Whoever said too many cooks spoil the broth must not have been a geneticist.
According to HHMI investigator Richard Lifton, a kitchen full of cooks—plus some very high-end cookware—was exactly what was needed to define the molecular defects underlying a severe form of hypertension, and a specific type of tumor.
Lifton, a human geneticist at Yale School of Medicine, has long been interested in a type of adrenal gland tumor that causes high blood pressure. Normally, the adrenal gland, located atop the kidney, secretes the steroid hormone aldosterone into the blood. The hormone instructs the kidney to retain water and salt (and hence elevate blood pressure) in times of acute physiological stress, such as blood loss or salt imbalance.
Some 5 to 10 percent of patients with extreme hypertension, however, have tumors of the adrenal gland. These benign, sometimes massive tumors pump out unregulated levels of aldosterone into the bloodstream. Unrestrained aldosterone production causes the kidney to retain sodium, keeping blood pressure high. The condition is curable only by surgical removal of the adrenal gland.
Lifton postdoctoral fellows Murim Choi and Ute Scholl and a team of collaborators recently discovered the surprising cause of a subset of those adrenal tumors: mutations in a gene encoding an ion channel protein. Normally that channel, known as KCNJ5, allows potassium ions to pass in and out of cells. But in the tumor cells, the channel also allows sodium ions to leak through, which activate signaling pathways that stimulate tumor cell growth and unregulated aldosterone production.
“It seemed so obvious that these tumors were caused by somatic mutations,” says Lifton, referring to mutations that are acquired rather than inherited. “But it wasn’t until we could sequence all the DNA in a tumor that we could identify what those mutations were.”
That’s where all the cooks come in, starting with physicians at Uppsala University in Sweden, who diagnosed hypertensive patients with adrenal tumors, surgically removed the tumors, and then meticulously stored tumor specimens to preserve their DNA.
Lifton then did a high-tech search through the 23,000 or so genes in the patients’ tumor cell DNA to find culprit mutations. That analysis, called whole exome sequencing, was a two-step process. First, the team used gene microchips to “capture” the approximately 1 percent of a cell’s 3 billion DNA letters that contains genes (the exome). Second, they sequenced that relatively small portion of the genome using next-generation machinery at the Yale Center for Genome Analysis.
Choi took on the daunting task of devising computer programs to make sense of all that DNA data—including whole exome analysis of 4 patients and partial analysis of 18 more. In the end, his analysis identified 8 patients harboring mutations in the KCNJ5 channel gene. According to Lifton, the odds of that happening by chance are 10–30.
But was it structural damage that made the mutant KCNJ5 channels leaky? Collaborating with Wenhui Wang at New York Medical College in Valhalla, Scholl answered the question experimentally. When she studied the mutant channels in cultured cell lines, she found that they allowed sodium ions to flow abnormally into the cells. The work was reported February 11, 2011, in Science.
Particularly notable to Lifton was how beautifully this story meshed with predictions made in the 1990s by fellow HHMI investigator Rod MacKinnon of Rockefeller University.
Murim Choi spent his hours outside the lab learning computer programming.
MacKinnon had shown that channel proteins allow only certain ions to pass through due to the configuration of protein building blocks that form a gate—a so-called “selectivity filter.” For that work, he was awarded the Nobel Prize in Chemistry in 2003.
“One mutation we found in KCNJ5 in tumors was in a residue that MacKinnon had defined as critical for potassium selectivity for similar channels,” says Lifton. MacKinnon’s work offers a satisfying architectural explanation for why sodium ions slip through the mutant channel, he adds.
The KCNJ5 story is the first report of an ion channel playing a role in the unbridled cell proliferation characteristic of tumors. Whether this knowledge will lead to less invasive treatment for adrenal tumors remains to be seen. In the meantime, Lifton envisions a simple blood test to detect KCNJ5 mutations to help diagnose this type of adrenal tumor.
Now, almost two decades after Lifton began his effort to discover the genetic basis of hypertension, his lab has identified 10 or so genes that when mutated increase blood pressure. All, including KCNJ5, control regulation of salt balance. He’s hoping these investigations will lead to better treatment strategies for the approximately 1 billion patients worldwide who have hypertension, a major risk factor for heart disease and stroke.
“To treat hypertensive disease, we often use three or more drugs per patient, and about two-thirds of those patients don’t improve under that kind of control,” says Lifton. “We must figure out a better way to treat these patients.”