Global Gene Expression Analysis Reveals Dynamic and Developmental Stage-Dependent Enrichment of Lead-Induced Neurological Gene Alterations

Article excerpt

BACKGROUND: The underlying genetic mechanisms specific to subtle neurological alterations associated with environmental lead (Pb) exposures have not been clearly elucidated.

OBJECTIVES: The goal of this study was to identify novel gene targets and the underlying genetic mechanisms associated with developmental Pb neurotoxicity.

METHODS: We first exposed zebrafish embryos to a range of Pb concentrations throughout early development to establish relative toxicity. Using the data from that experiment, we exposed another group of zebrafish embryos to a subledial dose of Pb (100 ppb) immediately after fertilization through 72 or 120 hr postfertilization (hpf). Global gene expression was then analyzed for molecular pathways and gene ontology enrichment, and Western blot analysis was performed to investigate the translation of gene expression changes to protein levels.

RESULTS: After 72 hpf, we identified 231 probes representing 90 nonredundant genes with well-established function or orthology to human genes as being altered by Pb exposure. This gene set was both confirmatory and novel in nature and was highly enriched for neurological development, function, and disease. Moreover, gene changes at this time point were correlated to altered protein levels. Alternatively, the gene set at 120 hpf did not share association with neurological development.

CONCLUSIONS: Global gene expression alterations associated with developmental Pb exposure were dynamic and dependent on developmental stage. Gene expression alterations at the 72-hpf time point were highly enriched with genes and molecular pathways associated with neurological development and disease. Moreover, we identified a number of novel targets for future exploration into their role in the genetic mechanisms underlying Pb-induced neurological alterations.

KEY WORDS: Danio rerio, gene expression, genomics, lead, microarray, Pb, toxicogenomics, zebrafish. Environ Health Perspect 119:615-621 (2011). doi:10.1289/ehp.1002590 [Online 8 December 2010]

The heavy metal lead (Pb) is a well-studied toxicant known to cause a wide array of adverse health effects. The developing nervous system is especially susceptible to Pb-induced alterations, and the Centers for Disease Control and Prevention (CDC) currently recommends that primary prevention strategies be implemented when a child's blood Pb level (BLL) exceeds 10 [micro]g/dL (CDC 2005). However, increasingly stringent regulatory actions have failed to limit exposures to a level documented as safe and without neurological impacts (Canfield et al. 2003; Min et al. 2007), including recent epidemiological studies that linked subinterventional BLLs to decreased IQ (intelligence quotient) and attention span and increased behavioral problems and odds ratio for the development of attention deficit hyperactivity disorder (Eubig et al. 2010, Surkan et al. 2007). Moreover, although the mechanisms of Pb toxicity have been studied for many years, the molecular mechanisms underlying the subtle neurological impacts are not understood.

The developing nervous system is particularly susceptible to Pb toxicity because of increased levels of cell proliferation, migration, and differentiation and the complexity of cell interactions (Rodier 1994, 2004). In addition, children absorb a greater percentage of ingested Pb, and circulating Pb has greater access to the brain because the blood-brain barrier is not yet fully established (Cory-Slechta and Schaumburg 2000; Leggett 1993). Also, Pb can be released from maternal bone stores and transferred to a developing fetus (Gardella 2001). As a result, Pb exposure can be initiated early in prenatal development, and studies have indicated that umbilical BLLs [less than or equal to] 10 [micro]g/dL are linked with deleterious effects on cognitive development (Gomaa et al. 2002). Thus, it is important to study Pb exposure in vivo during embryo-genesis at a relevant concentration. …