Optimizing Mouse Models of Neurodegenerative Disorders
Optimizing Mouse Models of Neurodegenerative Disorders
Many neurological diseases present with tremendous clinical heterogeneity. For example, some ALS patients can progress very rapidly in their disease, while others experience only weakness for more extended periods of time. In addition to variation in the age of onset, ALS patients may present with either bulbar or limb onset, and may even exhibit tremendous variation in the sequence of limbs affected and how the disease progresses. Given its high sporadic incidence, ALS is considered to be a complex disease that is influenced by many environmental triggers and likely many genetic susceptibility and resistance loci.
The SOD1G93A mouse model has been the premier model for preclinical testing to predict efficacy in ALS clinical trials. However, much of the preclinical work carried out in the SOD1G93A model has failed to translate to the clinic. This failure to translate is often attributed to poor preclinical trial design, as revealed in sobering and impactful publications by Benatar and Scott et al. However, even with proper preclinical trial designs, are we expecting too much from a single model? SOD1 mutations represent such a small proportion of the ALS patient population and are also likely under-represented in clinical trials, if they are represented at all. Is it, therefore, unrealistic to expect a single gene mutation on a single genetic mouse background to represent a diverse patient population of such a complex disease? Indeed, the field of ALS research has been hindered by a lack of animal models; validation across numerous models would increase the confidence that preclinical work will translate to human trials.
Today, advances in next-generation sequencing technologies and massive reductions in sequencing costs have generated a wealth of data that are revealing the genetic basis for many human neurodegenerative diseases. This is an especially exciting time for gene discoveries in ALS. With new gene discoveries comes a new opportunity to engineer new mouse models. However, getting the most out of these mouse models will also depend on how we leverage both sequencing and new engineering technologies in order to explore modifier genes in mice. It is known that genetic modifiers exist and can significantly influence disease onset in the SOD1G93A model. Male mice in this model on a B6SJL genetic background survive to 129 ± 9 days (mean ± standard deviation). On a C57BL/6J genetic background, male survival is significantly extended to 157 ± 9 days (mean ± standard deviation). Other genetic backgrounds have also been shown to influence survival in these mice, and gene identification is underway. Recent studies also demonstrated that survival of the transgenic mouse (Prnp-TARDBP*A315T)95Balo/J was also influenced by genetic background. In a dominant modifier screen, male survival was significantly different across a number of F1 genetic crosses and, in addition, males were always significantly more affected than females, regardless of genetic background, giving this mouse model both gender-specific and genetic background-specific influences. Advances in high-throughput exome sequencing will significantly enhance our ability to identify these genetic modifiers, even in complex genetic interactions.
New technologies in genome editing with site-specific nucleases will also play a powerful role in our ability to create and validate new disease genes and allelic variants associated with human ALS. Although homologous recombination proved to be a powerful tool in genetic engineering, the creation of the models through construct design and traditional targeting is often hindered by the low efficiency of the technology. Recent advances in site-specific nuclease technology allows for a more directed approach to introducing mutations into the mouse genome, with the significant advantage of introducing multiple mutations that are not restricted to the genetic background of the embryonic stem cell. This is potentially very interesting from the standpoint of genetic modifiers, as a single transcription activator-like effector nuclease, zinc finger or clustered regularly interspaced short palindromic repeat could be used to introduce mutations in multiple genetic backgrounds.
Leveraging Technologies to Explore Genetic Modifiers
Many neurological diseases present with tremendous clinical heterogeneity. For example, some ALS patients can progress very rapidly in their disease, while others experience only weakness for more extended periods of time. In addition to variation in the age of onset, ALS patients may present with either bulbar or limb onset, and may even exhibit tremendous variation in the sequence of limbs affected and how the disease progresses. Given its high sporadic incidence, ALS is considered to be a complex disease that is influenced by many environmental triggers and likely many genetic susceptibility and resistance loci.
The SOD1G93A mouse model has been the premier model for preclinical testing to predict efficacy in ALS clinical trials. However, much of the preclinical work carried out in the SOD1G93A model has failed to translate to the clinic. This failure to translate is often attributed to poor preclinical trial design, as revealed in sobering and impactful publications by Benatar and Scott et al. However, even with proper preclinical trial designs, are we expecting too much from a single model? SOD1 mutations represent such a small proportion of the ALS patient population and are also likely under-represented in clinical trials, if they are represented at all. Is it, therefore, unrealistic to expect a single gene mutation on a single genetic mouse background to represent a diverse patient population of such a complex disease? Indeed, the field of ALS research has been hindered by a lack of animal models; validation across numerous models would increase the confidence that preclinical work will translate to human trials.
Today, advances in next-generation sequencing technologies and massive reductions in sequencing costs have generated a wealth of data that are revealing the genetic basis for many human neurodegenerative diseases. This is an especially exciting time for gene discoveries in ALS. With new gene discoveries comes a new opportunity to engineer new mouse models. However, getting the most out of these mouse models will also depend on how we leverage both sequencing and new engineering technologies in order to explore modifier genes in mice. It is known that genetic modifiers exist and can significantly influence disease onset in the SOD1G93A model. Male mice in this model on a B6SJL genetic background survive to 129 ± 9 days (mean ± standard deviation). On a C57BL/6J genetic background, male survival is significantly extended to 157 ± 9 days (mean ± standard deviation). Other genetic backgrounds have also been shown to influence survival in these mice, and gene identification is underway. Recent studies also demonstrated that survival of the transgenic mouse (Prnp-TARDBP*A315T)95Balo/J was also influenced by genetic background. In a dominant modifier screen, male survival was significantly different across a number of F1 genetic crosses and, in addition, males were always significantly more affected than females, regardless of genetic background, giving this mouse model both gender-specific and genetic background-specific influences. Advances in high-throughput exome sequencing will significantly enhance our ability to identify these genetic modifiers, even in complex genetic interactions.
New technologies in genome editing with site-specific nucleases will also play a powerful role in our ability to create and validate new disease genes and allelic variants associated with human ALS. Although homologous recombination proved to be a powerful tool in genetic engineering, the creation of the models through construct design and traditional targeting is often hindered by the low efficiency of the technology. Recent advances in site-specific nuclease technology allows for a more directed approach to introducing mutations into the mouse genome, with the significant advantage of introducing multiple mutations that are not restricted to the genetic background of the embryonic stem cell. This is potentially very interesting from the standpoint of genetic modifiers, as a single transcription activator-like effector nuclease, zinc finger or clustered regularly interspaced short palindromic repeat could be used to introduce mutations in multiple genetic backgrounds.
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