p53

Outsmarting cancer’s deadly tricks

/ Todd Dubnicoff / Leave a comment

Cancer cells are devious monsters that kill people by sabotaging normal cell functions toward a path of uncontrolled cell growth. Without an effective treatment, aggressive cancers can crowd out healthy tissue and ultimately cause organ failure and death. This devastation by design makes it seem as though a cancer cell has a mind of its own but in reality it’s all due to mindless mutations in DNA. Gaining a deep understanding of those mutations provides scientists with insights into the molecular mechanisms of cancer which can help pinpoint targets for potential cancer treatments.

A team at The Scripps Research Institute (TSRI) followed the trail of such a mutation in a gene called POT1. Today in Cell Reports the researchers, funded in part by CIRM, describe their identification of a novel mechanism for cancer progression in cells carrying the POT1 mutation and they also speculate on the development on a unique therapeutic strategy.

Chromosomes go to pot without POT1
The POT1 protein is one component of shelterin, a multi-protein structure that binds to and protects telomeres, a region of DNA found at the ends of chromosomes. The team found that when POT1’s function is disrupted by mutation, the telomeres become vulnerable to damage which leads to chromosome instability. As a result, many regions of DNA on the chromosomes get rearranged leading to further gene mutations that in turn can accelerate the process of cancerous growth.

Telomere_caps

Human chromosomes (grey) capped by telomeres (white) Wikipedia

However, in the case of POT1 mutations, the DNA damage in the unstable chromosomes stimulates an enzyme called ATR that’s known to shut down cell division and initiate apoptosis, or programmed cell death. Now, unless I’m missing something, cells that have either stopped dividing or even died would seem to be the opposite of cancer progression. So why then are POT1 mutations found in a number of cancers such as leukemia, melanoma (skin cancer) and glioma (brain cancer)? As TSRI Associate Professor Eros Lazzerini Denchi, a co-leader on the publication, mentions in a press release, this conundrum presented an opportunity to better understand POT1 related cancers:

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Eros Lazzerini Denchi

“Somehow those cells found a way to survive—and thrive. We thought that if we could understand how that happens, maybe we could find a way to kill those cells.”

 

Mutant POT1 and p53: diabolical partners in cancer progression
The team looked for answers by studying the POT1 mutation in the presence of a mutated form of the p53 tumor suppressor gene, found in over 50% of all human cancers. Mice bred with the POT1 mutation alone formed no cancers while those animals with the p53 mutation alone developed T cell lymphomas, a type of immune system cancer, by 20 weeks and survived 24 weeks. Mice with both mutations fared much worse with median survival times of just 17 weeks. So somehow the p53 mutation was bringing out the potential of the POT1 mutation to cause aggressive cancer growth.

Further experiments revealed that the p53 mutation quashed the ATR enyzme’s programmed cell death signal which the team had shown was stimulated by the POT1 mutation. As a result, the cells avoided programmed cell death. Because the cells had no mechanism to die, more cancer-causing mutations had the opportunity to develop from the chromosome instability caused by the POT1 mutation.

The bright side to this diabolical cooperation between mutant POT1 and p53 is that it presents a possible opening for new treatment strategies. It turns out that no cell, not even a cancerous one, can survive in the complete absence of ATF. Since cells with the POT1 mutations already have a reduced level of ATF, the authors suggest that delivery of low doses of ATF inhibitors, which have already been developed for the clinic, could kill cancer cells without affecting healthy cells. No doubt the team is eager to follow up on this hypothesis.

It’s comforting to know that there are crafty scientists out there who are closing in on ways to outsmart the sneaky tactics of cancer cells. And it wouldn’t be possible without this fundamental research, as Lazzerini Denchi points out:

“This study shows that by looking at basic biological questions, we can potentially find new ways to treat cancer.”

 

Bringing down the gatekeeper for a stem cell-based Parkinson’s cure

/ Todd Dubnicoff / Leave a comment
Feng-dopamine-HI

University of Buffalo researchers converted these dopamine neurons directly from human skin cells. Image shows a protein found only in neurons (red) and an enzyme that synthesizes dopamine (green). Cell DNA is labeled in blue.

