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Identification of genes (outside PRNP) modulating scrapie susceptibility

Final Activity Report Summary - IGMSS (Identification of genes (outside PRNP) modulating scrapie susceptibility)

Scrapie is a fatal and progressive neurodegenerative disease of sheep and goats that belongs to Transmissible spongiform encephalopathies (TSE). These include animal and human diseases that share a common pathogenic mechanism. The principal feature of scrapie is the accumulation of PrPsc, mainly in nervous and in lymphatic tissue.

Scrapie was reported for the first time in Great Britain in 1730 and later in France and Germany. The nature of the scrapie agent remained a mystery for a long time. Since the 1980s, it has been considered an atypical virus, belonging to the group of lentiviruses. Later studies dismissed the viral theory. In 1997 Stanley Prusiner was awarded the Nobel Prize for a new theory, called 'the protein only hypothesis', suggesting that the TSE agent was a pathogenic protein with infectious features, which he called prion. This protein, named PrPsc, originated from the abnormal refolding of PrP, a glycoprotein normally associated to the cellular membrane.

The recent crisis of Bovine spongiform encephalopathy (BSE) clearly showed the need of keeping animal TSE under control in order to protect animal and human health. Susceptibility to scrapie TSEs is generally strongly influenced by genetic. The prion protein encoding (PRNP) gene has a critical role in this process. However, it was estimated that other genes could also be involved in the genetic susceptibility.

The existence of genes with quantitative effect (QTL) influencing the incubation period of scrapie-infected animals was demonstrated in mice, cattle and sheep. During the last years, after an entire genome scan, we could identify ovine chromosome regions where such genes were thought to be located.

A shortcut to pinpoint relevant genes in such regions could be the use of functional genomics, in case one accepted the idea that, at least in some cases, key genes could be expressed all along the prion contamination period, and could be differentially modulated in comparison to early steps of the infection and through the whole incubation time to the onset of symptoms.

In this study, we aimed to identify genes that could unambiguously mark some aspects of prion contamination in sheep neural tissue. We used the Subtractive suppressive hybridisation (SSH) technique on four regions of cerebral tissue from sheep harbouring a similar genomic background, which were either healthy or heavily contaminated, i.e. in the terminal phase of the disease. Surprisingly, following the subtraction, several discrete differential bands appeared between the two reciprocally subtracted states. These bands were cloned and sequenced, allowing for the identification of transcripts' encoding proteins which were mis-regulated during prion diseases in neural tissues, namely COX1, CHN1, PPP2CA, LRFN5, CAMK2A and RABEPK.

CHN1 was revealed by the SSH procedure owing to a highly specific differential splicing between affected and sane sheep. This gene was positioned inside a QTL region and was observed to be involved in prion-disease incubation time in mice on a large region of chromosome 2, along with RABEPK. In addition, another of the observed genes, namely LRFN5, mapped at the location of a QTL modulating scrapie incubation time in sheep chromosome 18. Our results, together with in silico data concerning CHN1 and LRFN5, suggested that these two genes could be interesting candidates for modulating prion incubation period in mice and, more generally, for prion physiopathology in mammals.