A missense variant in ASCL5 leads to lobodontia

Lobodontia, a rare dental anomaly marked by supernumerary cusps and a single pyramid-shaped molar root, has been previously linked to a variant in the CACNA1S gene without definitive evidence. This study investigates 17 patients with lobodontia from Thai and Croatian families. Microsatellite genotyping defines a 15.4 Mbp critical region encompassing CACNA1S and ASCL5 among Thai families. While genome sequencing confirms the CACNA1S variant only in the Thai patients, all 17 patients harbor the ASCL5 c.274 G > A (p.Glu92Lys) variant, which is absent in 12 unaffected members. Functional studies using CRISPR/Cas9-generated Ascl5 knock-in mutant mice demonstrate the dental anomalies resembling lobodontia in Ascl5^Mut/WT, while Ascl5^Mut/Mut display severe defects in tooth and jaw development, underscoring the essential role of ASCL5 in craniofacial patterning. Transcriptomic analysis of E17.5 mandibular dental arches reveals differential expression of key craniofacial developmental genes in Ascl5^Mut/Mut compared to Ascl5^WT/WT, including Dlx1as, and Dlx2. Luciferase assay shows that the p.Glu92Lys ASCL5 impairs DLX2 activation, further supporting the variant's pathogenicity. This study establishes ASCL5 as the gene responsible for lobodontia, revising its previously understood genetic basis, and highlights its crucial role in craniofacial development.