Abstracts

Rationale: Expression and upregulation of the multidrug resistance associated ABC transporters in the brain may play a role in the development of medically refractory epilepsy. Mitoxantrone resistance protein (MXR) encoded by ABCG2 is a member of the ABC transporter superfamily with known expression in endothelial cells (EC) of the blood brain barrier. Previously, we used immunohistochemistry (IHC) to examine MXR expression in brain tissue resected from six refractory epileptic patients with mesial temporal sclerosis (MTS) pathology. Contrary to earlier published reports, we demonstrated novel expression of MXR in neurons of these patients. To further validate our findings, we used different anti-MXR antibodies to reexamine the distribution and expression of MXR in brain tissue resected from additional MTS patients. To demonstrate the relative importance of MXR in the brain, we also examined ABCG2 mRNA expression relative to other ABC transporters. Methods: Diseased hippocampus and adjacent histologically normal lateral temporal cortex (LTC) were analyzed for each case. IHC was performed on 10 additional refractory MTS cases using two different anti-MXR antibodies, ABCG2 and BXP-21. QRT-PCR was performed on 33 cases to evaluate ABCG2 mRNA expression in the hippocampus and the cortex. Relative expression of ABCG2, ABCB1 and ABCC1-ABCC6 in the hippocampus were examined in 11 cases.Results: Prominent MXR immunoreactivity was observed in neurons of the hippocampus including granule cells of the dentate. MXR immunoreactivity was also observed in neurons of otherwise normal LTC. In some patients, a greater number of immunoreactive neurons were observed in the hippocampus than in the cortex. Both anti-MXR antibodies labeled neurons. However, ABCG2 antibody labeled neurons in all cases (10/10), while BXP-21 antibody labeled neurons in only half the cases (5/10). As expected, prominent MXR immunoreactivity was observed in EC of hippocampus and cortex in all cases with both anti-MXR antibodies. ¶Using qRT-PCR, ABCG2 mRNA copy numbers were calculated from a standard curve and assessed quantitatively. The range of ABCG2 mRNA expression was wide and no significant differences between ABCG2 mRNA expression in the hippocampus and cortex were observed. Relative ABCG2 mRNA expression between hippocampus and cortex was variable. ABCB1, ABCG2, and ABCC1-ABCC6 mRNA expression was quantitated relative to histone. High ABCG2 and ABCC4 mRNA expression was observed relative to other transporters in the hippocampus. Conclusions: Our findings provide conclusive evidence that neuronal MXR expression is indeed present in brains of refractory epileptic patients. MXR upregulation in neurons is widespread but may be more abundant in the epileptogenic hippocampal areas, particularly in some patients. These findings suggest that MXR may play a role in drug disposition in neurons and contribute to drug resistance in epilepsy. (Supported by The American Foundation for Pharmaceutical Education and NIH GM61390.)