Hyperglycosylation and reduced GABA currents of mutated GABRB3 polypeptide in remitting childhood absence epilepsy.
Childhood absence epilepsy (CAE) accounts for 10% to 12% of epilepsy in children under 16 years of age. We screened for mutations in the GABA(A) receptor (GABAR) beta 3 subunit gene (GABRB3) in 48 probands and families with remitting CAE. We found that four out of 48 families (8%) had mutations in GABRB3. One heterozygous missense mutation (P11S) in exon 1a segregated with four CAE-affected persons in one multiplex, two-generation Mexican family. P11S was also found in a singleton from Mexico. Another heterozygous missense mutation (S15F) was present in a singleton from Honduras. An exon 2 heterozygous missense mutation (G32R) was present in two CAE-affected persons and two persons affected with EEG-recorded spike and/or sharp wave in a two-generation Honduran family. All mutations were absent in 630 controls. We studied functions and possible pathogenicity by expressing mutations in HeLa cells with the use of Western blots and an in vitro translation and translocation system. Expression levels did not differ from those of controls, but all mutations showed hyperglycosylation in the in vitro translation and translocation system with canine microsomes. Functional analysis of human GABA(A) receptors (alpha 1 beta 3-v2 gamma 2S, alpha 1 beta 3-v2[P11S]gamma 2S, alpha 1 beta 3-v2[S15F]gamma 2S, and alpha 1 beta 3-v2[G32R]gamma 2S) transiently expressed in HEK293T cells with the use of rapid agonist application showed that each amino acid transversion in the beta 3-v2 subunit (P11S, S15F, and G32R) reduced GABA-evoked current density from whole cells. Mutated beta 3 subunit protein could thus cause absence seizures through a gain in glycosylation of mutated exon 1a and exon 2, affecting maturation and trafficking of GABAR from endoplasmic reticulum to cell surface and resulting in reduced GABA-evoked currents.
Department of Molecular & Medical Pharmacology, David Geffen School of Medicine, University of California Los Angeles, CA 90095, USA.
Miyabi
M
Tanaka
Richard W
RW
Olsen
Marco T
MT
Medina
Emily
E
Schwartz
Maria Elisa
ME
Alonso
Reyna M
RM
Duron
Ramon
R
Castro-Ortega
Iris E
IE
Martinez-Juarez
Ignacio
I
Pascual-Castroviejo
Jesus
J
Machado-Salas
Rene
R
Silva
Julia N
JN
Bailey
Dongsheng
D
Bai
Adriana
A
Ochoa
Aurelio
A
Jara-Prado
Gregorio
G
Pineda
Robert L
RL
Macdonald
Antonio V
AV
Delgado-Escueta
D000595:Amino Acid Sequence; D002460:Cell Line; D002648:Child; D002675:Child, Preschool; D004252:DNA Mutational Analysis; D018076:DNA, Complementary / Q000235:genetics; D004569:Electroencephalography; D004832:Epilepsy, Absence / Q000235:genetics* / Q000503:physiopathology*; D005260:Female; D008040:Genetic Linkage; D005838:Genotype; D006031:Glycosylation; D006801:Humans; D008297:Male; D008969:Molecular Sequence Data; D020125:Mutation, Missense; D010375:Pedigree; D010641:Phenotype; D011963:Receptors, GABA-A / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D014162:Transfection; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
D018076:DNA, Complementary; C546273:GABRB3 protein, human; D011963:Receptors, GABA-A; D005680:gamma-Aminobutyric Acid