Genetic influence on the structural variations of the abnormal prion protein

Prion diseases are characterized by the presence of the abnormal prion protein PrPSc, which is believed to be generated by the conversion of the α-helical structure that predominates in the normal PrP isoform into a β-sheet structure resistant to proteinase K (PK). In human prion diseases, two major types of PrPSc, type 1 and 2, can be distinguished based on the difference in electrophoretic migration of the PK-resistant core fragment. In this study, protein sequencing was used to identify the PK cleavage sites of PrPSc in 36 cases of prion diseases. We demonstrated two primary cleavage sites at residue 82 and residue 97 for type 1 and type 2 PrPSc, respectively, and numerous secondary cleavages distributed along the region spanning residues 74–102. Accordingly, we identify three regions in PrPSc: one N-terminal (residues 23–73) that is invariably PK-sensitive, one C-terminal (residues 103–231) that is invariably PK-resistant, and a third variable region (residues 74–102) where the site of the PK cleavage, likely reflecting the extent of the β-sheet structure, varies mostly as a function of the PrP genotype at codon 129.

Importance of epistasis as the genetic basis of heterosis in an elite rice hybrid

The genetic basis of heterosis was investigated in an elite rice hybrid by using a molecular linkage map with 150 segregating loci covering the entire rice genome. Data for yield and three traits that were components of yield were collected over 2 years from replicated field trials of 250 F2:3 families. Genotypic variations explained from about 50% to more than 80% of the total variation. Interactions between genotypes and years were small compared with the main effects. A total of 32 quantitative trait loci (QTLs) were detected for the four traits; 12 were observed in both years and the remaining 20 were detected in only one year. Overdominance was observed for most of the QTLs for yield and also for a few QTLs for the component traits. Correlations between marker heterozygosity and trait expression were low, indicating that the overall heterozygosity made little contribution to heterosis. Digenic interactions, including additive by additive, additive by dominance, and dominance by dominance, were frequent and widespread in this population. The interactions involved large numbers of marker loci, most of which individually were not detectable on single-locus basis; many interactions among loci were detected in both years. The results provide strong evidence that epistasis plays a major role as the genetic basis of heterosis.

Variations in frequencies of drug resistance in Plasmodium falciparum

Continual exposure of malarial parasite populations to different drugs may have selected not only for resistance to individual drugs but also for genetic traits that favor initiation of resistance to novel unrelated antimalarials. To test this hypothesis, different Plasmodium falciparum clones having varying numbers of preexisting resistance mechanisms were treated with two new antimalarial agents: 5-fluoroorotate and atovaquone. All parasite populations were equally susceptible in small numbers. However, when large populations of these clones were challenged with either of the two compounds, significant variations in frequencies of resistance became apparent. On one extreme, clone D6 from West Africa, which was sensitive to all traditional antimalarial agents, failed to develop resistance under simple nonmutagenic conditions in vitro. In sharp contrast, the Indochina clone W2, which was known to be resistant to all traditional antimalarial drugs, independently acquired resistance to both new compounds as much as a 1,000 times more frequently than D6. Additional clones that were resistant to some (but not all) traditional antimalarial agents acquired resistance to atovaquone at high frequency, but not to 5-fluoroorotate. These findings were unexpected and surprising based on current views of the evolution of drug resistance in P. falciparum populations. Such new phenotypes, named accelerated resistance to multiple drugs (ARMD), raise important questions about the genetic and biochemical mechanisms related to the initiation of drug resistance in malarial parasites. Some potential mechanisms underlying ARMD phenotypes have public health implications that are ominous.

Genetic control of resistance to experimental infection with virulent Mycobacterium tuberculosis

Over 2 billion people are estimated to be infected with virulent Mycobacterium tuberculosis, yet fewer than 10% progress to clinical tuberculosis within their lifetime. Twin studies and variations in the outcome of tuberculosis infection after exposure to similar environmental risks suggest genetic heterogeneity among individuals in their susceptibility to disease. In a mouse model of tuberculosis, we have established that resistance and susceptibility to virulent M. tuberculosis is a complex genetic trait. A new locus with a major effect on tuberculosis susceptibility, designated sst1 (susceptibility to tuberculosis 1), was mapped to a 9-centimorgan (cM) interval on mouse chromosome 1. It is located 10–19 cM distal to a previously identified gene, Nramp1, that controls the innate resistance of mice to the attenuated bacillus Calmette–Guérin vaccine strain. The phenotypic expression of the newly identified locus is distinct from that of Nramp1 in that sst1 controls progression of tuberculosis infection in a lung-specific manner. Mice segregating at the sst1 locus exhibit marked differences in the growth rates of virulent tubercle bacilli in the lungs. Lung lesions in congenic sst1-susceptible mice are characterized by extensive necrosis and unrestricted extracellular multiplication of virulent mycobacteria, whereas sst1-resistant mice develop interstitial granulomas and effectively control multiplication of the bacilli. The resistant allele of sst1, although powerful in controlling infection, is not sufficient to confer full protection against virulent M. tuberculosis, indicating that other genes located outside of the sst1 locus are likely also to be important for controlling tuberculosis infection.

Analysis of the genetic differences between Neisseria meningitidis and Neisseria gonorrhoeae: two closely related bacteria expressing two different pathogenicities.

We have investigated genetic differences between the closely related pathogenic Neisseria species, Neisseria meningitidis and Neisseria gonorrhoeae, as a novel approach to the elucidation of the genetic basis for their different pathogenicities. N. meningitidis is a major cause of cerebrospinal meningitis, whereas N. gonorrhoeae is the agent of gonorrhoea. The technique of representational difference analysis was adapted to the search for genes present in the meningococcus but absent from the gonococcus. The libraries achieved are comprehensive and specific in that they contain sequences corresponding to the presently identified meningococcus-specific genes (capsule, frp, rotamase, and opc) but lack genes more or less homologous between the two species, e.g., ppk and pilC1. Of 35 randomly chosen clones specific to N. meningitidis, DNA sequence analysis has confirmed that the large majority have no homology with published neisserial sequences. Mapping of the cloned DNA fragments onto the chromosome of N. meningitidis strain Z2491 has revealed a nonrandom distribution of meningococcus-specific sequences. Most of the genetic differences between the meningococcus and gonococcus appear to be clustered in three distinct regions, one of which (region 1) contains the capsule-related genes. Region 3 was found only in strains of serogroup A, whereas region 2 is present in a variety of meningococci belonging to different serogroups. At a time when bacterial genomes are being sequenced, we believe that this technique is a powerful tool for a rapid and directed analysis of the genetic basis of inter- or intraspecific phenotypic variations.