Loss-of-Function Mutations in FRRS1L Lead to an Epileptic-Dyskinetic Encephalopathy

Abstract

Glutamatergic neurotransmission governs excitatory signaling in the mammalian brain, and abnormalities of glutamate signaling have been shown to contribute to both epilepsy and hyperkinetic movement disorders. The etiology of many severe childhood movement disorders and epilepsies remains uncharacterized. We describe a neurological disorder with epilepsy and prominent choreoathetosis caused by biallelic pathogenic variants in FRRS1L, which encodes an AMPA receptor outer-core protein. Loss of FRRS1L function attenuates AMPA-mediated currents, implicating chronic abnormalities of glutamatergic neurotransmission in this monogenic neurological disease of childhood.

Figure 1

Families Affected by Choreoathetosis, Epilepsy, and Cerebral Volume Loss (A) A total of eight individuals (one deceased) from four unrelated families are affected by this progressive neurological disease. (B) Brain MRI shows diffuse cortical and cerebellar volume loss (long arrows), ex vacuo ventriculomegaly and flattening of the heads of the caudate nuclei (short arrows), and periventricular FLAIR (fluid attenuation inversion recovery) hyperintense signal (arrowheads).

Figure 2

Variants in Human FRRS1L and Effects on Protein Abundance (A) The three variants leading to premature truncation codons are all predicted to lead to loss of the FRRS1L transmembrane (TM) domain, whereas p.Ile146Asnfs^∗10 and p.Gly246del are predicted to disrupt the predicted transmembrane and extracellular DOMON (dopamine beta-monooxygenase N-terminal) domain (InterPro: IPR005018). (B) Immunoblot in affected fibroblasts demonstrates lower amounts of FRRS1L in individuals with premature stop variants than in age- and sex-matched control individuals (representative image); anti-FRRS1L (1:1,000, Atlas, Sigma-Aldrich) and anti-β-actin (1:1,000, Santa Cruz) served as loading controls.

Figure 3

Frrs1l Expression Pattern Murine Frrs1l expression detected by lacZ reporter. Frrs1l is expressed throughout the adult brain, predominantly in the cerebellum (A) and differentially throughout layers of the cortex (B), thalamus (C), hippocampus (D), substantia nigra (E), and anterior olfactory nucleus (F). There is expression in the dorsal horn of the lumbar spinal cord (G) and the trigeminal ganglion (H). Outside of the nervous system, Frrs1l is expressed in the epididymis (I) and seminiferous tubules (J) of the testis. During development (E12.5), Frrs1l is expressed in the ventral forebrain (K) and weakly in the spinal cord (L).

Figure 4

Effects of FRRS1L Loss on Electrophysiology (A) siRNA knockdown of FRRS1L in neuronally differentiated SH-SY5Y cells led to FRRS1L amounts comparable to those seen in affected fibroblasts (n = 3); transient receptor potential channel 1 (1:500, Alomone) and β-actin served as loading controls. (B) siFRRS1L-treated cells showed decreased Ca²⁺ influx after stimulation with 100 μM AMPA (representative tracing). (C) Mean Ca²⁺ influx was significantly decreased by FRRS1L knockdown (mean ± SEM; n = 30–50 cells per condition; ^∗p < 0.05). (D) A bath applied 100 μM AMPA-induced inward currents to control and siFRRS1L cells at a holding potential of −80 mV (representative tracings). (E) Current-voltage (I-V) curves under the conditions in (D). (F) Mean current intensity at −80 mV (mean ± SEM; n = 8–10 recordings per condition; ^∗p < 0.05).