Genetic abnormalities in somatic cells are emerging as a novel mechanism in neurodevelopmental diseases with previously unknown causes, such as focal malformation of cortical development (FMCD). FMCDs are the leading cause of pediatric epilepsies, especially medically intractable ‘catastrophic’ epilepsy. A potential future therapy may include pharmacologically targeting defective gene regulatory networks, and will require extensive understanding of the genetic, cellular, and molecular mechanisms controlling neurological development and function. We will take advantage of unique animal and stem cell models established previously to investigate the molecular mechanisms of FMCD-associated seizures and uncover fundamental processes governing normal brain development and function.
What is the mechanism of developmental neuropathology of FMCD?
- Mosaic mutations in the components of PI3K-AKT-mTOR pathway were found in FMCD brains. The enrichment of the mutation in the cells with downstream activation, led to the hypothesis that gain-of-function in the pathway is the underlying cause of FMCD. In order to investigate developmental pathogenesis, we have previously generated a mouse model that expressed the FMCD-associated mutation in neural progenitors of a single hemisphere, mimicking the mosaicism found in FMCD brains. The mouse model recapitulated most of the key clinical and histological abnormalities seen in human FMCDs. We found that the mutation caused premature differentiation and abnormal migration of neurons resulting in structural brain malformation.
How are cellular abnormalities connected to molecular changes?
- Gene expression profiling, using human neural progenitor cells, revealed four functional networks of genes that are misexpressed by the FMCD mutation: (i) neuronal development, (ii) migration, (iii) signaling and homeostasis and (iv) cell cycle regulation. Further investigation of FMCD functional networks pointed to multiple downstream pathways that are responsible for clinically important cellular abnormalities such as cytomegaly and neuronal migration defects. Importantly, genetic removal of FMCD-associated mutations restored the expression of two-thirds of the FMCD network genes, suggesting that the pathway that was disrupted by FMCD-associated mutation was reversible. Pharmacological inhibition of one of the major downstream pathways, mTOR, effectively rescued the cytological defects supporting the idea of pathway inhibitors as potential treatments or therapies.
How does a small percentage of mutant cells disrupt the architecture of entire hemisphere?
- In human FMCD brain tissue samples, We found somatic mutations in as few as one-percent of measured alleles. It remains unclear how a mutation in a small percentage of cells could produce such severe and widespread brain defects. Strikingly, analysis of the FMCD functional networks identified that misexpression of RELN mediated non-cell autonomous neuronal migration defects resulted in widespread structural malformation (Fig. 1B). Further investigation revealed that inactivation of FOXG1 by AKT3 is an underlying mechanism of RELN misregulation. The results suggest two possible scenarios for FMCD-related seizure: cortical dyslamination or neuronal hyperactivity. Although reelin signaling regulates dendrite growth, no epileptic phenotype has been reported in mice in which reelin is overexpressed. Thus the data argue in favor of the mTOR pathways as having a larger contribution to epilepsy than reelin.
The treatment of pediatric neurodevelopmental diseases presents significant challenges in medicine. The genetic mechanism of FMCD, somatic mutations in a targetable pathway, may provide a rationale for the development of novel therapies targeting either the disrupted pathway or the mutated cells. Using a combination of in vivo and in vitro approaches, We will examine what and how downstream pathways cause neuronal hyperactivity and seizures. Our ultimate goal is to use the information gathered by these studies to identify new avenues for targeted therapeutic intervention.
Project 1. How do FMCD mutations cause neuronal hyperactivity?
- While FMCD-associated mutations cause structural brain malformations, they also cause increased spontaneous neuronal activity. we hypothesize that neuronal hyperactivity is responsible for FMCD-associated seizures. To begin to understand the molecular mechanisms behind neuronal hyperactivity, we will take advantage of the functional FMCD networks identified in our previous work. By generating morphological and electrical profiles, we can identify the major pathway that is responsible for neuronal hyperactivity. Moreover, by applying genetic rescue strategies using human neuronal cultures, these studies will begin to tease out whether cellular defects induced during the developmental stages can be reversed after differentiation.
Project 2. How are cellular defects associated with seizure?
