SYNGAP1 Developmental and Epileptic Encephalopathy
Developmental and Epileptic Encephalopathies (DEEs)
Proper brain development requires precise levels of synaptic proteins. The consequences of synaptic protein abundance dysregulation are devastating neurologic disorders including Developmental and Epileptic Encephalopathies (DEEs). Precisely how SYNGAP1 mutations result in phenotypes is unclear, however, in published data, The Holder Lab discovered that complete deficiency of SynGAP in human neurons causes precocious and exuberant neuronal network firing. Additionally, our published data indicate cultured engineered human neurons completely deficient for SYNGAP1 have increased excitability and network connectivity.
The Holder Lab is working to identify candidate therapeutic approaches for this devastating epileptic encephalopathy.
Cultured Human Neurons
The Holder Lab characterized induced Pluripotent Stem Cells (iPSCs) with homozygous loss-of function mutations in SYNGAP1 developed in collaboration with Gavin Rumbaugh’s Lab. We evaluated two mutant iPSCs that have homozygous loss-offunction mutations in exon 7 resulting in frameshift and >90% loss of SynGAP protein as well as two isogenic wild-type iPSC lines. Loss of SynGAP in these engineered human neurons results in larger neurons than isogenic wild-type controls. These larger neurons also have stronger synapses with elevated amplitude of miniature Excitatory PostSynaptic Currents (mEPSCs). The stronger synapses of these neurons are associated with increased number of excitatory synapses as determined by PSD95 and GluA1 immunostaining.
SHANK3 and Phelan-McDermid syndrome
Phelan-McDermid syndrome is a devastating neurodevelopmental disorder characterized by intellectual disability, autism and epilepsy. It is cause by deletion of chromosome 22q13 that include SHANK3 or point mutations in the SHANK3 gene. SHANK3 encodes a scaffolding protein at neuronal synapse that organizes the post-synaptic density. The Holder Lab investigates the impact of SHANK3 mutations in patients, human neurons and mice with SHANK3 haploinsufficiency. In addition, we are actively seeking novel, precision-based treatment strategies for Phelan-McDermid syndrome.
Identifying novel therapeutic entry points
We previously identified the ERK pathway as a key regulator of SHANK3 protein stability. Genetic depletion of ERK increases SHANK3 abundance in cultured neurons and mice. We are currently exploring the feasibility of ERK inhibition as a treatment strategy for Phelan-McDermid syndrome.
We also performed a genome-wide screen to identify novel regulators of SHANK3 protein stability. We are currently investigating how these novel regulators influence SHANK3 abundance and function. We aim to determine if these SHANK3 regulators could serve as novel therapeutic strategies for Phelan-McDermid syndrome.
The natural history of epilepsy in Phelan-McDermid syndrome
Approximately 35% of individuals with Phelan-McDermid syndrome also have epilepsy. It is unclear why some individuals develop epilepsy and others do not. The spectrum of seizure frequency and severity is very broad. We are investigating the natural history of epilepsy in Phelan-McDermid syndrome for clues as to the reason for such large variability and the consequence of seizures on other symptoms such as developmental progression and sleep.
Neuropsychiatric gene discovery
Bipolar disorder has one of the most robust inheritances of the neuropsychiatric disorders. Despite this, confirming specific genomic variants that predispose to bipolar disorder have been elusive. The Holder Lab seeks to identify and validate genomic variants that cause bipolar disorder. We are recruiting individuals with early-onset bipolar disorder to perform advanced genomic analyses to identify the variants that drive development of bipolar disorder. We then validate the impact of these variants in human neurons, fruit flies and rodent models.
