitochondrial DNA from a host can be explored. “We know that without mitochondrial DNA the tumour cells won’t grow, but when it’s been picked up, they begin to grow as tumours. Our sequencing work shows that the growing tumours definitely contain the signature of the mitochondrial DNA of the mouse the cells were injected into. This confirms that mitochondrial DNA has been transferred from the host mouse. In collaborative experiments carried out in Prague, we have shown that whole mitochondria are transferred in this process.
Dr Melanie McConnell, Malaghan Institute Research Associate and Senior Lecturer at Victoria University, works closely with Prof Berridge and leads a team of researchers. “We had observed the transfer of whole mitochondria and mitochondrial DNA, but to study how often it was happening required us to develop better molecular tools,” she says. “These include quantitative mitochondrial genotyping techniques and the development of new cell lines, which enable us to see how often mitochondrial transfer occurs, and how big a contribution it makes to the host cell. The new techniques are sensitive enough to spot mitochondrial DNA from another cell at 1 in 5000–10,000 events.”
BRAIN CELL RESEARCH
Dr McConnell’s research team has carried out parallel studies with brain cancer cells and healthy brain cells that have been injured, then determined whether the exposure to other cells improves their recovery after injury – and whether mitochondrial transfer is implicated in the recovery. “The damaged cells have no choice – they either take what is available to them or they die. We’ve observed that about 20 percent of damaged normal brain cells take up mitochondria from another cell.” One puzzling observation is being investigated using tagged cells, confocal microscopy and clever computing. “We routinely see foreign mitochondria on the surface of a cell, but only very infrequently inside a cell. We developed a neural network that enables us process hundreds of images automatically and pull out the ones where the cells have foreign mitochondria inside. It may be that mitochondrial DNA transfer actually happens from the cell surface – we just don’t know.”
This research has important implications for finding new treatments cancer and neurodegenerative diseases. “In one case, injury is desirable to cause cell death and in the other, the goal is to prevent it. Understanding these complex processes can only be helpful in developing new therapies for both kinds of brain disease.”
Professor Mike Berridge, Dr James Baty, Dr David Eccles, Carole Grasso, Dr Patries Herst, Dr Robert Weinkove, Kate White.
Dr Melanie McConnell, Georgia Carson, Leticia Castro, Daniel Hudson, Matthew Rowe, Remy Schneider, Dinindu Senanayake, Marie-Sophie Fabre.