Alterations in the alpha2 isoform of Na,K-ATPase associated with familial hemiplegic migraine type 2

New roles for an old enzyme: Na,K-ATPase emerges as an interesting drug target

Na,K-ATPase (NKA) is well known for its role as a maintainer of electrolyte and fluid balance in cells, organs and whole body. Exciting new findings have revealed additional fundamental roles for NKA as a signal transducer and modulator of growth, apoptosis, cell adhesion and motility. The signal transduction function can be triggered by the binding of ouabain, the mammalian analogue of digitalis to NKA. The catalytic subunit of NKA exists in different forms and mutations in two of the forms that are expressed in brain can give rise to migraine, epilepsy and Parkinsonism-like symptoms. This review will present these new aspects of NKA and their clinical implications.

Genetic biomarkers for migraine

Biomarkers are physical signs or laboratory measurements that occur in association with a pathological process and have putative diagnostic and/or prognostic utility. In migraine, clinical, radiological, and biochemical biomarkers might be helpful to improve diagnosis, get insight in pathophysiology, and facilitate treatment choices. Genetic biomarkers are defined as genetic variations (mutations or polymorphisms) that can predict disease susceptibility, disease outcome, or treatment response. As yet, only a few genetic biomarkers for migraine are available. Mutations in 3 different genes responsible for familial hemiplegic migraine, a monogenetic subtype of migraine with aura, and the MTHFR C677T polymorphism in common forms of migraine are clear examples. Many positive findings from linkage studies and association studies in common forms of migraine have not been replicated, and are therefore of less clinical use. In this review, we will discuss genetic biomarkers in migraine.

Ion channel functional candidate genes in multigenic neuropsychiatric disease

Scores of monogenic Mendelian ion channel diseases serve to anchor the pathophysiology of the channelopathies, but there are also now clear examples of environmental, pharmacogenetic, and acquired channelopathy mechanisms. The cardinal feature of heritable ion channel disease is a periodic disturbance of rhythmic function in constitutionally hyperexcitable tissue. While the complexity of neuroanatomy obscures functional analysis of mutations causing monogenic seizure, ataxia, or migraine syndromes, extrapolation from the cardiac (Long QT [LQT]) and muscle (Periodic Paralysis) channelopathy syndromes provides a simplified predictive framework of molecular pathology: electrically stabilizing potassium ion (K(+)) and chloride ion (Cl(-)) channels, likely having lesions that diminish their current, and excitatory Na(+) channels, likely having gain-of-function lesions. The voltage-gated calcium channel gene family that contains CACNA1C, the newest LQT locus, causing Timothy Syndrome with a phenotype including autism, has proven to be particularly informative for its members' ability to tie the various central nervous system (CNS) phenotypes together in an interpretable fashion, now including direct extension to the classically multigenic neuropsychiatric phenotypes. Features of a promising ion channel candidate gene arise from its broad locus, gene family, nature of alleles, physiology and pharmacology, tissue expression profile, and phenotype in model organisms. KCNN3 is explored as a paradigm to consider.

Two de novo mutations in the Na,K-ATPase gene ATP1A2 associated with pure familial hemiplegic migraine

Familial hemiplegic migraine (FHM) is a rare autosomal dominantly inherited subtype of migraine, in which hemiparesis occurs during the aura. The majority of the families carry mutations in the CACNA1A gene on chromosome 19p13 (FHM1). About 20% of FHM families is linked to chromosome 1q23 (FHM2), and has mutations in the ATP1A2 gene, encoding the alpha2-subunit of the Na,K-ATPase. Mutation analysis in a Dutch and a Turkish family with pure FHM revealed two novel de novo missense mutations, R593W and V628M, respectively. Cellular survival assays support the hypothesis that both mutations are disease-causative. The identification of the first de novo mutations underscores beyond any doubt the involvement of the ATP1A2 gene in FHM2.

Mutations Phe785Leu and Thr618Met in Na+,K+-ATPase, associated with familial rapid-onset dystonia parkinsonism, interfere with Na+ interaction by distinct mechanisms

The Na(+),K(+)-ATPase plays key roles in brain function. Recently, missense mutations in the Na(+),K(+)-ATPase were found associated with familial rapid-onset dystonia parkinsonism (FRDP). Here, we have characterized the functional consequences of FRDP mutations Phe785Leu and Thr618Met. Both mutations lead to functionally altered, but active, Na(+),K(+)-pumps, that display reduced apparent affinity for cytoplasmic Na(+), but the underlying mechanism differs between the mutants. In Phe785Leu, the interaction of the E(1) form with Na(+) is defective, and the E(1)-E(2) equilibrium is not displaced. In Thr618Met, the Na(+) affinity is reduced because of displacement of the conformational equilibrium in favor of the K(+)-occluded E(2)(K(2)) form. In both mutants, K(+) interaction at the external activating sites of the E(2)P phosphoenzyme is normal. The change of cellular Na(+) homeostasis is likely a major factor contributing to the development of FRDP in patients carrying the Phe785Leu or Thr618Met mutation. Phe785Leu moreover interferes with Na(+) interaction on the extracellular side and reduces the affinity for ouabain significantly. Analysis of two additional Phe(785) mutants, Phe785Leu/Leu786Phe and Phe785Tyr, demonstrated that the aromatic function of the side chain, as well as its exact position, is critical for Na(+) and ouabain binding. The effects of substituting Phe(785) could be explained by structural modeling, demonstrating that Phe(785) participates in a hydrophobic network between three transmembrane segments. Thr(618) is located in the cytoplasmic part of the molecule near the catalytic site, and the structural modeling indicates that the Thr618Met mutation interferes with the bonding pattern in the catalytic site in the E(1) form, thereby destabilizing E(1) relative to E(2)(K(2)).