Small fitness effects and weak genetic interactions between deleterious mutations in heterozygous loci of the yeast Saccharomyces cerevisiae

Rare, random mutations were induced in budding yeast by ethyl methanesulfonate (EMS). Clones known to bear a single non-neutral mutation were used to obtain mutant heterozygotes and mutant homozygotes that were later compared with wild-type homozygotes. The average homozygous effect of mutation was an approximately 2% decrease in the growth rate. In heterozygotes, the harmful effect of these relatively mild mutations was reduced approximately fivefold. In a test of epistasis, two heterozygous mutant loci were paired at random. Fitness of the double mutants was best explained by multiplicative action of effects at single loci, with little evidence for epistasis and essentially excluding synergism. In other experiments, the same mutations in haploid and heterozygous diploid clones were compared. Regardless of the haploid phenotypes, mildly deleterious or lethal, fitness of the heterozygotes was decreased by less than half a per cent on average. In general, the results presented here suggest that most mutations tend to exhibit small and weakly interacting effects in heterozygous loci regardless of how harmful they are in haploids or homozygotes.     

Concentration dependence of protein diffusion.

The concentration dependence of protein self-diffusion constants is described by a free volume diffusion theory which accounts for the effects of local protein concentration fluctuations.     

A PEDIGREE OF ONE FAMILY WITH DELAYED SLEEP PHASE SYNDROME

The prevalence of delayed sleep phase syndrome (DSPS) has been estimated to be quite low. Although no genetic inheritance pattern has been described, it has been reported that close to 50% of DSPS patients have biological relatives with similar symptoms. A pedigree of one extended family with symptoms suggestive of DSPS has been identified. Morningnesseveningness questionnaires were administered to all first- and second-degree relatives of a proband identified with DSPS. A total of 51 (86%) questionnaires were returned, and 6 adult biological relatives of 27 (22%) showed a preference for eveningness, which is much higher than reported in the general population. Both the paternal and maternal branches contained affected individuals, suggesting the possibility of a bilineal mode of inheritance. While the trait did not obey simple Mendelian inheritance, the vertical patterns of transmission were consistent with either an autosomal dominant mode of inheritance with incomplete penetrance or a multifactorial mode of inheritance. These data provide some preliminary support to the notion that eveningness, and thus DSPS, may have a genetic component. The prevalence of symptoms suggestive of DSPS is higher in this family than reported in the general population. Case reports such as this support the utility of larger, more systematic studies. It is unclear whether this degree of familiarity is representative of that in the general population. (Chronobiology International, 18(5), 831-840, 2001)     

New substrates and enzyme assays for the detection of mucopolysaccharidosis III (Sanfilippo Syndrome) types A, B, C, and D by tandem mass spectrometry

The clinical phenotype of Sanfilippo Syndrome is caused by one of four enzyme deficiencies that are associated with a defect in mucopolysaccharide metabolism. The four subtypes (A, B, C, and D) are each caused by an enzyme deficiency involved in the degradation of heparan sulfate. We have developed a highly efficient synthesis of the substrates and internal standards required for the enzymatic assay of each of the four enzymes. The synthesis of the substrates involves chemical modification of a common intermediate. The substrates and internal standards allow the measurement of the enzymes relevant to heparan N-sulfatase (type A); N-acetyl-α-glucosaminidase (type B); acetyl-CoA:α-glucosamide N-acetyltransferase (type C); and N-acetylglucosamine 6-sulfatase (type D). The internal standards are similar to the substrates and allow for the accurate quantification of the enzyme assays using tandem mass spectrometry. The synthetic substrates incorporate a coumarin moiety and can also be used in fluorometric enzyme assays. We confirm that all four substrates can detect the appropriate Sanfilippo Syndrome in fibroblast lysates, and the measured enzyme activities are distinctly lower by a factor of 10 when compared to fibroblast lysates from unaffected persons.     

Schwann Cells: Development and Role in Nerve Repair

Schwann cells develop from the neural crest in a well-defined sequence of events. This involves the formation of the Schwann cell precursor and immature Schwann cells, followed by the generation of the myelin and nonmyelin (Remak) cells of mature nerves. This review describes the signals that control the embryonic phase of this process and the organogenesis of peripheral nerves. We also discuss the phenotypic plasticity retained by mature Schwann cells, and explain why this unusual feature is central to the striking regenerative potential of the peripheral nervous system (PNS).The myelin and nonmyelin (Remak) Schwann cells of adult nerves originate from the neural crest in well-defined developmental steps (Fig. 1). This review focuses on embryonic development (for additional information on myelination, see Salzer 2015). We also discuss how the ability to change between differentiation states, a characteristic attribute of developing cells, is retained by mature Schwann cells, and explain how the ability of Schwann cells to change phenotype in response to injury allows the peripheral nervous system (PNS) to regenerate after damage.Open in a separate windowFigure 1.Main transitions in the Schwann cell precursor (SCP) lineage. The diagram shows both developmental and injury-induced transitions. Black uninterrupted arrows, normal development; red arrows, the Schwann cell injury response; stippled arrows, postrepair reformation of myelin and Remak cells. Embryonic dates (E) refer to mouse development. (Modified from Jessen and Mirsky 2012; reprinted, with permission and with contribution from Y. Poitelon and L. Feltri.)