Cohesin complex-associated holoprosencephaly

Abstract

Marked by incomplete division of the embryonic forebrain, holoprosencephaly is one of the most common human developmental disorders. Despite decades of phenotype-driven research, 80-90% of aneuploidy-negative holoprosencephaly individuals with a probable genetic aetiology do not have a genetic diagnosis. Here we report holoprosencephaly associated with variants in the two X-linked cohesin complex genes, STAG2 and SMC1A, with loss-of-function variants in 10 individuals and a missense variant in one. Additionally, we report four individuals with variants in the cohesin complex genes that are not X-linked, SMC3 and RAD21. Using whole mount in situ hybridization, we show that STAG2 and SMC1A are expressed in the prosencephalic neural folds during primary neurulation in the mouse, consistent with forebrain morphogenesis and holoprosencephaly pathogenesis. Finally, we found that shRNA knockdown of STAG2 and SMC1A causes aberrant expression of HPE-associated genes ZIC2, GLI2, SMAD3 and FGFR1 in human neural stem cells. These findings show the cohesin complex as an important regulator of median forebrain development and X-linked inheritance patterns in holoprosencephaly.

Keywords: X-linked inheritance; cohesin complex; forebrain division; holoprosencephaly.

Figure 1

Patient images. (A) Patient 3 with alobar HPE and a c.436C>T p.(Arg146*) variant in STAG2; (B) Patient 5 with microform HPE and a c.2898_2899del p.(Glu968Serfs*15) in STAG2; (C) Patient 2 with semi-lobar HPE and a c.205C>T p.(Arg69*) variant in STAG2; (D) Patient 9 with semi-lobar HPE and a c.2683C>G p.( Arg895Gly) variant in SMC1A; (E) Patient 7 with middle interhemispheric variant HPE and a c.3285+1G>C variant in SMC1A; (F) Patient 8 with microform HPE and a c.1495C>T p.(Arg499*) variant in SMC1A; (G) Patient 15 with semi-lobar HPE and an SMC3 variant c.1138_1152del p.(Gly380_Gln384del). See Table 1 for further details.

Figure 2

Gestational day (GD) 8.25 mouse embryos were stained by in situ hybridization to determine gene expression patterns. A ventral view is shown for whole mounts. Transverse sections through the prosencephalic neural folds (at the level of the dashed line in schematic) were stained to visualize gene expression in specific cellular compartments. eem = extra-embryonic membranes; h = heart; hm = head mesenchyme; ne = neuroectoderm; nf = neural folds. Scale bar = 100 µm.