Characterization of human TSE strains after passage in humanized transgenic mice

Transmissible Spongiform Encephalopathies (TSEs) are fatal neurodegenerative diseases affecting humans and animals caused by a transmissible agent whose nature is still under debate. In spite of this uncertainty, the distinctive features of each infectious strain can be highlighted and compared through the experimental passage in a given species of laboratory animals. By using this approach, it was discovered that the strain responsible for variant Creutzfeldt-Jakob disease (vCJD) is directly related to the bovine spongiform encephalopathy infectious strain, thus helping to improve prophylactic measures to stop the spread of the disease. This observation proved the possibility of zoonotic transmission of animal TSEs and stimulated health authorities and researchers to intensify their efforts for the definition of strain properties with the aim to define their epidemiological history, their virulence and to understand possible correlations between atypical human and animal forms. The subject of my PhD thesis is the characterization of the strains associated to several atypical Italian cases of sporadic Creutzfeldt-Jakob Disease (sCJD) by the transmission in transgenic mice which express the human prion protein gene (PRNP) with three alternative genotypes at the polymorphic codon 129 (MM, MV, VV), the most recognised factor of susceptibility and pathological phenotype in sCJD. The three human cases analyzed, all heterozygous at position 129 of the prion protein, were characterized by unusual clinical signs (two of them, by an early onset, long duration of the disease and psychiatric symptoms), or by unusual biochemical properties for what concern the pathological prion protein (absence of the high glycosylated form). The three lines of transgenic mice, engineered by “gene targeting” technique, posses the human prion protein gene in the correct position within murine genome, under the strict control of natural expression modifiers. This genetic modification allowed to evaluate the “strain” characteristics and to study the susceptibility of the different human genotypes (129 MM, MV, VV) to the single prion strains. Results showed that the first two cases transmitted the disease to all three genotypes with an high efficiency, yet with a particular tropism towards the valine homozygous mice which were clinically sick with incubation times 50% shorter than mice expressing the other genotypes. The case with defective glycosylation displayed an exceedingly low attack rate, producing overt disease only in few, valine homozygous, mice. When comparing the efficiency of infection (sick animals/inoculated), the incubation times and the neurophatological and biochemical profiles, between our animals and their counterparts in the literature expressing identical PRNP genotypes but injected with other forms of sCJD, we observed that the two cases with atypical clinical features are associated to an infectious strain already known, whilst the case with atypical glycosylation seems to belong to an absolutely original strain, never found before. Concerning the latter case, further investigations are in course with the aim to understand its spread, its danger and its possible connection with animal forms. The study confirmed the relevant value of these transgenic lines, not only to identify new strains with an unknown pathogenic potential for humans, but also to investigate the influence of the polymorphic codon 129 on the host’s susceptibility and the pathological process of the prion diseases.