Hochedlinger pushes cells' developmental rewind buttons to examine their epigenetic history.

Text and Interview by Ruth Williams

Transferring a terminally differentiated cell nucleus into an egg cell that has had its own nucleus removed wipes away the epigenetic marks of differentiation and allows the nucleus once again to code for any cell type. Whether this trick was even possible was a fundamental question of developmental biology until Konrad Hochedlinger, as a Ph.D. student, finally delivered unequivocal proof (1).

Nuclear transfer is one of the most sophisticated, fickle, and challenging techniques in cell biology. Attempting it as a fresh-faced Ph.D. student is brave. Even his supervisor was worried. “He thought it was very risky and may not work at all, but I tried it anyway,” Hochedlinger says with disarming nonchalance.

At just 31 years old, Hochedlinger has already worked on therapeutic cloning in a mouse model (2), reprogramming cancer nuclei (3), and the molecular mechanisms controlling stem cell pluripotency (4). He has now been running his own lab at Harvard University for a year and a half. In a recent interview, he spoke with great enthusiasm about the new challenges of being a lab leader, about his remarkable career so far, and about the new direction in which he thinks reprogramming research is headed.

How did you get started in science?

My older sister got me into science. She studied biology at university. When I was deciding what to study, I was torn between medicine and biology. My sister told me that biology was really cool and that I should try it out. So it's my sister's fault. [laughs]

Since then, it has always been biology. As soon as I took my first genetics class at university (University of Vienna, Austria), I got really interested. Genetics provides an ideal tool for dissecting what humans or animals are about. As soon as I had my first lecture in genetics, I knew I definitely wanted to continue on that route.

What was the next step on that route?

I went to the Institute of Molecular Pathology (IMP, Vienna) to do my Master's thesis. I joined Erwin Wagner's lab. He studies bone development in mice. He makes knockout and transgenic mice to study the function of genes in bone development and cancer. Erwin's lab was where I got exposed to real science for the first time.

You went from studying bone development to asking more fundamental questions about development and its reversibility. How did you get interested in that?

I got interested in cloning during my undergraduate course, when I learned about John Gurdon's classic frog cloning experiments. It fascinated me. Then in '97 the paper by Ian Wilmut came out on the cloning of Dolly the sheep. Before that, nuclear reprogramming hadn't been shown in mammals. It was thought that mammalian cells might be refractory to cloning.

The reason I then was drawn into epigenetics and stem cell biology was a lecture at the IMP by Rudolf Jaenisch. I was really fascinated by the data he presented on the role of epigenetics in cloning and reprogramming.

That inspired you to apply to his laboratory?

Yes, I decided to come to Cambridge (Massachusetts) to visit my sister, and I stopped by MIT and talked to Rudolf. I started in his lab in March of 2000.

What was your project there?

For my Ph.D. thesis, I worked on nuclear transfer. I asked whether a terminally differentiated cell is still amenable to reprogramming and able to give rise to a cloned animal. This question had not been resolved unequivocally by the cloning of Dolly or other mammals.

What were the limitations of experiments such as Dolly?

They used adult cells, but it was possible that the cells that gave rise to successfully cloned animals were derived from rare adult stem cells. This also might have explained why cloning is inefficient: only 1–3% of cloned embryos eventually develop into an adult clone.

I took advantage of lymphocytes. These cells carry specific genetic marks that indicate their maturity—the genetic rearrangements responsible for antibody production. I was successful in cloning mice from lymphocytes and could show that the genetic marks were present in all the cells of the cloned mouse.

It's quite an ambitious project for a new Ph.D. student!

A pipette (right) holds the egg steady as a new nucleus (left) is injected.

It was a very risky project. Rudolf has since told me that he thought I wouldn't pull it off. He thought it might not work at all, that you might not be able to clone from fully differentiated cells.

After your Ph.D. you stayed in the same lab for your postdoc?

I stayed on in Rudolf's lab for another three years to do a postdoc, because I knew I wanted to stay in the reprogramming field. And at that point, there wasn't any other lab where I could really learn more. Rudolf's lab is one of the few that has all of these technologies together. Anything you can think of in mouse genetics or mouse embryology has happened in his lab, whether it's making mice from embryonic stem cells, making transgenic mice, nuclear transfer, embryonic stem cell biology, studying cancer models, or embryonic development.

What was life like in the Jaenisch Lab?

It was very rewarding. I would say most of what I know now I learned in his lab.

It's also a very critical lab. You learn very quickly in the lab meetings that you have to justify your interpretation of data. You learn how to think critically. I think that's a very important part of the training that everyone goes through. Rudolf pushes you hard, and that's good.

You've recently set up your own lab at Harvard. Have you continued the tradition of tough-love lab meetings?

Probably, subconsciously. I think you do things in the way you've been trained by your mentor. I try to be as critical as Rudolf used to be with me, and I hope it pays off with my people.

How has it been setting up the new lab?

It was stressful in the beginning in terms of hiring people, getting your experiments to work again, your cells to grow, your mice to breed, getting adjusted to a new environment, and to new colleagues. There are new responsibilities. All of a sudden you have to manage people. You have to make sure that their salaries are paid, that they're happy, and that they are making progress in their experiments.

But the more I'm experiencing it, the more I like it. It's really very rewarding to see people excited about the work they do.

Nuclear transfer can be a very fickle technique. Have you managed to set that up in your new lab?

It was a little difficult at the beginning to optimize it and to make all the special culture media for growing the cloned embryos. But the system is working now.

But actually I've recently become interested in an alternative way of reprogramming: using defined factors or genes. This is based on a landmark study by Shinya Yamanaka, published last year, that showed that just four defined factors, when introduced into skin cells, are sufficient to turn these cells back into embryonic stem cells. We've recently reproduced this data and extended the original findings.

Four factors? That's all you need to be embryonic again?

People were very skeptical that this was true and wondered whether there might be an alternative interpretation to the results. But recently, three groups, Yamanaka again, Rudy Jaenisch, and our group, independently reproduced the data. I think the field will now believe it. I certainly do.

Where do you plan to go with this?

Reprogrammed fibroblasts (green) can develop into different tissue types in a newborn chimeric mouse.

It allows us now to study reprogramming at a molecular level, which was impossible to do with nuclear transfer or cell fusion, because you had such limited cellular material. You can take large numbers of cells, expose them to these four factors, and ask what happens at the molecular level, what genes are turned on.

We know very little about the process, about what exactly goes on at the level of DNA and chromatin. What is downstream of those four factors? That's something I'm very interested in pursuing.

We also want to ask whether different cell types can be reprogrammed by the same four factors. If it works in humans, there are therapeutic implications. It may circumvent the ethical and logistical limitations associated with nuclear transfer.

This finding has revolutionized the entire stem cell and reprogramming field and has opened up many new avenues of research. I think we'll see a lot more exciting research in that area in the next few years, or even months.

Hochedlinger, K., and R. Jaenisch.
Rideout, W.M., III, et al.
Hochedlinger, K., et al.
Genes Dev.
Hochedlinger, K., et al.