Erythrocyte inosine triphosphatase activity is decreased in HIV-seropositive individuals

Metabolic events in HIV-infected patients using abacavir are associated with erythrocyte inosine triphosphatase activity

Objectives

Abacavir use has been associated with an increased risk of cardiovascular disease (CVD) and metabolic events in HIV-infected patients, although this finding was not consistently found. It is unclear whether abacavir only increases this risk in subpopulations of HIV-infected patients. It may be hypothesized that inosine 5'-triphosphate pyrophosphohydrolase (ITPase), an enzyme involved in the metabolism of purine analogues used in HIV treatment, plays a role in the risk of CVD and metabolic events in HIV-infected patients.

Methods

ITPase activity and ITPA genotype were determined in 393 HIV-infected patients. ITPase activity <4 mmol IMP/mmol Hb/h was considered decreased. ITPA polymorphisms tested were: c.94C>A (rs1127354) and c.124 + 21A>C (rs7270101). ORs were determined using generalized estimating equation models for developing CVD in patients who had ever been exposed to abacavir, tenofovir or didanosine and for developing metabolic events in patients currently using these drugs.

Results

In patients using abacavir, metabolic events were associated with ITPase activity. No association was demonstrated for tenofovir or didanosine. The OR for metabolic events was 3.11 in patients using abacavir with normal ITPase activity (95% CI 1.34-7.21; P = 0.008) compared with patients with decreased ITPase activity [adjusted for age, BMI, cumulative duration of combination ART (cART) use and the use of PI and NNRTI]. CVD was not associated with ITPase activity or ITPA genotype.

Conclusions

This study shows, for the first time, that ITPase activity is associated with the occurrence of metabolic events in patients using abacavir. Further studies are needed to confirm this association and to elucidate the possible mechanism.

RNAi mediated gene silencing of ITPA using a targeted nanocarrier: Apoptosis induction in SKBR3 cancer cells

A pure nucleotide pool is required for high-fidelity DNA replication and prevention of carcinogenesis in living cells. Human inosine triphosphatase (ITPase), encoded by the ITPA gene, plays a critical role in maintaining the purity of the cellular nucleotide pool by excluding nucleotides that enhance mutagenesis. ITPase is a nucleoside triphosphate pyrophosphatase that hydrolyzes the non-canonical nucleotides inosine triphosphate (ITP) and xanthine triphosphate (XTP). The monophosphate products of ITPase reactions are subsequently excluded from the nucleotide pool and the improper substitution of ITP and XTP into DNA and RNA is prevented. Previous studies show that deficiency in ITPA can suppress cellular growth and enhance DNA instability. In this study, we evaluated the influence of effective ITPA down-regulation on the induction of apoptosis in a human cancer cell line using folate-single wall nanotubes (SWNT) as a targeted nanocarrier. We assessed whether SWNT enhances IPTA-siRNA transfection efficiency in cancer cells using folate as a homing device. Since folate receptor is considerably overexpressed in cancer cells, conjugation of SWNTs to folate could enhance their cancer-specific penetrance. We found that nanocarrier mediated ITPA-siRNA transfection into SKBR3 cells caused significant reduction of ITPA mRNA expression level and complete down-regulation of the ITPase protein product. The silencing of ITPA led to promotion of apoptosis in SWNT-treated SKBR3 cancer cells.

Erythrocyte Inosine triphosphatase activity: A potential biomarker for adverse events during combination antiretroviral therapy for HIV

The purine analogues tenofovir and abacavir are precursors of potential substrates for the enzyme Inosine 5'-triphosphate pyrophosphohydrolase (ITPase). Here, we investigated the association of ITPase activity and ITPA genotype with the occurrence of adverse events (AEs) during combination antiretroviral therapy (cART) for human immunodeficiency virus (HIV) infection. In 393 adult HIV-seropositive patients, AEs were defined as events that led to stop of cART regimen. ITPase activity ≥4 mmol IMP/mmol Hb/hour was considered as normal. ITPA genotype was determined by testing two ITPA polymorphisms: c.94C>A (p.Pro32Thr, rs1127354) and c.124+21A>C (rs7270101). Logistic regression analysis determined odds ratios for developing AEs. In tenofovir-containing regimens decreased ITPase activity was associated with less AEs (p = 0.01) and longer regimen duration (p = 0.001). In contrast, in abacavir-containing regimens decreased ITPase activity was associated with more AEs (crude p = 0.02) and increased switching of medication due to AEs (p = 0.03). ITPA genotype wt/wt was significantly associated with an increase in the occurrence of AEs in tenofovir-containing regimens. Decreased ITPase activity seems to be protective against occurrence of AEs in tenofovir-containing cART, while it is associated with an increase in AEs in abacavir-containing regimens.