- Previous results suggest that the focus for the neurocognitive defects resides within the FMCD-associated lesion. We intend to use FMCD mouse models to answer the following questions: 1) Does continuous activation of the downstream pathways of FMCD mutations lead to seizure? 2) Are the severity or patterns of seizure associated with the extent of the brain area expressing the FMCD mutation? 3) Do different FMCD mutations share common mechanisms for seizure? Furthermore, we will systematically introduce FMCD mutations during multiple developmental time points critical for brain formation. By measuring electrographic activity over postnatal time periods, we can dissect the time window critical for seizure development, and define cell types responsible for abnormal neuroelectrical activities.
Project 3. Targeted approaches to relieve the burden of epilepsy
- The reversibility of key developmental defects in FMCD models suggests the use of pathway inhibitors as potential treatments or therapies for some forms of FMCD. To explore the idea, we will first test whether the removal of cells expressing FMCD-associated mutations can relieve the burden of epilepsy, using genetic tools to selectively remove the cells expressing the mutation postnatally. By histological and electrographical profiling, we will characterize the broader effects of targeted-cell removal which will help to develop effective strategies for the treatment of intractable pediatric epilepsies associated with FMCD.
These studies will provide insights on the mechanisms of seizures associated with FMCD. By exploring the previously uncharted area of seizures caused by somatic genetic abnormalities, we may be able to identify critical mechanisms driving cellular and molecular defects, and may reveal effective strategies for inhibiting or reversing the course. The long-term goal is help the field advance to a point where therapies can be developed for what were previously considered untreatable conditions.
|2011년 이후 논문
Baek ST, Copeland B, Yun EJ, Kwon SK, Guemez-Gamboa A, Schaffer AE, Kim S, Kang HC, Song S, Mathern GW, Gleeson JG. (2015)
AnAKT3-FOXG1-Reelin Network Underlies Defective Migration in Human Focal Malformations of Cortical Development.
Nat. Med., 21(12):1445-54
Kang HC, Baek ST, Song S, Gleeson JG. (2015)
Clinical and genetic aspects of the segmental overgrowthspectrum due to somatic mutations in PIK3CA.
J Pediatr., 167(5):957-62
Conti V, Pantaleo M, Barba C, Baroni G, Mei D, Buccoliero AM, Giglio S, Giordano F, Baek ST, Gleeson JG, Guerrini R. (2015)
Focal dysplasia of the cerebral cortex and infantilespasms associated with somatic 1q21.1-q44 duplication including the AKT3 gene.
Clin Genet., 88(3):241-7
Yun EJ, Baek ST, Xie D, Tseng SF, Dobin T, Zhang L, Sun H, Xiao G, He D, Kittler R, Hernandez E, Zhou J, Yang J, Hsieh JT. (2015)
DAB2IP regulates cancer stem cell phenotypes throughmodulating stem cell factor receptor and ZEB1.
Baek ST*, Kerjan G*, Bielas SL, Lee JE, Fenstermaker AG, Novarino G, Gleeson JG. (2014)
Off-Target Effect of doublecortinFamily shRNA on Neuronal Migration Associated with Endogenous MicroRNADysregulation.
Neuron, 82(6):1255-62. (* equal contribution)
Novarino G, Baek ST, Gleeson JG. (2013)
The sacred disease: the puzzling genetics of epileptic disorders.
Baek ST, Gibbs EM, Gleeson JG and Mathern GW. (2013)
Hemimegalencephaly, a paradigm for somatic postzygoticneurodevelopmental disorders.
Curr Opin Neurol., 26(2):122-7
Acharya A, Baek ST*, Huang G*, Eskiocak B, Goetsch S, Sung CY, Banfi S, Sauer MF, Olsen GS, Duffield JS, Olson EN, Tallquist MD. (2012)
The bhlhtranscription factor Tcf21, is required for lineage-specific EMT of cardiac fibroblastprogenitors.
Development, 139(12):2139-2149. (* equal contribution) [Research Highlight. Development. 2012]
Baek ST and Tallquist MD. (2012)
Nf1limits epicardial derivative expansion by regulating epithelial to mesenchymaltransition and proliferation.
Smith CL, Baek ST, Sung CY, Tallquist MD. (2011)
Epicardial-derived cell epithelial to mesenchymaltransition and fate specification require PDGF receptor signaling.
Circ Res., 108(12):e15-26. [Best Manuscript Award. Circ Res. 2011]
Acharya A, Baek ST, Banfi S, Eskiocak B, Tallquist MD. (2011)
Efficient inducible Cre-mediatedrecombination in Tcf21 cell lineages in the heart and kidney.