Rationale: Although children with primary ciliary dyskinesia (PCD) typically have low nasal nitric oxide (nNO), some children with indisputable PCD may have unexplained high nNO concentrations. Objectives: To look for relationships between nNO measures and genetic findings (and cilia motility or ultrastructure when available) in children with PCD with known genotypes. Methods: We retrospectively studied 73 children with PCD (median age, 9.5 [range, 2.1-18.2] yr). nNO was the mean value of a plateau reached while the velum was closed (nNO-VC; threshold, 77 nl ⋅ min^-1) or was calculated as the average of five peaks obtained during tidal breathing (threshold, 40 nl ⋅ min^-1). Ciliary beat was classified as either motile (including dyskinetic pattern) or immotile, depending on whether motility was present or absent in all cilia, or as a mixture of motile and immotile cilia. Genotypes were classified as pathogenic mutations in genes known to be associated with high nNO (mild genotype), biallelic truncating mutations in other genes (severe mutations), or putative hypomorphic pathogenic mutations (missense, single amino acid deletion, or moderate splicing mutations) in at least one allele believed to be possibly associated with residual production of a functional protein. Results: nNO was above the discriminant threshold in 16 of 73 (21.9%) children (11 nNO-VC and 5 nNO during tidal breathing). High nNO was less frequent in children with severe mutations (2 of 42) than in those with mild genotypes (7 of 10) or at least one hypomorphic mutation (7 of 21) (P < 0.0001). Median (interquartile range) nNO-VC values (n = 60) were significantly different in the three genotype groups: severe mutations, 18 (10-26) nl ⋅ min^-1 (n = 36); possible residual functional protein production (putative hypomorphic mutations), 23 (16-68) nl ⋅ min^-1 (n = 17); and mild genotypes, 139 (57-216) nl ⋅ min^-1 (n = 7) (P = 0.0002). The higher the cilia motility, the higher the nNO-VC (16 [10-23], 23 [17-56], and 78 [45-93] nl ⋅ min^-1 in patients with immotile, dyskinetic motile and immotile, and dyskinetic motile cilia, respectively; P < 0.0001), while nNO values were scattered across different ultrastructure defects (P = 0.07). Conclusions: In children with PCD, high nNO values were linked not only to specific genes but also to potentially hypomorphic mutations in other genes (with possible functional protein production). nNO values increased with the proportion of motile cilia.

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