Testing the Cre-mediated genetic switch for the generation of conditional knock-in mice

Phox2b-expressing neurons contribute to breathing problems in Kcnq2 loss- and gain-of-function encephalopathy models

Loss- and gain-of-function variants in the gene encoding KCNQ2 channels are a common cause of developmental and epileptic encephalopathy, a condition characterized by seizures, developmental delays, breathing problems, and early mortality. To understand how KCNQ2 dysfunction impacts behavior in a mouse model, we focus on the control of breathing by neurons expressing the transcription factor Phox2b which includes respiratory neurons in the ventral parafacial region. We find Phox2b-expressing ventral parafacial neurons express Kcnq2 in the absence of other Kcnq isoforms, thus clarifying why disruption of Kcnq2 but not other channel isoforms results in breathing problems. We also find that Kcnq2 deletion or expression of a recurrent gain-of-function variant R201C in Phox2b-expressing neurons increases baseline breathing or decreases the central chemoreflex, respectively, in mice during the light/inactive state. These results uncover mechanisms underlying breathing abnormalities in KCNQ2 encephalopathy and highlight an unappreciated vulnerability of Phox2b-expressing ventral parafacial neurons to KCNQ2 pathogenic variants.

Genetic and Molecular Quality Control of Genetically Engineered Mice

Genetically engineered mice are used as avatars to understand mammalian gene function and develop therapies for human disease. During genetic modification, unintended changes can occur, and these changes may result in misassigned gene-phenotype relationships leading to incorrect or incomplete experimental interpretations. The types of unintended changes that may occur depend on the allele type being made and the genetic engineering approach used. Here we broadly categorize allele types as deletions, insertions, base changes, and transgenes derived from engineered embryonic stem (ES) cells or edited mouse embryos. However, the methods we describe can be adapted to other allele types and engineering strategies. We describe the sources and consequ ences of common unintended changes and best practices for detecting both intended and unintended changes by screening and genetic and molecular quality control (QC) of chimeras, founders, and their progeny. Employing these practices, along with careful allele design and good colony management, will increase the chance that investigations using genetically engineered mice will produce high-quality reproducible results, to enable a robust understanding of gene function, human disease etiology, and therapeutic development.

Conditional switching of KIF2A mutation provides new insights into cortical malformation pathogeny

By using the Cre-mediated genetic switch technology, we were able to successfully generate a conditional knock-in mouse, bearing the KIF2A p.His321Asp missense point variant, identified in a subject with malformations of cortical development. These mice present with neuroanatomical anomalies and microcephaly associated with behavioral deficiencies and susceptibility to epilepsy, correlating with the described human phenotype. Using the flexibility of this model, we investigated RosaCre-, NestinCre- and NexCre-driven expression of the mutation to dissect the pathophysiological mechanisms underlying neurodevelopmental cortical abnormalities. We show that the expression of the p.His321Asp pathogenic variant increases apoptosis and causes abnormal multipolar to bipolar transition in newborn neurons, providing therefore insights to better understand cortical organization and brain growth defects that characterize KIF2A-related human disorders. We further demonstrate that the observed cellular phenotypes are likely to be linked to deficiency in the microtubule depolymerizing function of KIF2A.

Droplet digital PCR or quantitative PCR for in-depth genomic and functional validation of genetically altered rodents

Gene targeting and additive (random) transgenesis have proven to be powerful technologies with which to decipher the mammalian genome. With the advent of CRISPR/Cas9 genome editing, the ability to inactivate or modify the function of a gene has become even more accessible. However, the impact of each generated modification may be different from what was initially desired. Minimal validation of mutant alleles from genetically altered (GA) rodents remains essential to guarantee the interpretation of experimental results. The protocol described here combines design strategies for genomic and functional validation of genetically modified alleles with droplet digital PCR (ddPCR) or quantitative PCR (qPCR) for target DNA or mRNA quantification. In-depth analysis of the results obtained with GA models through the analysis of target DNA and mRNA quantification is also provided, to evaluate which pitfalls can be detected using these two methods, and we propose recommendations for the characterization of different type of mutant allele (knock-out, knock-in, conditional knock-out, FLEx, IKMC model or transgenic). Our results also highlight the possibility that mRNA expression of any mutated allele can be different from what might be expected in theory or according to common assumptions. For example, mRNA analyses on knock-out lines showed that nonsense-mediated mRNA decay is generally not achieved with a critical-exon approach. Likewise, comparison of multiple conditional lines crossed with the same CreER^T2 deleter showed that the inactivation outcome was very different for each conditional model. DNA quantification by ddPCR of G0 to G2 generations of transgenic rodents generated by pronuclear injection showed an unexpected variability, demonstrating that G1 generation rodents cannot be considered as established lines.

Sulfation pathways from red to green

Sulfur is present in the amino acids cysteine and methionine and in a large range of essential coenzymes and cofactors and is therefore essential for all organisms. It is also a constituent of sulfate esters in proteins, carbohydrates, and numerous cellular metabolites. The sulfation and desulfation reactions modifying a variety of different substrates are commonly known as sulfation pathways. Although relatively little is known about the function of most sulfated metabolites, the synthesis of activated sulfate used in sulfation pathways is essential in both animal and plant kingdoms. In humans, mutations in the genes encoding the sulfation pathway enzymes underlie a number of developmental aberrations, and in flies and worms, their loss-of-function is fatal. In plants, a lower capacity for synthesizing activated sulfate for sulfation reactions results in dwarfism, and a complete loss of activated sulfate synthesis is also lethal. Here, we review the similarities and differences in sulfation pathways and associated processes in animals and plants, and we point out how they diverge from bacteria and yeast. We highlight the open questions concerning localization, regulation, and importance of sulfation pathways in both kingdoms and the ways in which findings from these "red" and "green" experimental systems may help reciprocally address questions specific to each of the systems.