Biallelic DAW1 variants reveal a tissue-specific role in heterotaxy without primary ciliary dyskinesia

Defects in motile cilia cause a range of disorders, including heterotaxy (HTX), congenital heart disease (CHD), and primary ciliary dyskinesia (PCD). Although these conditions often co-occur, the genetic and mechanistic bases for tissue-specific manifestations remain poorly understood. Here, we identify compound heterozygous variants in DAW1, a dynein arm assembly factor, in a proband with an HTX/laterality defect spectrum, presenting with complex CHD without extracardiac organ malposition and no evidence of PCD. Whole-genome sequencing revealed a maternally inherited canonical splice-site variant (c.648+1 G > A, chr2:227,903,110 G > A (GRCh38)) and a paternally inherited missense variant (c.341 G > A; p.Arg114Gln, chr2: 227,893,818 G > A (GRCh38)), both classified as variants of uncertain significance under ACMG/AMP guidelines. Using Xenopus tropicalis, we show that Daw1 depletion disrupts left-right patterning, cardiac looping, and mucociliary flow, all of which are rescued by wild-type human DAW1. Functional testing of patient alleles showed notable tissue specificity: p.Arg114Gln rescued mucociliary flow but did not restore left-right patterning, while the splice-site variant resulted in a complete loss of function in both contexts. These findings closely match the proband's clinical phenotype and provide strong functional evidence to support reclassifying c.648+1 G > A as likely pathogenic and p.Arg114Gln as a context-dependent hypomorphic allele. This study establishes functional criteria for interpreting DAW1 variants, shows how developmental context clarifies genotype-phenotype relationships, and highlights how in vivo models can support ACMG reclassification of unresolved HTX-related variants.