Mootha and others don’t yet know whether dysfunctional mitochondria cause diabetes, are a by-product of the disease or are simply an unrelated correlative condition. Still, these results strengthen the hope for one simple remedy. “It has long been known that exercise increases mitochondrial content and efficiency,” Mootha says.
Mitochondria’s role in cancer has been hypothesized since the 1930s, when Otto Warburg, a Nobel Prize–winning German physician, discovered that mitochondria in cancer cells do a poor job of producing energy. And although the past 70 years have seen often halting progress in research on the subject, it’s now clear there is at least a coincidental connection between mitochondria and cancer. “All cancer cells examined so far have mutations in mitochondrial DNA,” says Keshav Singh, a mitochondrial geneticist at the Roswell Park Cancer Institute in Buffalo and founder of the Mitochondria Research Society. “And most cancer cells have defective mitochondria.”
But as with diabetes, the question is whether mitochondrial mutations or defects cause cancer or are merely correlated with it. Singh believes that defective mitochondria harm other parts of the cell, particularly the nucleus. “When mitochondria don’t work properly, they cause genetic instability in the nuclear genome,” Singh says, “and instability is a major cause of cancer.” The result is that mutations occur more frequently than normal, increasing the likelihood that genes involved in cell division or cell death will fail to work. (Uncontrolled cell division leads to cancer; programmed cell death prevents it.)
That instability may stem from mitochondria that produce an excess of free radicals, which can damage nuclear DNA. Or, Singh says, deficient mitochondria may trigger a process called error-prone repair—instead of fixing damaged DNA, it inserts incorrect nucleotides into the genetic code, increasing the likelihood that a cell will mutate and become cancerous.
No one yet knows which comes first—a malignant tumor or mitochondrial mutations—but the association itself may aid cancer detection. The existence of mutations could signal a cancer’s presence, and with recent breakthroughs in scanning bodily fluids for mitochondrial mutations, it may be possible to screen for early signs of disease. (Because there is a lot of mitochondrial DNA in cancer cells, mutations are easy to spot, and they appear early in cancer development.)
One diagnostic approach involves the MitoChip, a gene chip created by Johns Hopkins researchers in 2004 to detect mutations in mitochondrial genes. Singh is working on another method, the NucleoMito Chip, which looks for changes in expression of mitochondrial proteins encoded by nuclear genes. Both methods can potentially detect a variety of cancers rapidly, although clinical use is a few years away.
Singh is also working with scientists at the Sandia National Laboratories in Albuquerque on a device known as the Biocavity laser, which detects fluorescence in individual mitochondria. Some proteins in the mitochondria’s energy production machinery glow; when mitochondria malfunction, the glow is affected. “We can detect changes in a single cell,” Singh says. “Most of our success against cancer has been due to early detection, and if our tests become sensitive enough to find one bad mitochondrion, that may help us find cancer very early on.”
If this and other clinical outgrowths of research into the role of mitochondria succeed, they could open a fruitful focus for detecting and treating disease. “Mitochondria are like a bridge connecting two island suburbs—one island represents the many causes of a disease, the other the resulting symptoms,” says Auckland Hospital’s Barry Snow. “Knocking out all the houses on either side”—in other words, relieving all symptoms or eliminating all environmental and genetic causes—“is difficult. Targeting the narrow bridge”—mitochondrial dysfunction, which appears to link causes and symptoms—“is much easier.”