Identification of a novel CYP26A1 mutation in a Chinese family with congenital microtia

A Pragmatic Workflow for Human Relevance Assessment of Toxicological Pathways and Associated New Approach Methodologies

Currently, safety assessments of chemical substances are predominantly based on animal data. Multiple considerations call for the use of alternative testing strategies that are based on new approach methodologies (NAMs). However, the human relevance of these testing strategies is usually uncertain. This necessitates a harmonized and accepted workflow for assessing their applicability for regulatory purposes. This report proposes such a workflow, applicable for assessing the human relevance of a toxicological pathway and the relevance of NAMs related to the different components of the pathway. The workflow starts with an established toxicological pathway, of which the adverse outcome is relevant for human health risk assessment and that has sufficient weight of evidence. Human relevance is assessed through three main questions, related to the different components (steps) of the pathway, the pathology of human syndromes that have a similar adverse outcome, and quantitative aspects. The latter comprise both interspecies differences and in vitro - in vivo differences. The combined evidence is scored as 'strong', 'moderate' or 'weak' support of human relevance, based on expert judgement. The workflow developed was tested in a case study, through application to an AOP describing craniofacial malformations after in utero exposure to triazoles. Based on evidence collected for two of the three main questions, the case study provided moderate to strong support for human relevance of both the various components of the AOP and its associated NAMs. Furthermore, it demonstrated that the workflow is a promising approach that allows for a more transparent scientific evaluation of human relevance of toxicological pathways and associated NAMs. Therefore, despite some areas for improvement, we consider the workflow an important step forward for application of AOPs and related NAMs in human health risk assessment.

An innovative CRISPR/Cas9 mouse model of human isolated microtia indicates the potential contribution of CNVs near HMX1 gene

BACKGROUND

Microtia is a prevalent congenital malformation, the precise etiology and pathogenesis of which remain elusive. Mutations in the non-coding region of the HMX1 gene have been implicated in isolated cases of microtia, emerging as a significant focus of contemporary research. Several pathogenic copy number variations (CNVs) proximal to the HMX1 gene have been documented in wild animal populations, whereas only a single large segmental duplication in this region has been identified in humans. However, the absence of a gene-edited animal model has impeded the investigation of the unclear gene function associated with HMX1 mutations in human isolated microtia. In this study, we sought to precisely identify the pathogenic mutation by analyzing three pedigrees alongside population controls. Subsequently, our objective was to develop a CRISPR/Cas9 gene-edited mouse model to elucidate the functional implications of the identified mutation.

METHODS

Genomic DNA was collected from 32 affected individuals across three pedigrees, as well as from 2000 control subjects. Comprehensive genomic analyses, including genome-wide linkage analysis, targeted capture, second-generation sequencing, and copy number analysis, were conducted to identify potential mutations associated with congenital auricle malformation. CRISPR/Cas9 gene-edited murine models were generated in response to the identified mutation. The auricular phenotypes of these gene-edited mice were systematically monitored. Small-animal Micro-CT scanning was employed to identify potential craniofacial or skeletal abnormalities. Furthermore, the expression of the HMX1 gene in the PA2 region of mouse embryos was quantified using RT-qPCR.

RESULTS

A co-segregated 600 base pair duplication located on chromosome 4 (chr4:8701900-8702500, hg19) was identified in affected individuals across three pedigrees, but was absent in healthy controls. Two types of CRISPR/Cas9 gene-edited mice were subsequently generated. The knock-in (KI) mouse model was engineered by inserting one copy of the duplicated sequence directly adjacent to the mutated site, whereas the knockout (KO) mouse model was created by excising the mutation sequence. The phenotypes of different group of CRISPR/Cas9 gene-edited mice demonstrated distinct auricular deformities. Furthermore, an increase in the copy number of the mutated sequence was associated with elevated expression levels of HMX1 in the gene-edited mouse model.

CONCLUSIONS

In this study, we further narrowed down and identified a 600 base pair copy number variation (CNV) located at chr4:8701900-8702500 (hg19), which is implicated in human bilateral, isolated microtia. Utilizing CRISPR/Cas9 technology, we developed novel mouse models harboring the identified mutation. These models serve as a robust platform for the comprehensive investigation of the underlying mechanisms of the disease.

Multiple roles for retinoid signaling in craniofacial development

Retinoic acid (RA) signaling plays multiple essential roles in development of the head and face. Animal models with mutations in genes involved in RA signaling have enabled understanding of craniofacial morphogenic processes that are regulated by the retinoid pathway. During craniofacial morphogenesis RA signaling is active in spatially restricted domains defined by the expression of genes involved in RA production and RA breakdown. The spatial distribution of RA signaling changes with progressive development, corresponding to a multiplicity of craniofacial developmental processes that are regulated by RA. One important role of RA signaling occurs in the hindbrain. There RA contributes to specification of the anterior-posterior (AP) axis of the developing CNS and to the neural crest cells (NCC) which form the bones and nerves of the face and pharyngeal region. In the optic vesicles and frontonasal process RA orchestrates development of the midface, eyes, and nasal airway. Additional roles for RA in craniofacial development include regulation of submandibular salivary gland development and maintaining patency in the sutures of the cranial vault.