A number sign (#) is used with this entry because of evidence that developmental and epileptic encephalopathy-98 (DEE98) is caused by heterozygous mutation in the ATP1A2 gene (182340) on chromosome 1q23.

Developmental and epileptic encephalopathy-98 (DEE98) is characterized by onset of seizures in the first decade (range infancy to late childhood) associated with variable global developmental delay. Other features may include hypotonia, spasticity, and quadriparesis. Brain imaging may be normal or show nonspecific and variable abnormalities, including polymicrogyria. The severity is variable; some patients may die of refractory status epilepticus (summary by Vetro et al., 2021).

For a general phenotypic description and a discussion of genetic heterogeneity of DEE, see 308350.

Vetro et al. (2021) reported 6 unrelated children, ranging from 1 month to 12 years of age, with onset of various types of seizures between 10 days and 8 years of age. Seizure types included focal dyscognitive, clonic, tonic, migrating focal seizures, and generalized tonic-clonic seizures. Two patients died within the first year of life due to respiratory complications of refractory status epilepticus. EEG was abnormal in all patients, showing slow background with multifocal sharp waves and discharges; 1 patient had a burst-suppression pattern. Brain imaging was normal in 3 patients, and showed variable abnormalities in the other 3. Patient 1 had peritrigonal high signal intensities; patient 2 had a thin corpus callosum, dilated ventricles, and frontal and cerebellar atrophy; and patient 6 had perisylvian polymicrogyria with a thickened corpus callosum. Other features included hypotonia, spasticity, and quadriparesis. All but 1 (patient 5) had global developmental delay with variably impaired intellectual development. Patient 4 also had Down syndrome. Patient 5 was the least affected: she had onset of seizures at age 8 years, no additional neurologic features, and normal brain imaging. Patient 2 was the most severely affected individual. At age 5 years, he was profoundly delayed and nonverbal with severe axial hypotonia and dyskinetic quadriparesis; brain imaging showed cerebral atrophy.

The heterozygous mutations in the ATP1A2 gene that were identified in patients with DEE98 by Vetro et al. (2021) occurred de novo.

In 6 unrelated patients with DEE98, Vetro et al. (2021) identified 5 de novo heterozygous missense mutations in the ATP1A2 gene (see, e.g., 182340.0020-182340.0022). The mutations, which occurred at conserved residues, were not present in the gnomAD database. In vitro functional expression studies showed all of the mutations caused variable functional defects in the Na+/(K+)ATPase. Variants with more severe functional deficits were associated with a more severe phenotype. Vetro et al. (2021) estimated that about 5% of ATP1A2 mutations may be associated with DEE. Polymicrogyria was estimated to occur in about 1% of patients with ATP1A2 mutations.