Next, the investigators screened for PA-824-resistant M. tb that retained sensitivity to a close relative of PA-824. Within this subgroup of PA-824-resistant bacteria, the team identified those mutant strains with FGD1 and F420. The investigators reasoned that resistance to PA-824 in mutants possessing FGD1 and F420 must be due to a mutation in the M. tb protein that directly interacts with PA-824.
But determining exactly which of M. tb's thousands of proteins was changed in these mutants proved difficult, says Dr. Barry. Conventional genetic techniques for comparing normal and mutant strains of M. tb failed, so the team turned to a specially modified microarray-based technique, called comparative genome sequencing, developed by NimbleGen Systems, Inc. (Madison, WI). This was the first time the technique has been used to identify a protein involved in TB drug resistance, notes Dr. Barry.
Using the NimbleGen technique, which effectively re-sequences the entire genome of the bacterium, the scientists quickly pinpointed the protein altered in the PA-824-resistant mutant strains of M. tb. In the past, such a complete genome comparison might have taken many months of work; this new technology enables scientists to zero in on the specific genetic difference between mutant and normal bacterial strains in just days, says Dr. Barry.
The scientists found a total of four PA-824-resistant mutant strains: two lacked the newly described M. tb protein altogether, while the remaining two mutants evidently acquired resistance to PA-824 through a mutation that made the protein unable to bind to the drug, Dr. Barry says.
With the discovery of the specific protein that interacts with PA-824, Dr. Barry and colleagues, including researchers at the Novartis Institute for Tropical Diseases in Singapore, have information they can use to produce improved PA-824 relatives and acce
Contact: Anne A. Oplinger
NIH/National Institute of Allergy and Infectious Diseases