On the surface, a stem cell-based cure for Parkinson’s disease seems pretty straight-forward. This age-related neurodegenerative disorder, which leads to progressively worsening tremors, slowness of movement and muscle rigidity, is caused by the death of a specific type of nerve cell, or neuron, that produces the chemical dopamine in a specific region of the brain. So it would seem that simply transplanting stem cell-derived dopamine-producing neurons (DA neurons) in the brains of Parkinson’s patients to replace the lost cells would restore dopamine levels and alleviate Parkinson’s symptoms.

Easier said than done
Well, it hasn’t turned out to be that easy. After initial promising results using fetal brain stem cell transplants in the 80’s and 90’s, larger clinical trials showed no significant benefit and even led to a worsening of symptoms in some patients. One potential issue with those early trials could have been variable cell composition of the fetal cell-based therapy. On top of that, the availability of fetal tissue is limited and the quantities of transplantable cells obtained from these samples are very low.

More recently, researchers have been busy at generating more pure populations of DA neurons from human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs). Great progress has been made so far, but the field is still hampered by not being able to make enough DA neurons from hESCs and iPSCs in a timely manner.

Cutting out the pluripotent “middle man”
This week a research team at the University of Buffalo reported in Nature Communications about a much more efficient method for producing DA neurons. It’s a finding that could provide a strong push towards stem cell-based therapy development for Parkinson’s disease.

The team bypassed the need to start with hESCs or iPSCs and instead converted skin cells directly into DA neurons. A thorough analysis of the cells confirmed that they were functional and matched that characteristics of the specific dopamine neurons that are lost in Parkinson’s.

This direct reprogramming of skin cells into DA neurons as well as other cells is a technique pioneered by several independent researchers including some of our own grantees. This method is thought to have a few advantages over the specialization of immature hESCs or iPSCs into tissue-specific cells. Not only is the direct reprogramming process faster it also doesn’t require cell division so there’s less concern about the introduction of DNA mutations and the potential of tumor formation. Another plus for direct reprogramming is the possibility of inducing the direct conversion of one cell type into another inside the body rather than relying on the manipulation of hESCs and iPSCs in the lab. Still, despite these advantages the efficiency of direct reprogramming is still very low. That’s where the University of Buffalo team comes into the picture.

Bringing down the gatekeeper
The researchers led by physiology and biophysics professor Jian Feng, made a few key modifications to increase the efficiency of the current skin cell to DA neuron direct reprogramming methods. They first reduced the level of a protein call p53. This protein has several nicknames like “guardian of our genes” and “tumor suppressor” because it plays critical roles in controlling cell division and DNA repair and, in turn, helps keep a clamp on cell growth.

Reducing the presence of p53 during the direct reprogramming process led to a much more efficient conversion of skin cells to DA neurons. And because the conversion from a skin cell to a neuron happens quickly – just a couple days – timing the introduction of cell nutrients specific to neurons had to be carefully watched. Together, these tweaks improved upon previous studies as Feng mentioned in a University of Buffalo press release:

“The best previous method could take two weeks to produce 5 percent dopamine neurons. With ours, we got 60 percent dopamine neurons in ten days.”

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Jian Feng, PhD, professor of physiology and biophysics, Jacobs School of Medicine and Biomedical Sciences, University of Buffalo

IMHO (In my humble opinion)
I imagine there’s a lot more work ahead to get this method of deriving DA neurons from skin ready for the clinic. This reprogramming technique relied on the introduction of neuron-specific genes into the skin cells using a deactivated virus as the means of delivery. Even though the virus is inactive, its viral DNA randomly inserts into the cells’ chromosomes which can turn on genes that cause cancer. Therefore, a non-viral version of this method would need to be developed for clinical use.

Also, as mentioned earlier, since p53 inhibits tumors by suppressing uncontrolled cell division, it would be important to make sure that a reduction of p53 didn’t lead to any long-term negative consequences, like the transplantation of potentially cancerous cells into the patient.

