Combined generalized and focal epilepsy with reflex features in Adaptor protein complex 4-associated hereditary spastic paraplegias: A cohort observational study

Background

Patients with genetic deficiency of the adaptor protein complex 4 (AP-4) exhibit earlyonset developmental delay, spastic diplegia, intellectual disability, speech impairment. The phenotype overlaps with other hereditary spastic paraplegias and cerebral palsies. Febrile seizures are common at onset. Epilepsy has been described in more than half of cases, arising in early infancy often with status epilepticus, but no typical seizure semiology or electroencephalographic features have been identified thus far.

Purpose

We aimed to specifically investigate the epileptological characteristics of the syndrome to unveil possible biomarkers of seizure development and prognosis in AP-4 deficiency.

Methods

Observational cohort study on patients with bi-allelic pathogenic variants in AP-4 subunits and epilepsy. We focused on the seizure semiology, electroencephalographic characteristics and response to antiseizure medications.

Results

Patients harboured pathogenic variants in AP4S1 (n = 5) or AP4M1 (n = 1). The phenotype included spastic paraparesis, intellectual disability, speech/language impairment, microcephaly, and MRI evidence of hypoplasia of the corpus callosum. In 66 % of the patients, febrile seizures preceded the onset of epilepsy, which spanned from infancy to adolescence (range=14 months-13 years). Absences (66 %) and focal motor seizures (50 %) were common. No patient met the criteria for drug-resistance. Peculiar electroencephalographic features arose after the epilepsy onset and persisted at long-term follow-up: bilateral and asynchronous focal discharges combined with independent diffuse spike-wave-discharges (100 %) and reflex abnormalities (66 %).

Conclusion

In AP-4 complex disease, epilepsy could arise beyond early infancy, until adolescence, with variable combination of generalized and focal seizures. The prognosis was favourable. We observed a common electroencephalographic signature - combined focal/generalized and reflex abnormalities - which may constitute a biomarker of AP-4 deficiency with epilepsy, applicable to inform genetic testing and disentangle the differential diagnosis.