MEF2C-related disorders (aka MEF2C-haploinsufficiency) are caused by variations in or involving the MEF2C gene and are characterized by intellectual disability, developmental delay, lack of speech, limited walking, and seizures. Despite these findings, the disorder is not easily recognized clinically. We performed a systematic review following Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines to assemble the most comprehensive list of patients and their phenotypes. Through searching PubMed, Web of Science, and MEDLINE, 43 articles met the inclusion criteria and were fully reviewed. One hundred and seventeen patients were identified from these publications with most having a phenotype of intellectual disability, developmental delay, seizures, hypotonia, absent speech, inability to walk, stereotypic movements, and MRI abnormalities. Nonclassical findings included one patient with a question mark ear, two patients with a jugular pit, one patient with a unique neuroendocrine finding, and nine patients that did not have MEF2C deletions or disruptions but may be affected due to a positional effect on MEF2C. This systematic review characterizes the phenotype of MEF2C-related disorders, documents the severity of this condition, and will help providers to better diagnose and care for patients and their families. Additionally, this compiled information provides a comprehensive resource for investigators interested in pursuing specific genotype-phenotype correlations.

OBJECTIVE: MEF2C-related epilepsy has been poorly described in the literature, despite a consistent MEF2C haploinsufficiency phenotype characterized by severe language impairment and motor delay (MIM# 613443). We aimed to delineate the spectrum of electroclinical manifestations of MEF2C-related epilepsy from an illustrative case and literature review.
METHODS: A retrospective chart review of our case was performed followed by a literature review on PubMed and OMIM. Publications including patients with MEF2C pathogenic, likely pathogenic variants, or microdeletions without involvement of other genes were selected.
RESULTS: The index case is a 2-year-old male with global developmental delay who presented at 7 months with atypical febrile seizures, generalized myoclonias, and focal impaired awareness seizures. Neuroimaging studies were unremarkable and electroencephalograms showed high voltage 200-400uV, 2-2.5 Hz generalized spike-and-waves and polyspikes with alternating frontal predominance, and multifocal spike-and-slow waves. Whole exome sequencing showed an unreported de novo likely pathogenic variant in the MEF2C gene c.236 G > C (p.Arg79Pro). Data from ten additional publications including 22 patients were gathered. From the 23 patients in total, 19 (82%) had seizures. Febrile seizures were most common, but myoclonic, focal-onset and generalized seizures were also reported. Electroencephalogram findings were described in eleven, and nine (82%) showed epileptiform abnormalities.
CONCLUSIONS: MEF2C-related epilepsy may be described as a spectrum of manifestations including febrile seizures, myoclonia, and focal-onset or generalized seizures. Electroencephalogram is consistently abnormal, showing findings such as background slowing, multifocal and generalized epileptiform discharges and polyspikes. It remains unclear whether most patients are responsive or refractory to treatment with anti-epileptic medications.

Mutations in the MEF2C ( myocyte enhancer factor 2 ) gene have been established as a cause for an intellectual disability syndrome presenting with seizures, absence of speech, stereotypic movements, hypotonia, and limited ambulation. Phenotypic overlap with Rett\'s and Angelman\'s syndromes has been noted. Following the first reports of 5q14.3q15 microdeletions encompassing the MEF2C gene, further cases with point mutations and partial gene deletions of the MEF2C gene have been described. We present the clinical phenotype of our cohort of six patients with MEF2C mutations and compare our findings with previously reported patients as well as with a growing number of genetic conditions presenting with a severe neurodevelopmental, Rett-like, phenotype. We aim to add to the current knowledge of the natural history of the \"MEF2C haploinsufficiency syndrome\" as well as of the differential diagnosis, clinical management, and genetic counseling in this diagnostically challenging group of patients.

BACKGROUND: MEF2C haploinsufficiency syndrome is characterized by severe intellectual disability, epilepsy, stereotypic movements, minor dysmorphisms and brain abnormalities. We report the case of a patient with a new MEF2C mutation, comparing his clinical and imaging features to those previously reported in the literature.
METHODS: A 10 year-old boy first came to pediatric neurology clinic at the age of 11 months because of severe psychomotor delay, without regression. He presented generalized hypotonia, poor eye contact, hand-mouth stereotypies, strabismus and minor facial dimorphisms. Epileptic seizures started at 26 months of age and were refractory. Brain MRI showed a slight increase in periventricular white matter signal and globally enlarged CSF spaces. Molecular analysis revealed a de novo, pathogenic and causative MEF2C mutation.
CONCLUSIONS: MEF2C haploinsufficiency syndrome was recently recognized as a neurodevelopmental disorder. Severe intellectual disability with inability to speak and epilepsy are universal features in patients with MEF2C mutations, although mild cognitive and speech disorders have been reported to occur in patients with duplications. Epilepsy might be absent in patients with partial deletions. Abnormal movement patterns are very common in patients with MEF2C haploinsufficiency. Delayed myelination seems to be more commonly observed in patients with MEF2C mutations, while malformations of cortical development were only reported in patients with microdeletions. Although MEF2C haploinsufficiency prevalence is yet to be determined, it should be considered in the differential diagnosis of patients with severe intellectual disability and Rett-like features.

Deletions or intragenic mutations involving the MEF2C gene on chromosome 5q14.3 have generally been associated with a relatively uniform phenotype characterized by severe developmental delay, absent speech, stereotypies, absent or limited gait abilities, lack of a typical facial gestalt and scarcity of major malformations. We report on a patient of Cypriot descent with a de novo, approximately 147 kb in size, partial MEF2C deletion removing exons 1 to 3. He had a history of severe intellectual disability with absent speech, poor eye contact, hand stereotypies and a wide-based gait. A broad-based, shallow jugular pit with an overlying vascular malformation was also present. Partial MEF2C deletions have only been reported in a very small number of patients and have on occasion been associated with relatively milder phenotypes. We present a patient of Cypriot descent with such a deletion and review previously published literature on partial MEF2C gene deletions postulating a key role of the first few exons in the pathogenesis of the disease.