Inosine Triphosphate Pyrophosphohydrolase Expression: Decreased in Leukocytes of HIV-Infected Patients Using Combination Antiretroviral Therapy

OBJECTIVE

In HIV-infected patients, the enzyme Inosine triphosphate pyrophosphohydrolase (ITPase), involved in purine nucleotide homeostasis, was found to be decreased in erythrocytes. Since purine analogues are pivotal in the HIV treatment, a better understanding of ITPase expression in CD4 lymphocytes may lead to better understanding of nucleotide metabolism and (adverse) effects.

DESIGN

Cross-sectional, cohort, observational study.

METHODS

HIV-infected and control patients above 18 years were included. All DNA samples were genotyped for the 2 functional ITPA SNPs; c.94C>A (rs1127354) and g.IVS+21A>C (rs7270101). ITPase expression was determined by flow cytometry in all leukocyte subsets.

RESULTS

Fifty-nine HIV-infected patients and 50 controls were included. Leukocyte subtype distribution showed no difference in monocytes and granulocytes, but lymphocytes were higher in HIV-infected patients (P < 0.001). ITPase expression was highest in activated monocytes and lowest in lymphocytes. In HIV-infected patients, the percentage of ITPase positive cells was less in all leukocyte and lymphocyte subsets compared with controls (P < 0.01). In HIV-infected patients, 97.4% of CD4 lymphocytes were ITPase positive versus 99.9% in controls (P = 0.002) and 85.9% versus 99.6% of CD8 lymphocytes (P < 0.0001), respectively. Stratification according to genotype revealed no significant differences in ITPase expression in leukocytes in HIV-infected and control patients.

CONCLUSIONS

HIV-infection seems to be interfering with the nucleotide metabolism in leukocytes, including CD4 lymphocytes, by decreasing ITPase expression, independently of ITPA genotype. Given that active metabolites of purine-analogue reverse transcriptase inhibitors are potential substrates for ITPase, these results warrant further research towards effectiveness and adverse events of purine analogues and ITPase activity.

Recessive ITPA mutations cause an early infantile encephalopathy

OBJECTIVE

To identify the etiology of a novel, heritable encephalopathy in a small group of patients.

METHODS

Magnetic resonance imaging (MRI) pattern analysis was used to select patients with the same pattern. Homozygosity mapping and whole exome sequencing (WES) were performed to find the causal gene mutations.

RESULTS

Seven patients from 4 families (2 consanguineous) were identified with a similar MRI pattern characterized by T2 signal abnormalities and diffusion restriction in the posterior limb of the internal capsule, often also optic radiation, brainstem tracts, and cerebellar white matter, in combination with delayed myelination and progressive brain atrophy. Patients presented with early infantile onset encephalopathy characterized by progressive microcephaly, seizures, variable cardiac defects, and early death. Metabolic testing was unrevealing. Single nucleotide polymorphism array revealed 1 overlapping homozygous region on chromosome 20 in the consanguineous families. In all patients, WES subsequently revealed recessive predicted loss of function mutations in ITPA, encoding inosine triphosphate pyrophosphatase (ITPase). ITPase activity in patients' erythrocytes and fibroblasts was severely reduced.

INTERPRETATION

Until now ITPA variants have only been associated with adverse reactions to specific drugs. This is the first report associating ITPA mutations with a human disorder. ITPase is important in purine metabolism because it removes noncanonical nucleotides from the cellular nucleotide pool. Toxicity of accumulated noncanonical nucleotides, leading to neuronal apoptosis and interference with proteins normally using adenosine triphosphate/guanosine triphosphate, probably explains the disease. This study confirms that combining MRI pattern recognition to define small, homogeneous patient groups with WES is a powerful approach for providing a fast diagnosis in patients with an unclassified genetic encephalopathy.