Comment: Genotype–phenotype correlation with CRISPR-Cas9—

Technological improvements and decreasing costs have led to increased use of next-generation sequencing as an a priori approach to clinical diagnosis. This approach lends itself to important discoveries of novel genotypic etiologies and phenotypic associations.

In the current report, the authors present a case of congenital arthrogryposis in an infant with a de novo missense mutation in the NALCN gene identified with whole-exome sequencing.¹ Two groups originally reported NALCN mutations in 2013 in association with congenital contractures of the limbs and face with hypotonia and developmental delay (CLIFAHDD) syndrome.^2,3 In contrast to earlier reports, Bend et al. describe in their patient the clinical electrophysiologic features of peripheral motor system hyperexcitability, thus expanding the phenotypic spectrum of NALCN-related disorders.

These findings are further investigated by probing the functional consequences of the orthologous missense NALCN mutation from their patient, as well as other previously reported NALCN mutations, using the CRISPR-Cas9 system in the model organism Caenorhabditis elegans. Consistent with their patient's clinical features of peripheral motor system overactivity, the authors nicely demonstrate in C elegans a gain of function as a consequence of the patient's mutation. Furthermore, other mutations previously reported in association with CLIFAHDD also had either loss- or gain-of-function consequences.

The authors' approach is an excellent example of how to use the CRISPR-Cas9 in a model system to investigate the functional consequences of missense mutations. The authors' use of motor behavior of C elegans as a straightforward readout and the conserved nature of the NALCN gene makes the studies more easily interpreted. However, this paradigm may be less suited to the study of other disorders with more complex phenotypic–genotypic relationships, or in the study of less well conserved genes.