We delineated the phenotypic spectrum of epilepsy in individuals with NGLY1 deficiency from an international cohort. We collected detailed clinical and electroencephalographic data from 29 individuals with bi‐allelic (likely) pathogenic variants in NGLY1 as part of an ongoing prospective natural history study. Participants were evaluated in‐person at a single center and/or remotely. Historical medical records were reviewed. Published cases were included for comprehensive phenotyping. Of 29 individuals (mean 11.4 years, range 3–27 years), 17 (58.6%) participants had a history of epilepsy. Seizure onset was in early childhood (mean 43 months, range 2 months to 19 years). The most common seizure types were myoclonic and atonic. Epilepsy course was variable, but 35.2% (6/17) of participants with epilepsy achieved seizure freedom. The most common medications included levetiracetam, valproate, lamotrigine, and clobazam. Electroencephalogram (EEGs) were abnormal in 80% (12/15) of participants with or without epilepsy, although encephalopathy was uncommon. There was a trend in neurodevelopmental outcomes that participants with epilepsy had more developmental delays. In summary, epilepsy is common in NGLY1 deficiency. Over half of the participants had a history of epilepsy and nearly all had EEG abnormalities indicating an increased risk of epilepsy. This work expands the electroclinical phenotype of NGLY1 deficiency and supports a high clinical suspicion for seizures. Some of the more common seizure types (epileptic spasms, myoclonic, and atonic seizures) can be subtle and require counseling to ensure early recognition and treatment to ensure the best possible outcomes. Despite transient liver enzyme abnormalities in this disorder, hepatically metabolized medications were well tolerated.

N-glycanase 1 (NGLY1) Deficiency is a debilitating, ultra-rare autosomal recessive disorder caused by loss of function of NGLY1, a cytosolic enzyme that deglycosylates other proteins. It is characterized by severe global developmental delay and/or intellectual disability, hyperkinetic movement disorder, transient elevation of transaminases, (hypo)alacrima, and progressive, diffuse, length-dependent sensorimotor polyneuropathy. A prospective natural history study (NHS) was conducted to elucidate clinical features and disease course. Twenty-nine participants were enrolled (15 onsite, 14 remotely) and followed for up to 32 months, representing ~29% of the ~100 patients identified worldwide. Participants exhibited profound developmental delays, with almost all developmental quotients below 20 on the Mullen Scales of Early Learning, well below the normative score of 100. Increased difficulties with sitting and standing suggested decline in motor function over time. Most patients presented with (hypo)alacrima and reduced sweat response. Pediatric quality of life was poor except for emotional function. Language/communication and motor skill problems including hand use were reported by caregivers as the most bothersome symptoms. Levels of the substrate biomarker, GlcNAc-Asn (aspartylglucosamine; GNA), were consistently elevated in all participants over time, independent of age. Liver enzymes were elevated for some participants but improved especially in younger patients and did not reach levels indicating severe liver disease. Three participants died during the study period. Data from this NHS informs selection of endpoints and assessments for future clinical trials for NGLY1 Deficiency interventions. Potential endpoints include GNA biomarker levels, neurocognitive assessments, autonomic and motor function (particularly hand use), (hypo)alacrima, and quality of life.

N-glycanase 1 (NGLY1) Deficiency is a debilitating, ultra-rare autosomal recessive disorder caused by loss of function of NGLY1, a cytosolic enzyme that deglycosylates other proteins. It is characterized by severe global developmental delay and/or intellectual disability, hyperkinetic movement disorder, transient elevation of transaminases, (hypo)alacrima, and progressive, diffuse, length-dependent sensorimotor polyneuropathy. A prospective natural history study (NHS) was conducted to elucidate clinical features and disease course. Twenty-nine participants were enrolled (15 onsite, 14 remotely) and followed for up to 32 months, representing ~ 29% of the ~ 100 patients identified worldwide. Participants exhibited profound developmental delays, with almost all developmental quotients below 20 on the Mullen Scales of Early Learning, well below the normative score of 100. Increased difficulties with sitting and standing suggested decline in motor function over time. Most patients presented with (hypo)alacrima and reduced sweat response. Pediatric quality of life was poor except for emotional function. Language/communication and motor skill problems including hand use were reported by caregivers as the most bothersome symptoms. Levels of the substrate biomarker, GlcNAc-Asn (aspartylglucosamine; GNA), were consistently elevated in all participants over time, independent of age. Liver enzymes were elevated for some participants but improved especially in younger patients and did not reach levels indicating severe liver disease. Three participants died during the study period. Data from this NHS informs selection of endpoints and assessments for future clinical trials for NGLY1 Deficiency interventions. Potential endpoints include GNA biomarker levels, neurocognitive assessments, autonomic and motor function (particularly hand use), (hypo)alacrima, and quality of life.