No clear monogenic links between left-handedness and situs inversus

Left-handedness is a complex trait which might sometimes involve rare, monogenic contributions. Situs inversus (SI) of the visceral organs can occur with Primary Ciliary Dyskinesia (PCD), due to mutations which affect left-right axis formation. Roughly 10% of people with SI and PCD are left-handed, similar to the general population. However, in non-PCD SI, the rate of left-handedness may be elevated. We sequenced the genomes of nine non-PCD SI people who show an elevated rate of left-handedness (five out of nine). We also sequenced six SI people with PCD as positive controls, and fifteen unaffected people as technical controls. Recessive mutations in known PCD genes were found in all positive controls with PCD. Of the nine non-PCD SI cases, two had recessive mutations in known PCD genes, suggesting reduced penetrance for PCD, and one had a recessive mutation in the non-PCD laterality gene PKD1L1. However, six of the nine non-PCD SI cases, including most of the left-handers, had no mutations in likely candidate genes, nor any significant biological pathways affected by their mutations. Therefore we did not identify a molecular link between visceral and brain laterality. While we cannot exclude a monogenic basis for non-PCD SI with left-handedness, multifactorial and non-genetic models must also be considered.