Temporal Lobe Epilepsy and Retrotransposons

Idiopathic (cryptogenic) epilepsy is a common, chronic group of brain disorders, affecting 1-2% of the US population, creating a significant public health problem because of the associated morbidity and mortality. One subtype of this complex group of brain disorders, temporal lobe epilepsy (TLE), is particularly burdensome, because a substantial fraction of patients do not respond to anticonvulsant medications. If the seizure focus can be localized by imaging and/or EEG, the TLE patient often elects to have a partial resection of the affected temporal lobe, because they have daily seizures, despite multiple pharmacotherapy regimens. Although genetic risk is thought to play a role in TLE, identification of common risk alleles has had limited success. In the past 5 years, data have accumulated to prove that neuronal embryogenesis is accompanied by activation of LINE1 (L1) retrotransposons (RTPs) to an unexpected degree, such that developing neurons may accumulate de novo L1 insertions. This results in a substantial mosaicism within populations of CNS neurons. While most of these somatic de novo L1s will have little effect on neuronal function (perhaps because they occur in gene deserts or in large introns or in genes not required for that cell's function), some may interfere with normal neuronal activity because they have inserted into a gene needed by that particular neuron for normal function. If one or more functional de novo L1s occur early in CNS development, all the daughter neurons that derive from that neuronal precursor will also carry the L1 insertion, perhaps leading to a dysfunctional population of neurons destined to become an epileptic focus. This study leveraged CNS tissue (obtained at craniotomy) from patients with intractable TLE, who have undergone a therapeutic partial resection of one temporal lobe. Using fluorescence-assisted cell sorting of temporal lobe neuronal nuclei, followed by high-throughput sequencing and alignment to the reference genome, detection of de novo L1s will be done. In this manner, it is expected that de-novo intragenic L1s, contributing to idiopathic TLE, will be discovered.]]>
This project employed previously collected temporal cortex from TLE patients with medically refractory idiopathic TLE. These 33 patients (cases) were referred to surgery if they had no evidence of tumor, malformation, infection, traumatic brain injury or other causes of TLE and met specified clinical criteria (Sperling et al, 1996). They had ictal and interictal EEG, neuropsychologic tests, and MRI to identify the seizure focus for excision. Patients were candidates for temporal lobectomy if >90% of seizures could be localized to one temporal lobe. TLE cases and controls were matched as best as possible for age, sex and ethnicity. The 33 postmortem control temporal cortex tissues were obtained from the Neurobiobank (https://neurobiobank.nih.gov). Control tissue is not matched to TLE cases for multiple variables, including postmortem status, non-CNS diseases, medication given at time of surgery, or during agonal state.]]>
1996-1999: TLE patient brain tissue procurement at craniotomies 2015-2016: Postmortem control brain tissue procurement from Neurobiobanks 2016-2017: Brain tissue processing into neuronal and non-neuronal nuclei populations 2016-2017: Targeted LINE-1 amplification from neuronal gDNA 2016-2017: Illumina sequencing of LINE-1 libraries at UPenn next generation sequencing core ]]>