martedì 16 ottobre 2007

DNA Sequencing Becomes Much Quicker


Science Daily — A new technique that combines gene chip technology with the latest generation of gene sequencing machines to allow fast and accurate sequencing of selected parts of the genome has been developed by researchers from the Human Genome Sequencing Center at Baylor College of Medicine in Houston and NimbleGen Systems, Inc., a Wisconsin-based company recently purchased by Roche Applied Science.
"This new technology will replace polymerase chain reaction (PCR) for many purposes," said Dr. Richard Gibbs, director of the HGSC and senior author of the report. "If the aim is to sequence a whole genome for everybody, this is a huge step in that direction."
The report, which appears in Nature Methods, describes the use of microarrays to enrich or increase the volume of specific genomic sequences. High throughput DNA sequencing machines made by 454 Life Sciences then determine the exact genetic code of the material.
For example, if scientists were looking for a mutation in a particular cancer-causing gene (such as BRCA1 that is associated with breast and ovarian cancer), they could make a microarray that is complementary to the part of the genome in which one is interested. This takes advantage of the fact that the adenine (A) bases always attach to the thymine (T) and the cytosine (C) always attaches to the guanine (G) in reactions.
"You take the DNA and you hybridize it (allow the DNA to stick to its complement) on the chip," said Dr. George Weinstock, co-director of the HGSC. "Then you wash away everything that doesn't stick. This can enrich the portion of the genome to be studied by factors of three hundred or more."
The new process is simpler, more accurate and efficient than the multiplex PCR that was previously used to sequence portions of the genome. In one experiment, more than 6,400 exons (the part of the genetic code that carries the instructions for making proteins), were analyzed. Using the old technology this would have taken at least six months.
"We hope to be able to use this to sequence all the exons in the genome," Gibbs said.
Resequencing of genes or other genomic regions of interest is a key step in detecting mutations associated with various complex human diseases, such as cancer, asthma and heart disease. The predominant method for selection of specific genomic regions for resequencing has primarily relied on PCR (polymerase chain reaction) to enrich for specific DNA fragments.
However, PCR is limited in the length of sequence it can amplify, is difficult to scale or multiplex for the enrichment of thousands of fragments, and has limited performance in the repetitive regions typical of complex genomes, such as human. The sequence capture microarray technology bridges the gap between next-generation DNA sequencing technology and current sample preparation methods by providing an adaptable, massively parallel method for selective enrichment of genomic regions of interest. Roche NimbleGen's sequence capture technology enables high-performance targeting of thousands of specific genes or loci using a single microarray hybridization-based enrichment process.
The Nature Methods paper published by Baylor demonstrates that the sequence capture process is simpler, more accurate, more efficient and more cost-effective than the multiplex PCR that was previously used to prepare genomic samples for sequencing. In one experiment, more than 6,700 exons (the part of the genetic code that together form genes), were enriched and analyzed, as well as contiguous genomic regions of up to 5 million bases.
The study, entitled "Direct Selection of Human Genomic Loci by Microarray Hybridization," appears online (14 October 2007 ahead of print) in the journal Nature Methods.
The authors are Albert TJ, Molla MN, Muzny DM, Nazareth L, Wheeler D, Song X, Richmond TA, Middle CM, Rodesch MJ, Packard CJ, Weinstock GM, and Gibbs RA. (DOI:10.1038/NMETH1111)
Others who took part in this study include David Wheeler, Donna Muzny and Xingzhi Song of BCM and Thomas J. Albert, Michael N. Molla, Lynne Nazareth, Todd. A. Richmond, Chris M. Middle, Matthew J. Rodesch and Charles J. Packard of NimbleGen.
Funding for this work came from the U.S. National Human Genome Research Institute and the National Cancer Institute.
Note: This story has been adapted from material provided by Baylor College Of Medicine.

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