Markers for AIDS
In the course of an AIDS infection, the HIV virus can infiltrate the central nervous system and bring about dementia and encephalitis. With antiretroviral therapy, patients are living longer and may encounter significant neurocognitive disorders. According to William Wikoff, Ph.D., research associate at Scripps Research Institute, metabolomic profiling has the potential to provide early diagnosis of neural involvement and to predict the progression of the condition to a full-blown CNS disease. Moreover, it may eventually be possible to determine response to treatment and provide insights into the pathogenesis, treatment, and prevention of AIDS.
Although there have been numerous proteomic investigations of urine, serum, and CNS fluid from patients, these studies have provided few insights, driving the search for alternative screening tools. For neuroinfectious, neurodegenerative, and psychiatric disorders, the cellular and phenotypic complexity of the brain has hindered biomarker identification. Dr. Wikoff and his colleagues have invoked a new strategy, focusing on metabolomic studies to reveal, until now, undiscovered markers.
The Scripps group uses SIV infection of rhesus macaque monkeys as a model for human AIDS. The animals show the same pattern of symptoms, including dementia, CNS degeneration, and encephalitis observed in human AIDS patients. The experimental design that the team followed involved collecting cerebrospinal fluid before and after viral infection.
“The screening platform used a nontargeted, mass-based metabolomics approach with online data base screening to identify metabolites,” explains Dr. Wikoff.
Because the blood-brain barrier is partially compromised during HIV infection, the concentration of albumin and many metabolites such as fatty acids and phospholipids are increased. For instance, carnitine and acylcarnitines are elevated in infection as are various other unknown metabolites.
The Scripps researchers have searched for molecules that are specifically altered in monkeys showing encephalitis in the course of their SIV infection. They subsequently performed a microarray analysis to track down changes that relate to lipids or lipid processing, and identified increased expression of phospholipases in the brains of animals with encephalitis. Through the use of in situ hybridization and immunohistochemistry of brain tissue from infected animals it was determined that elevated phopholipases occurred in astrocytes and in the ventricles of the choroid plexus.
“We believe the identification of specific metabolites illustrates the potential of mass-based metabolomics to elucidate neurodegenerative diseases,” Dr. Wikoff concludes.
Various omics approaches have been used to develop databases of biomarkers for a number of years, yet it has proven extremely difficult to assemble consistent bodies of information that can be used to develop new disease biomarkers. In numerous cases, studies cannot be replicated and putative biomarkers cannot be validated. There are a number of reasons for these failures, but inadequate sample size, failure to control methodological variation, and inadequate quality assurance have been contributing factors.
Success within the omics field is absolutely essential for the development of a robust biomarkers. If we have learned anything from the advance of biomedical science in the last half century it is that diseases that are virtually untreatable by any available technology when they reach an advanced stage can be cured by essentially trivial and low-tech intervention during their early stages. It is virtually assured that no significant progress will be made until rapid, sensitive, cheap, and noninvasive methods are available for early detection.