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

## TL;DR

This study shows how DAW1 gene variants cause specific developmental issues in some tissues but not others, helping explain why some patients have heterotaxy and heart defects without ciliary problems.

## Contribution

The study provides functional evidence for tissue-specific roles of DAW1 variants in heterotaxy and supports reclassification of variants under ACMG guidelines.

## Key findings

- DAW1 variants cause left–right patterning and cardiac defects but not primary ciliary dyskinesia.
- The p.Arg114Gln variant rescues mucociliary flow but not left–right patterning.
- The splice-site variant leads to complete loss of DAW1 function in all tested contexts.

## Abstract

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 HTX and complex congenital heart disease but no clinical signs of PCD. Whole-genome sequencing revealed a maternally inherited canonical splice-site variant (c.648 + 1G > A) and a paternally inherited missense variant (c.341G > A; p.Arg114Gln), 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 fully 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 + 1G > A as 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.