MAC Mice: Novel Insights into the Link Between Trisomy 21 and Epilepsy

Down syndrome (DS) is among the most common genetic causes of intellectual disability, occurring in 1:800 births.^1,2 DS results from the presence of a third chromosome 21, most often due to meiotic nondisjunction.^1,2 In addition to a range of cardiac, pulmonary, oncologic, and other conditions, epilepsy occurs in 8% of individuals with DS, far greater than the prevalence of epilepsy in the general population. ¹ Certain seizure types predominate, particularly infantile spasms and generalized tonic–clonic seizures. Epilepsy onset prior to age 1 year is common—leading to life-long consequences, including evolution of epilepsy to medication-resistant Lennox-Gastaut syndrome. ³ However, there are no precision or disease-modifying epilepsy treatments specific to DS, and no biomarkers to predict which people with DS will develop epilepsy. These gaps result from limited knowledge of fundamental biological mechanisms underlying increased epilepsy risk in DS. Therefore, informative experimental models of DS are needed.

Transgenic mouse models have been employed to study epilepsy in DS, but fully recapitulating the human syndrome, particularly the presence of a third chromosome 21, has been challenging. Some mouse models are partial (segmental) trisomies of the mouse ortholog of human chromosome 21 (chromosome 16). Full trisomy of chromosome 16 in mice is incompatible with life. ⁴ Therefore, segmental trisomy 16 models have been employed. Epilepsy has been particularly well studied in the segmental trisomy 16 model Ts65Dn, which shows increased susceptibility to seizures resembling infantile spasms, including with administration of GABA-B agonists or GABA-A antagonists. ⁵ These findings suggest that excessive circuit excitability is a cause of epilepsy in DS, and that interventions to balance excess excitation with inhibition may be useful, though more precise mechanisms are unclear. Whereas hundreds of papers have been published using the Ts65Dn model, major caveats include incomplete recapitulation of human trisomy 21 (with ∼40% of human chromosome 21 genes absent on mouse chromosome 16, and the presence of >30 extra trisomic genes unrelated to human chromosome 21), which may limit the clinical relevance of findings in this model.

By contrast, the more recently developed TcMAC21 model carries an MAC (mouse artificial chromosome) containing a near-complete copy of human chromosome 21. ⁶ Previous work by the authors of the present paper demonstrated that TcMAC21 mice exhibit heightened susceptibility to GABA-B induced infantile spasms-like seizures. ⁷ In the current study, the authors examined susceptibility to generalized tonic–clonic seizures. ⁸ Exposure to the GABA-A antagonist flurothyl elicited generalized tonic–clonic seizures with shorter latencies and longer durations in TcMAC21 mice, compared to euploid controls. ⁸ The authors next employed neurophysiological approaches in hippocampal and cortical slices to interrogate circuit mechanisms underlying increased susceptibility to generalized tonic–clonic seizures in TcMAC21 mice. While spontaneous hippocampal or cortical epileptiform bursting did not differ between TcMAC21 and euploid mice, GABA-A antagonism with gabazine led to evoked cortical responses that were larger in magnitude in TcMAC21 mice. ⁸ This difference was seen at post-natal days 17–20, but not at earlier timepoints. Collectively, these results point to an exaggerated excitatory output when inhibitory input is compromised in TcMAC21 mice. The findings also indicate that this cortical hyperexcitability emerges during a specific developmental window. This study thus links increased seizure susceptibility to excessive cortical excitability in DS. The finding of a developmental window for hyperexcitability in TcMAC21 mice may relate to the early life onset of epilepsy in infants with DS.

The authors’ studies of TcMAC21 mice represent a major advance in uncovering biological mechanistic links between Trisomy 21 and epilepsy, because humanized, TcMAC21 mice present a more faithful replication of the human genetic condition, eliminating limitations inherent to less complete mouse segmental trisomy models of DS, such as Ts65Dn. These findings are intriguing clues to epilepsy pathogenesis in DS to shape future investigations and potentially clinical care. At the same time, these studies in TcMAC21 mice reveal similar mechanisms to those described in Ts65Dn (excessive circuit excitability), providing a useful validation of prior research in that model.

TcMAC21 mice can now be employed to study additional, clinically relevant, and pressing questions about biological mechanisms underlying DS. It remains to be established whether some proportion of TcMAC21 mice, like humans with DS, develop epilepsy (ie, spontaneous, recurrent seizures). This question could be answered with long-term, continuous electroencephalographic (EEG) monitoring, which would further enhance translational relevance to DS patients. Longitudinal EEG studies, potentially coupled with artificial intelligence tools, could aid in identifying biomarkers of excitation–inhibition imbalance and predicting seizure risk during high risk periods such as infancy. A second question is why circuit hyperexcitability emerges at P17–20 (similar to the increased risk of IS in infants with DS). It is possible that this period of susceptibility relates to the developmental expression of certain genes on chromosome 21. If so, identifying those genes using modern molecular approaches could shed light on mechanisms of epilepsy that are specific to DS, providing new pathways for precision therapeutics. A third, critically important question is whether and how circuit hyperexcitability is linked to the high rate additional neurological comorbidities in DS, which include autism, intellectual disability, Alzheimer's disease-type histopathological changes, and early onset dementia in people with DS.^1,3 Notably, premature dementia in DS is more likely in those individuals with DS and epilepsy, ⁹ suggesting a link between excessive excitation and other neurological disorders. Thus, the TcMAC21 model should catalyze greatly needed, new basic and translational research to identify and target biological mechanisms of epilepsy and other neurological diseases in DS.