High Depth Exome Sequencing on DNA From a Salivary Sample by Mouth Smear.

Despite technological advances, a genetic etiology has been identified in only about 50% to 60% of patients with Neurodevelopmental disorders (NDDs), with a higher diagnostic yield in the syndromic NDD and IDD subgroups. However, identifying a precise etiological diagnosis is essential to optimize patient care, clarify their prognosis, consider targeted therapies, refer families to appropriate resources and support, and provide genetic counseling to relatives. The tests typically offered as part of the etiological assessment of syndromic NDDs and IDD include DNA microarray analysis, testing for fragile X syndrome and genome sequencing from a blood sample. When this assessment remains negative, the cause usually remains unknown.

Mosaic genomic abnormalities (or post-zygotic variations) are a common cause of negative results in current diagnostic genetic tests and represent a field of research that has yet to be fully explored outside of skin disorders. Identifying mosaic genomic abnormalities remains technically complex due to the difficulty of detecting low levels of mosaicism and limited access to the tissue of interest when the variation is absent from blood tissue.

High-depth exome sequencing is the technique of choice for detecting low levels of mosaicism. In the case of NNDs, as the affected tissue is not available, the buccal epithelium is an interesting alternative to blood, as it is easily accessible and inexpensive.

The objective of our study is to evaluate the diagnostic yield of high-depth exome sequencing technology on a DNA extracted from a buccal swab in the etiological assessment of patients with IDD or syndromic NDD whose reference analysis (genome sequencing on blood) proved inconclusive.