The impact of molecular genetics on clinical neurology

Pre- and postmortem influences on brain RNA

Many potentially valuable techniques for the understanding of human neurobiological and neuropathological processes require the use of RNA obtained from postmortem tissue. As with earlier neurochemical studies, there are two particular problems posed by such tissue in comparison with tissue from experimental animals. These are the postmortem interval and the condition of the patient prior to death, referred to as the agonal state. We review the nature and extent of the effects of postmortem interval and agonal state on RNA in brain tissue, with particular reference to the study of neuropsychiatric disorders. Perhaps surprisingly, postmortem interval has at most a modest effect on RNA. Abundant intact and biologically active RNA is present in tissue frozen 36 h or more after death. Postmortem interval does not account for the marked variability observed among human brains in all RNA parameters. Despite the overall stability of RNA after death, some evidence suggests that individual RNAs may undergo postmortem decay. Less attention has been paid to the effects of agonal state. The existing data indicate that events in the premortem period such as hypoxia and coma can affect the amount of some messenger RNAs. The nature of agonal state influences depends on the messenger RNA in question, though the basis for this selective vulnerability is unknown. No agonal state effect on overall RNA level or activity has been found. The data show that postmortem brain tissue can be used for RNA research. However, considerable attention must be paid to controlling for the influences of pre- and postmortem factors, especially when quantitative analyses are performed.

Alzheimer's disease-associated reduction of polysomal mRNA translation

Polysomes from the frontal cortices of individuals who had histopathologically confirmed Alzheimer's disease were compared with polysomes from individuals who exhibited no neuropathological conditions. The cytosolic polysome yield from Alzheimer's disease frontal cortex was reduced 40% compared with that obtained from control frontal cortex. The translational activity per unit polysome of the Alzheimer's disease polysomes was only 50% of control in a reticulocyte lysate in vitro translation assay in which human polysomes do not undergo reinitiation. These differences exhibited brain region specificity in that polysomes isolated from Alzheimer's disease cerebellum were not different from control cerebellar polysomes. Thus, the disruptions are not due to a secondary and general response of the entire brain to the disease. These reductions were reflected by similar decreases in the translation of the mRNA for high molecular weight neurofilament polypeptide. Thus, the inhibition of polysomal mRNA translation is a mechanism by which gene expression is impaired in pathologically involved brain regions of individuals afflicted by Alzheimer's disease.

Membrane excitability expressed in human neuroblastoma cell hybrids

The voltage-sensitive Na+ channel is responsible for the action potential of membrane electrical excitability in neuronal tissue. Three methods were used to demonstrate the presence of neurotoxin-responsive Na+ channels in two hybrid cell lines resulting from the fusion of excitable human neuroblastoma cells with mouse fibroblasts. Only one of the two electrically active hybrid cell lines maintained the sensitivity of the neuroblastoma parent to tetrodotoxin (TTX). The other hybrid, although electrically active, was not responsive to TTX or scorpion venom. Comparisons of the patterns of expression of membrane excitability and of chromosome complements in these human neuroblastoma cell hybrids suggest that the phenotype of membrane excitability is composed of genetically distinct elements.