International Cooperation to Enable the Diagnosis of All Rare Genetic Diseases.

Provision of a molecularly confirmed diagnosis in a timely manner for children and adults with rare genetic diseases shortens their "diagnostic odyssey," improves disease management, and fosters genetic counseling with respect to recurrence risks while assuring reproductive choices. In a general clinical genetics setting, the current diagnostic rate is approximately 50%, but for those who do not receive a molecular diagnosis after the initial genetics evaluation, that rate is much lower. Diagnostic success for these more challenging affected individuals depends to a large extent on progress in the discovery of genes associated with, and mechanisms underlying, rare diseases. Thus, continued research is required for moving toward a more complete catalog of disease-related genes and variants. The International Rare Diseases Research Consortium (IRDiRC) was established in 2011 to bring together researchers and organizations invested in rare disease research to develop a means of achieving molecular diagnosis for all rare diseases. Here, we review the current and future bottlenecks to gene discovery and suggest strategies for enabling progress in this regard. Each successful discovery will define potential diagnostic, preventive, and therapeutic opportunities for the corresponding rare disease, enabling precision medicine for this patient population.

Multiple Slits regulate the development of midline glial populations and the corpus callosum.

The Slit molecules are chemorepulsive ligands that regulate axon guidance at the midline of both vertebrates and invertebrates. In mammals, there are three Slit genes, but only Slit2 has been studied in any detail with regard to mammalian brain commissure formation. Here, we sought to understand the relative contributions that Slit proteins make to the formation of the largest brain commissure, the corpus callosum. Slit ligands bind Robo receptors, and previous studies have shown that Robo1(-/-) mice have defects in corpus callosum development. However, whether the Slit genes signal exclusively through Robo1 during callosal formation is unclear. To investigate this, we compared the development of the corpus callosum in both Slit2(-/-) and Robo1(-/-) mice using diffusion magnetic resonance imaging. This analysis demonstrated similarities in the phenotypes of these mice, but crucially also highlighted subtle differences, particularly with regard to the guidance of post-crossing axons. Analysis of single mutations in Slit family members revealed corpus callosum defects (but not complete agenesis) in 100% of Slit2(-/-) mice and 30% of Slit3(-/-) mice, whereas 100% of Slit1(-/-); Slit2(-/-) mice displayed complete agenesis of the corpus callosum. These results revealed a role for Slit1 in corpus callosum development, and demonstrated that Slit2 was necessary but not sufficient for midline crossing in vivo. However, co-culture experiments utilising Robo1(-/-) tissue versus Slit2 expressing cell blocks demonstrated that Slit2 was sufficient for the guidance activity mediated by Robo1 in pre-crossing neocortical axons. This suggested that Slit1 and Slit3 might also be involved in regulating other mechanisms that allow the corpus callosum to form, such as the establishment of midline glial populations. Investigation of this revealed defects in the development and dorso-ventral positioning of the indusium griseum glia in multiple Slit mutants. These findings indicate that Slits regulate callosal development via both classical chemorepulsive mechanisms, and via a novel role in mediating the correct positioning of midline glial populations. Finally, our data also indicate that some of the roles of Slit proteins at the midline may be independent of Robo signalling, suggestive of additional receptors regulating Slit signalling during development.

Analysis of the growth cone turning assay for studying axon guidance.

The "pipette" or "growth cone turning" assay is widely used for studying how axons respond to diffusible guidance cues in their environment. However, little quantitative analysis has been presented of the gradient shapes produced by this assay, or how they depend on parameters of the assay. Here we used confocal microscopy of fluorescent gradients to characterize these shapes in 3 dimensions. We found that the shape, and more specifically the concentration at the position usually occupied by the growth cone in this assay, varied in sometimes unexpected ways with the molecular weight of the diffusible factor, charge, pulse duration and pulse frequency. These results suggest that direct observation of the gradient of the particular guidance factor under consideration may be necessary to quantitatively determine the signal to which the growth cone is responding.

Autologous adult bone marrow stem cell transplantation in an animal model of huntington's disease: behavioral and morphological outcomes.

We investigated the effects of autologous bone marrow stem cell transplantation in a rat model of Huntington's Disease. Thirteen days after bilateral quinolinic lesions (QA), bone marrow was implanted into the damaged striatum. The ability of the transplants to reverse QA-induced cognitive deficits in the radial-arm water maze (RAWM) was examined. The transplants significantly reduced working memory deficits. Most of the transplanted cells appeared quite primitive. Because only a few cells expressed neural phenotypes, we suggest that the release of growth factors by the transplants allowed surviving cells within the caudate to function more efficiently and to facilitate other compensatory responses.