Polymorphisms in Genes Related to Oxidative Stress Are Associated with Inferior Cognitive Function after Therapy for Childhood ALL

Introduction: Outcome studies consistently document increased rates of neurocognitive deficits among survivors of childhood acute lymphoblastic leukemia (ALL). This study was conducted to test whether some of the observed interpatient variability in cognitive outcomes can be explained by inherited polymorphisms in genes related to the multifactorial pathophysiology of treatment-induced cognitive decline.

Methods: Neurocognitive testing was conducted for 350 leukemia survivors, at a target of 5 years after completion of treatment. All patients had been enrolled on DFCI ALL Consortium Protocols 95-01 or 00-01, between 1995 and 2005. The median age at diagnosis was 4.5 years (range 1-18); 54% were male. Neurocognitive outcomes included Wechsler estimated IQ, Vocabulary and Digit Span and parent-reported Attention and Hyperactivity scores (BASC-2). Impaired performance for all measures was defined as ³1 standard deviation below the reference population mean. Genomic DNA was isolated from bone marrow collected at remission. Twenty-nine candidate polymorphisms were selected, targeting variants within genes related to drug metabolism, oxidative damage, altered neurotransmission, neuroinflammation, and folate physiology. Single nucleotide polymorphisms were detected using either a customized multiplexed Sequenom MassARRAY assay or PCR-based allelic discrimination assays. The number of 28bp nucleotide repeats within the 5' untranslated region of the gene for thymidylate synthase was assessed by PCR-product length analysis. Multivariable logistic regression models were calculated to estimate the effects of genotype on neurocognitive outcomes, adjusting for the effects of age at diagnosis, sex, risk group, race, CNS prophylaxis (radiation vs. intrathecal chemotherapy), corticosteroid (prednisone vs. dexamethasone), and socioeconomic status.

Results: For all target genes, the distribution of genotypes was consistent with Hardy-Weinberg equilibrium. Minor allele frequencies were similar to published values. Inferior cognitive or behavioral outcomes were significantly associated with polymorphisms in four genes related to oxidative stress and/or neuroinflammation: endothelial nitric oxide synthase (NOS3), prostaglandin transporter (SLCO2A1), hemochromatosis (HFE), and catechol-O-methyltransferase (COMT). Lower mean IQ scores were observed among survivors homozygous for the NOS3 T allele (95.8; 95% CI, 89.9-101.7; n = 30 vs. 103.9; 95% CI, 102.1-105.8; n = 264; multivariable P=0.011) and among those with at least one SLCO2A1 G allele (100.4; 95% CI, 97.5-103.2; n=117 vs. 104.6; 95% CI 102.4-107; n=177; P =0.014).

Conclusions: Inferior cognitive and behavioral outcomes among childhood leukemia survivors were associated with prevalent polymorphisms in four genes associated with susceptibility to oxidative stress. These data are consistent with the hypothesis that oxidative damage contributes to chemotherapy-associated neurotoxicity among children with leukemia. Furthermore, our results suggest that detecting these polymorphisms can identify those children with ALL who are at greatest risk for treatment-induced cognitive decline, and who would therefore benefit from a clinical trial testing a protective intervention.