ARTÍCULOS DE REVISIÓNTabla 4. Evolución de las frecuencias alélicas ARR y ARQ al variar la susceptibilidad decada raza al “scrapie” (Datos tomados de Barillet et al., 2003).RAZA INCIDENCIA FRECUENCIA DE ARR FRECUENCIA DE ARQManechcabeza rojacambios estructurales en otra, modificandosu estructura secundaria. Mucho másimprobable resulta aún la posibilidad deque la proteína patógena pueda, por símisma, transmitir a otras proteínas no solosu capacidad patógena, sino determinadosaspectos diferenciales en cuando a la patogeniade la enfermedad, dando lugar a lasdenominadas “cepas”.En conclusión, parece más probablela existencia de elementos adicionales a laproteína PrP Sc en el proceso patológico.2. La consideración de genotipos del genPRNP resistentes a la tembladera y noportadores de proteína PrP Sc , comienzaa ser más discutible. Será necesariointroducir el concepto de “cepa” y nuevasevidencias.La más sensible 0,17 0,80Basco-Béarnaise Bastante sensible 0,35 0,65Manechcabeza negraMenos sensible 0,48 0,50Corsa Menos sensible 0,50 0,45Lacaune La más resistente 0,60 0,37Está demostrado que la infección experimentalde ovejas ARR/ARR es posible.También existen evidencias de que ovejasde genotipo ARR/ARR pueden ser portadorasde proteína PrP Sc , aunque no muestrensíntomas de la enfermedad.En conclusión, debería considerarse larelación entre otros codones del gen PRNP yla susceptibilidad a padecer la tembladera.3. El gen PRNP no parece ser el únicoimplicado en el control genético de lasusceptibilidad a padecer la tembladera.Individuos con el mismo genotipo para elgen PRNP, situados en el mismo rebaño ysometidos a ambientes muy semejantes,en presencia de la proteína patógena,unos enferman y otros no.Individuos con el mismo genotipopara el gen PRNP y pertenecientes a rebañosdiferentes, se comportan de formadistinta. De hecho, existe una susceptibilidaddiferente en animales con el mismogenotipo para el gen PRNP, pero pertenecientesa razas diferentes.En conclusión, es necesario buscarotros genes que podrían estar implicadosen los procesos de susceptibilidad y deresistencia a la tembladera.4. La raza es un factor importante en lasusceptibilidad a la tembladera. Determinadasrazas, que se muestran másresistentes que otras a esta enfermedad,pueden ser portadoras de genotipospara genes aún desconocidos queconfieran una mayor resistencia.La existencia de razas diferentes, portadorasde todos los genotipos conocidos parael gen PRNP y compartiendo ambientesmuy semejantes, pero que muestrandiversos grados de susceptibilidad a laenfermedad, parecen apoyarlo.En resumen, factores asociados a laraza se muestran muy importantes sobrela susceptibilidad y la resistencia a la tembladera,por lo que deben ser estudiadosmás a fondo.BIBLIOGRAFÍA34BARILLET, F., PALHIERRE, I., ASTRUC, J.M., BROCHARD, M., et al. (2003). 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