Hyperekplexia 2

A number sign (#) is used with this entry because of evidence that hyperekplexia-2 (HKPX2) is caused by compound heterozygous or homozygous mutation in the GLRB gene (138492) on chromosome 4q32.

For a general phenotypic description and a discussion of genetic heterogeneity of hyperekplexia, see HKPX1 (149400).

Clinical Features

Rees et al. (2002) reported a male neonate who presented with generalized hypertonia and hyperreactivity 3 hours after birth. He had an exaggerated startle response and myoclonus elicited by testing the tendon reflexes or tapping on the back. Moro reflex was absent. Hyperreflexia and exaggerated startle response were noted until about 1 year of age, and then decreased somewhat. He had slightly delayed motor development, but normal mental development. By age 3 years, the phenotype had improved such that an abnormal startle response could only be invoked by tactile stimulus of the perioral region. He also had a hiatal hernia with gastroesophageal reflux.

Al-Owain et al. (2012) reported a large consanguineous Saudi Arabian kindred in which 9 individuals had hyperekplexia. All showed increased fetal movements and generalized stiffness at birth. The stiffness tended to ameliorate with age. Three patients had a positive reflex test, 2 of whom showed a positive glabellar test. Seven patients had esotropia. Two girls had social phobia and 1 boy had attention-deficit hyperactivity disorder. All had normal cognition, except 1 girl with seizures and a possible history of meningitis in infancy. The patients had a favorable response to clonazepam or vigabatrin, and 2 patients were able to stop treatment at ages 8 and 3 years, respectively.

Molecular Genetics

In a patient with hyperekplexia-2, Rees et al. (2002) identified compound heterozygosity for 2 mutations in the GLRB gene (138492.0001-138492.0002). Each unaffected parent was heterozygous for 1 of the mutations. Electrophysiologic studies showed reduced sensitivity to agonist-mediated activation of the alpha-1-beta (G229D) glycine receptor. The authors suggested that glycine receptor beta subunits may not be restricted to conferring modulatory influences and maintaining structural integrity, but may also play a functional role in human glycine receptor ligand binding.

Al-Owain et al. (2012) identified a homozygous missense mutation in the GLRB gene (M177R; 138492.0003) in 9 affected members of 3 branches of a large consanguineous Saudi Arabian kindred with hyperekplexia-2.