More powerful gene-sequencing tools have increasingly been uncovering disease secrets in DNA within the cell nucleus. Now a research team is expanding those rapid next-generation sequencing tests to analyze a separate source of DNA—within the genes inside mitochondria, cellular power plants that, when abnormal, contribute to complex, multisystem diseases.
The study team, headed by a specialist in mitochondrial medicine at The Children's Hospital of Philadelphia (CHOP), adapted next-generation sequencing to simultaneously analyze the whole exome (all the protein-coding DNA) of nuclear genes and the mitochondrial genome. "A first step in developing treatments for a disease is to understand its precise cause," said Marni J. Falk, M.D., the director and attending physician in the Mitochondrial-Genetic Disease Clinic at Children's Hospital. "We have developed a one-step, off-the-shelf tool that analyzes both nuclear and mitochondrial DNA to help evaluate the genetic cause of suspected mitochondrial disease."
Falk and colleagues describe their customized, comprehensive test, which they call the "1:1000 Mito-Plus Whole-Exome" kit, in the journal Discovery Medicine, published Dec. 26, 2012. Her co-corresponding author, biostatistician Xiaowu Gai, Ph.D., now of the Loyola University Stritch School of Medicine, collaborated on developing the test while at Children's Hospital.
While each mitochondrial disease is very rare in the population, hundreds of causes of mitochondrial diseases are known. Some originate in mutations in DNA specific to the mitochondria, tiny structures located outside the cell nucleus, while many other mitochondrial diseases are based in nuclear DNA genes that affect mitochondrial function. The role of mitochondria in human disease has been recognized only since the 1980s, based on pioneering research by Douglas C. Wallace, Ph.D., now at Children's Hospital, and a co-author of the current study.
Many mitochondrial diseases remain poorly understood. One complicating factor is heteroplasmy—a mixture of mutated and normal mitochondrial genomes within the same cells or tissues. In contrast to conventional gene sequencing, which can detect only heteroplasmic mutations that reach levels of at least 30 to 50 percent, the customized kit has the sensitivity to detect mitochondrial genome mutations present at levels as low as 8 percent. To achieve their results, the study team adapted an existing whole-exome sequencing kit from Agilent Technologies, expanding it to encompass the mitochondrial genome.
The availability of the new kit, said Falk, if used for either clinical or research purposes, may shorten the "diagnostic odyssey" experienced by many patients and families seeking the cause of debilitating and puzzling symptoms. "Many families travel from one specialist to another for years, searching for the cause of their rare disease," she says. Specific treatments are not always available, but identifying their disease cause may be the first step toward discovering treatments.