High-tech laboratory tools, like computers, are often updated publicly as their analytical capabilities expand. In the September issue of the Journal of Molecular Diagnostics, NIH grantees report they have developed a second generation "lab on a silicon chip" called the MitoChip v2.0 that for the first time rapidly and reliably sequences all mitochondrial DNA.
Mitochondria, the energy-producing organelles that power our cells, are unique because they are equipped with their own genetic instructions distinct from the DNA stored in the cell nucleus.
The authors say their full-sequence chip will be a key tool in accelerating research on mitochondrial DNA, a growing area of scientific interest. This interest stems from data that suggests natural sequence variations and/or mutations in each person's mitochondrial DNA could be biologically informative in fields as diverse as cancer diagnostics, gerontology, and criminal forensics.
According to Dr. Joseph Califano, a scientist at Johns Hopkins University School of Medicine in Baltimore and senior author on the paper, the MitoChip v2.0 showed in his group's hands better sensitivity that its predecessor to sequence variations in head and neck cancer samples. The v2.0 also detected nearly three dozen variations in the non-coding D-loop, long considered to be a sequencing no-man's land and which the original MitoChip did not include.
"At this point, we don't foresee a MitoChip v3.0," said Califano, whose research was supported by the NIH's National Institute of Dental and Craniofacial Research. "The v2.0 is a very good tool in that we've also arrayed 500 of the most common haplotypes - or grouped patterns of known DNA variations - banked in the mitochondrial public database."
Mitochondria are oblong, thread-like structures dispersed throughout the cell's cytoplasm. Hundreds to thousands of mitochondria exist in each human cell, occupying up to a quarter of their cytoplasm. Sometimes informally described as "cellular power plants," mitochondria convert organic materials into ATP, the cell's energy currency and without which life would cease.
As early as the 1920s, scientists uncovered clues that mitochondria might play a role in causing cancer. But like the other DNA in the cell nucleus, scientists lacked the needed research tools throughout most of the 20th century to systematically study the chemical composition of the mitochondrial genome, or complete set of genes, and its association to human disease.
In the early 1980s, scientists in England performed the then-Herculean feat of sequencing the complete human mitochondrial genome. The genome consisted of 16,568 base pair, or units, of DNA and encoded 37 contiguous genes.
By 1996, new technology brought new opportunity. Scientists with the company Affymetrix in Santa Clara, Calif. developed the first mitochondrial sequencing microarray. Roughly the size of a quarter, the silicon chip had lithographically annealed to it up to 135,000 short, arrayed bits of DNA sequence that, collectively, spanned most of a single strand of mitochondrial DNA.
The chip exploited the fact that DNA exists naturally as a double-stranded molecule. By gathering mitochondrial DNA and breaking it into short, single-stranded bits, the scientists showed that each bit would pair, or hybridize, with its complementary sequence arrayed on the chip. By crude analogy, each bit is like a unique magnet that sticks to its mirror image.
But if the extracted DNA contains mutations or other variations from the standard consensus sequence annealed to the chip, the bits with those changes would appear abnormal to the specially designed computer software programs that read the chip. The software programs will read not only the identity of the expected bases of DNA in a process called "base calling" but those of the variations.