A BIOPHYSICAL CHARACTERIZATION OF HOMOZYGOUS AND HETEROZYGOUS ALPHA-1 GABA RECEPTOR SUBUNIT MUTATIONS ASSOCIATED WITH A FAMILIAL JUVENILE MYOCLONIC EPILEPSY
Abstract number :
H.07
Submission category :
Year :
2003
Submission ID :
3611
Source :
www.aesnet.org
Presentation date :
12/6/2003 12:00:00 AM
Published date :
Dec 1, 2003, 06:00 AM
Authors :
Martin J. Gallagher, Luyan Song, Robert L. Macdonald Neurology, Vanderbilt University, Nashville, TN
Recently, a missense mutation ([alpha]1-A322D) in the [alpha]1 subunit of GABA-A receptor (GABAR) was found in affected members of a family with an autosomal dominant form of juvenile myoclonic epilepsy. Because each patient has one wild type and one mutant [alpha]1-subunit allele, and because each GABAR has two [alpha] subunits, four possible GABAR combinations can be assembled (1. both [alpha]-subunits wild type, 2. both [alpha]-subunits mutant, 3. wild type [alpha]-subunit between the [beta]-subunits and an A322D mutant between the [gamma]- and [beta]-subunits, and 4. and an A322D mutant [alpha]-subunit between the [beta]-subunits and a wild type [alpha]-subunit between the [gamma]- and [beta]-subunits) . We characterized the whole-cell kinetics of all homozygous and heterozygous GABAR combinations to determine if changes in GABAR physiology can account for the patients[rsquo] epilepsy
HEK293T cells were transfected with wild type human [beta]2- and [gamma]2S- subunits and with wild type and/or mutant (A322D) [alpha]1 subunits. Whole cell voltage- clamp currents were measured using lifted cells and with rapid GABA perfusion (exchange time 0.5-1.0 msec) to measure the whole cell GABAR kinetics. Whole cell current measurements included peak currents, 10-90% activation times, four phases of desensitization (fast -20 ms, intermediate - 200 ms, slow- 1 sec, and ultraslow-10 sec), and two phases of deactivation. In addition, the change in peak current in response to repetitive stimulation was determined.
1.) GABAR with two mutant (homozygous) GABA [alpha]1-subunits had 20-fold smaller peak currents, 10-fold longer activation times, no fast (20 ms) or intermediate (200 ms) phases of desensitization and two-fold faster deactivation rates compared with wild-type (homozygous) receptors.
2.) GABAR with one mutant and one wild type (heterozygous) [alpha]1-subunits had decreased peak currents, longer activation times, alterered desensitization and deactivation kinetics and decreased peak currents with repeptive stimulation.
GABARs with both [quot]homozygote[quot] and [quot]heterozygote[quot] mutant [alpha]1-A322D have distinctly different kinetics than wild type GABAR. These biophysical differences would all result in decreased GABA-mediated neuronal inhibition and thus could account for the patients[apos] epilepsy.
[Supported by: 1. NIH
NS 39479]