A team led by The Children's Hospital of Philadelphia's Marni J. Falk, MD, has expanded next-generation gene sequencing tools designed to unlock the secrets hidden in nuclear DNA to analyze a separate source of DNA — that found within mitochondria. Mitochondria, which contain their own DNA, are key suppliers of the energy needed for the multiple functions of our cells, and play a pivotal role in human health and disease.

"A first step in developing treatments for a disease is to understand its precise cause," said Dr. Falk, director of 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."

The investigators described their customized, comprehensive test — which they call the "1:1000 Mito-Plus Whole-Exome" kit — recently in Discovery Medicine. To achieve their results, the study team adapted an existing whole-exome sequencing kit, expanding it to encompass the mitochondrial genome.

While individual mitochondrial diseases are very rare, hundreds of causes of mitochondrial diseases are known. Some originate in mutations in DNA specific to the mitochondria, 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, and is largely based on the pioneering research by CHOP's Douglas C. Wallace, PhD, a co-author of the current study. However, 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, Dr. Falk's kit has the sensitivity to detect mitochondrial genome mutations present at levels as low as 8 percent.

A second study led by Dr. Falk reviews progress made in diagnosing mitochondrial disease at a single center over a rapidly changing three-year period before whole-exome sequencing was generally available. The retrospective review, published recently in Neurotherapeutics, covers 152 child and adult patients evaluated at CHOP's Mitochondrial-Genetics Diagnostic Clinic from 2008 to 2011.