Two studies published this month have produced conflicting findings on the value of testing mitochondrial DNA levels in patients receiving nucleoside analogue treatment, muddying the already murky waters of research into the relationship between NRTIs and a spectrum of physical problems attributed to the toxic effects of this drug class on mitochondrial DNA.
Mitochondria produce energy within cells, but nucleoside analogues inhibit their replication by inhibiting the action of polymerase gamma.
Mitochondrial toxicity contributes to the development of:
- Peripheral neuropathy
- Lactic acidosis
- Pancreatitis
- Hepatic steatosis
- Myopathy
It has also been blamed for the development of lipaotrophy (fat wasting), although the mechanisms of fat wasting and fat redistribution are still highly contentious.
Dr Julio Montaner and colleagues at the University of British Columbia reported on the use of a mitochondrial DNA (mtDNA) assay to assess mtDNA levels in 8 patients with hyperlactatemia assumed to be caused by mitochondrial toxicity. All eight were receiving d4T and and six were also receiving ddI, and all had symptoms of hyperlactatemia such as fatigue, nausea, abdominal discomfort and weight loss. Two control groups, of 24 people without HIV and 47 treatment-naive people with HIV, were also evaluated.
The assay looked at levels of mtDNA in the peripheral blood mononuclear cells, and found that the ratio of mtDNA to nuclear DNA was nearly 70% lower in the patients with hyperlactatemia when compared with HIV-negative people, and 43% lower when compared with HIV-positive untreated people.
Mitochondrial DNA levels improved when nucleoside analogue treatment was stopped, and did not deteriorate when NRTI treatment without d4T was resumed, leading the authors to suggest that d4T may have an especially toxic effect on mitochondrial DNA.
The authors suggest that given the frequency of hyperlactatemia in patients receiving NRTI treatment, the test could be used to monitor mitochondrial toxicity.
However, research at Case Western Reserve University Hospital in Cleveland, United States failed to show any association between NRTI treatment and mitochondrial DNA depletion.
Dr Grace McComsey investigated mtDNA levels in peripheral blood leukocytes from ten patients with lipoatrophy. In no case was the ratio of mtDNA to nuclear DNA reduced when compared with four antiretroviral-naïve patients and ten people without HIV. Indeed, mtDNA levels were higher in the NRTI-treated patients when compared with the healthy control group, leading the authors to suggest that either mitochondrial DNA levels are not a good marker for mitochondrial dysfunction, or else mitochondrial damage inflicted by NRTIs is tissue-specific and will not show up in blood tests.
Recent research from Germany suggests that mitochondrial DNA depletion is detectable in the subcutaneous fat of people with lipoatrophy
Dr Ulrich Walker and colleagues compared levels of mitochondrial DNA in the subcutaneous buttock fat of 24 HIV-positive patients and 8 HIV-negative controls. Mitochondrial DNA levels did not differ between the control group and five patients receiving no NRTI therapy, but was 44% lower in those receiving NRTIs (p=0.01). Patients were also stratified according to lipoatrophy; in the eleven patients with lipoatrophy, mtDNA levels were 43% lower than the no-lipoatrophy group (p=0.01).
Lipoatrophy was associated with d4T treatment, and the duration of stavudine treatment (p=0.003) and a longer duration of protease inhibitor-containing HAART (33 months versus 19 months, p=0.01).
In order to determine whether reduced mtDNA levels were a consequence of increased levels of apoptosis, apoptosis was induced in Jurkat cells and an increase in the ratio of mitochondrial DNA to nuclear DNA consistent with the early destruction of nuclear DNA was observed, suggesting that mitochondrial depletion is not a consequence of apoptosis.
Cote HCF et al. Changes in mitochondrial DNA as a marker of nucleoside toxicity in HIV-infected patients. N Engl J Med 346 (11): 811-820, 2002.
McComsey G et al. Analysis of the mitochondrial DNA genome in the peripheral blood leukocytes of HIV-infected patients with or without lipoatrophy. AIDS 16: 513-518, 2002.
Walker U et al. Evidence of nucleoside analogue reverse transcriptase inhibitor-associated genetic and structural defects of mitochondria in adipose tissue of HIV-infected patients. JAIDS 29: 117-121, 2002.