<i>De novo</i> variants in <i>LDB1</i> are linked to distinct neurodevelopmental phenotypes determined by variant location and differing pathomechanisms

Abstract LDB1 encodes transcriptional regulator protein LIM-domain-binding protein 1, which plays an important role in neurogenesis. Few C-terminal likely gene disrupting (LGD) variants have been reported in the literature in individuals with congenital ventriculomegaly. Through international collaboration, we now assembled a cohort of 16 individuals with de novo variants affecting various regions of LDB1. Eleven variants affect either the whole gene or the N-terminal dimerization domain (including gene deletions, NMD-sensitive LGD-, and missense variants) and five variants (missense or NMD-escaping LGD variants) affect only the C-terminus of LDB1 containing the LIM interaction domain. All individuals showed variable neurodevelopmental phenotypes, including developmental delay and behavioral anomalies. In line with literature reports, individuals harboring C-terminal variants additionally presented with ventriculomegaly, establishing a genotype-phenotype correlation. In accordance, we found diverging pathomechanisms in vitro : N-terminal missense variants disrupt homodimerization of LDB1, likely leading to a loss of function, while C-terminal variants impair interaction with the essential partner LHX2 in a dominant-negative manner. These findings were confirmed in vivo in Drosophila melanogaster . Toxicity of overexpressed human LDB1 in Drosophila was not observed with N-terminal missense variants but was exacerbated by C-terminal variants. Similarly, phenotypes associated with LDB1/chi loss were rescued by overexpression of wild-type LDB1, but neither by LDB1 harboring N-terminal missense variants nor by C-terminal variants that even worsened phenotypes. In summary, our findings link de novo variants in LDB1 to two overlapping, but distinct neurodevelopmental phenotypes based on variant location, and highlight two separate pathomechanisms underlying LDB1 -related neurodevelopmental disorders.