Aberrant regulation of a poison exon caused by a non-coding variant in a mouse model of Scn1a-associated epileptic encephalopathy.

Dravet syndrome (DS) is a developmental and epileptic encephalopathy that results from mutations in the Nav1.1 sodium channel encoded by SCN1A. Most known DS-causing mutations are in coding regions of SCN1A, but we recently identified several disease-associated SCN1A mutations in intron 20 that are within or near to a cryptic and evolutionarily conserved "poison" exon, 20N, whose inclusion is predicted to lead to transcript degradation. However, it is not clear how these intron 20 variants alter SCN1A expression or DS pathophysiology in an organismal context, nor is it clear how exon 20N is regulated in a tissue-specific and developmental context. We address those questions here by generating an animal model of our index case, NM_006920.4(SCN1A):c.3969+2451G>C, using gene editing to create the orthologous mutation in laboratory mice. Scn1a heterozygous knock-in (+/KI) mice exhibited an ~50% reduction in brain Scn1a mRNA and Nav1.1 protein levels, together with characteristics observed in other DS mouse models, including premature mortality, seizures, and hyperactivity. In brain tissue from adult Scn1a +/+ animals, quantitative RT-PCR assays indicated that ~1% of Scn1a mRNA included exon 20N, while brain tissue from Scn1a +/KI mice exhibited an ~5-fold increase in the extent of exon 20N inclusion. We investigated the extent of exon 20N inclusion in brain during normal fetal development in RNA-seq data and discovered that levels of inclusion were ~70% at E14.5, declining progressively to ~10% postnatally. A similar pattern exists for the homologous sodium channel Nav1.6, encoded by Scn8a. For both genes, there is an inverse relationship between the level of functional transcript and the extent of poison exon inclusion. Taken together, our findings suggest that poison exon usage by Scn1a and Scn8a is a strategy to regulate channel expression during normal brain development, and that mutations recapitulating a fetal-like pattern of splicing cause reduced channel expression and epileptic encephalopathy.

Improved Oviduct Transfer Surgery for Genetically Modified Rat Production.

Rat embryo transfer surgeries are becoming more common with targeted nucleases increasing the demand for rat models. This protocol details pre-surgical preparation, improved surgical techniques for placing embryos into the oviduct, and post-surgical care of rats to parturition. Direct application of mouse oviduct transfer protocols results in limited success in the rat. By combining techniques from several widely used protocols in the field, increased yield of live pups born to healthy dams is possible. This protocol is distinct from previously published protocols by the use of a modified anesthesia protocol (Smith et al., 2004), use of analgesia, the addition of peritoneal sutures (Filipiak and Saunders, 2006), incision location and number of transfers per animal (Krinke et al., 2000).

Generation of Mitochondrial-nuclear eXchange Mice via Pronuclear Transfer.

The mitochondrial paradigm for common disease proposes that mitochondrial DNA (mtDNA) sequence variation can contribute to disease susceptibility and progression. To test this concept, we developed the Mitochondrial-nuclear eXchange (MNX) model, in which isolated embryonic pronuclei from one strain of species are implanted into an enucleated embryo of a different strain of the same species (e.g., C57BL/6 and C3H/HeN, Mus musculus), generating a re-constructed zygote harboring nuclear and mitochondrial genomes from different strains. Two-cell embryos are transferred to the ostia of oviducts in CD-1 pseudopregnant mice and developed to term. Nuclear genotype and mtDNA haplotype are verified in offspring, and females selected as founders for desired MNX colonies. By utilizing MNX models, many new avenues for the in vivo study for mitochondrial and nuclear genetics, or mito-Mendelian genetics, are now possible.

Increased trabecular bone and improved biomechanics in an osteocalcin-null rat model created by CRISPR/Cas9 technology.

Osteocalcin, also known as bone γ-carboxyglutamate protein (Bglap), is expressed by osteoblasts and is commonly used as a clinical marker of bone turnover. A mouse model of osteocalcin deficiency has implicated osteocalcin as a mediator of changes to the skeleton, endocrine system, reproductive organs and central nervous system. However, differences between mouse and human osteocalcin at both the genome and protein levels have challenged the validity of extrapolating findings from the osteocalcin-deficient mouse model to human disease. The rat osteocalcin (Bglap) gene locus shares greater synteny with that of humans. To further examine the role of osteocalcin in disease, we created a rat model with complete loss of osteocalcin using the CRISPR/Cas9 system. Rat osteocalcin was modified by injection of CRISPR/Cas9 mRNA into the pronuclei of fertilized single cell Sprague-Dawley embryos, and animals were bred to homozygosity and compound heterozygosity for the mutant alleles. Dual-energy X-ray absorptiometry (DXA), glucose tolerance testing (GTT), insulin tolerance testing (ITT), microcomputed tomography (µCT), and a three-point break biomechanical assay were performed on the excised femurs at 5 months of age. Complete loss of osteocalcin resulted in bones with significantly increased trabecular thickness, density and volume. Cortical bone volume and density were not increased in null animals. The bones had improved functional quality as evidenced by an increase in failure load during the biomechanical stress assay. Differences in glucose homeostasis were observed between groups, but there were no differences in body weight or composition. This rat model of complete loss of osteocalcin provides a platform for further understanding the role of osteocalcin in disease, and it is a novel model of increased bone formation with potential utility in osteoporosis and osteoarthritis research.