Still, this dramatic increase in efficiency for making functional DA neurons and the identification of p53 as a key control point for direct reprogramming are very exciting developments for a disease field that is committed to finding cures for its patients.

Related links:

From the Stem Cellar archives: blogs about direct reprogramming
Video: CIRM Grantee Marius Wernig discusses direct reprogramming
Video: Thirty second elevator pitch describing direct reprogramming

Stem Cell Scientists Reconstruct Disease in a Dish; Gain Insight into Deadly Form of Bone Cancer

/ cirmweb / Leave a comment

The life of someone with Li-Fraumeni Syndrome (LFS) is not a pleasant one. A rare genetic disorder that usually runs in families, this syndrome is characterized by heightened risk of developing cancer—multiple types of cancer—at a very young age.

People with LFS, as the syndrome is often called, are especially susceptible to osteosarcoma, a form of bone cancer that most often affects children. Despite numerous research advances, survival rates for this type of cancer have not improved in over 40 years.

shutterstock_142552177 But according to new research from Mount Sinai Hospital and School of Medicine, the prognosis for these patients may not be so dire in a few years.

Reporting today in the journal Cell, researchers describe how they used a revolutionary type of stem cell technology to recreate LFS in a dish and, in so doing, have uncovered the series of molecular triggers that cause people with LFS to have such high incidence of osteosarcoma.

The scientists, led by senior author Ihor Lemischka, utilized induced pluripotent stem cells, or iPSCs, to model LFS—and osteosarcoma—at the cellular level.

Discovered in 2006 by Japanese scientist Shinya Yamanaka, iPSC technology allows scientists to reprogram adult skin cells into embryonic-like stem cells, which can then be turned into virtually any cell in the body. In the case of a genetic disorder, such as LFS, scientists can transform skin cells from someone with the disorder into bone cells and grow them in the lab. These cells will then have the same genetic makeup as that of the original patient, thus creating a ‘disease in a dish.’ We have written often about these models being used for various diseases, particularly neurological ones, but not cancer.

“Our study is among the first to use induced pluripotent stem cells as the foundation of a model for cancer,” said lead author and Mount Sinai postdoctoral fellow Dung-Fang Lee in today’s press release.

The team’s research centered on the protein p53. P53 normally acts as a tumor suppressor, keeping cell divisions in check so as not to divide out of control and morph into early-stage tumors. Previous research had revealed that 70% of people with LFS have a specific mutation in the gene that encodes p53. Using this knowledge and with the help of the iPSC technology, the team shed much-needed light on a molecular link between LFS and bone cancer. According to Lee:

“This model, when combined with a rare genetic disease, revealed for the first time how a protein known to prevent tumor growth in most cases, p53, may instead drive bone cancer when genetic changes cause too much of it to be made in the wrong place.”

Specifically, the team discovered that the ultimate culprit of LFS bone cancer is an overactive p53 gene. Too much p53, it turns out, reduces the amount of another gene, called H19. This then leads to a decrease in the protein decorin. Decorin normally acts to help stem cells mature into healthy, bone-making cells, known as osteoblasts. Without it, the stem cells can’t mature. They instead divide over and over again, out of control, and ultimately cause the growth of dangerous tumors.

But those out of control cells can become a target for therapy, say researchers. In fact, the team found that artificially boosting H19 levels could have a positive effect.

“Our experiments showed that restoring H19 expression hindered by too much p53 restored “protective differentiation” of osteoblasts to counter events of tumor growth early on in bone cancer,” said Lemischka.

And, because mutations in p53 have been linked to other forms of bone cancer, the team is optimistic that these preliminary results will be able to guide treatment for bone cancer patients—whether they have LFS or not. Added Lemischka:

“The work has implications for the future treatment or prevention of LFS-associated osteosarcoma, and possibly for all forms of bone cancer driven by p53 mutations, with H19 and p53 established now as potential targets for future drugs.”

Learn more about how scientists are using stem cell technology to model disease in a dish in our special video series: Stem Cells In Your Face: