Background Although splicing is an integral part of the expression of many genes in our body, genetic syndromes with spliceosomal defects affect only specific tissues. To help understand the mechanism, we investigated the expression pattern of a core protein of the major spliceosome, SmB/B′ (Small Nuclear Ribonucleoprotein Polypeptides B/B′), which is encoded by SNRPB. Loss‐of‐function mutations of SNRPB in humans cause cerebro‐costo‐mandibular syndrome (CCMS) characterized by rib gaps, micrognathia, cleft palate, and scoliosis. Our expression analysis focused on the affected structures as well as non‐affected tissues, using chick and mouse embryos as model animals. Results Embryos at young stages (gastrula) showed ubiquitous expression of SmB/B′. However, the level and pattern of expression became tissue‐specific as differentiation proceeded. The regions relating to CCMS phenotypes such as cartilages of ribs and vertebrae and palatal mesenchyme express SmB/B′ in the nucleus sporadically. However, cartilages that are not affected in CCMS also showed similar expressions. Another spliceosomal gene, SNRNP200, which mutations cause retinitis pigmentosa, was also prominently expressed in cartilages in addition to the retina. Conclusion The expression of SmB/B′ is spatiotemporally regulated during embryogenesis despite the ubiquitous requirement of the spliceosome, however, the expression pattern is not strictly correlated with the phenotype presentation.

Transverse section of a mouse embryo at 13.5 dpc stained for SmB (red) and nuclei (blue), dorsal to the left. The ventral neural tube is seen on the left side of the panel, and cartilaginous primordia of the vertebra and rib are developing. SmB is one of the core proteins of the major spliceosome components. Although splicing is ubiquitously required in all cells, the expression of SmB is found to be spatiotemporally restricted; From: Non‐ubiquitous expression of core spliceosomal protein SmB/B′ in chick and mouse embryos; Turner BRH, Mellor C, McElroy C, Bowen N, Gu W, Knill C, Itasaki N. Developmental Dynamics. 252(1):276‐293. https://doi.org/10.1002/dvdy.537

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Although gene splicing occurs throughout the body, the phenotype of spliceosomal defects is largely limited to specific tissues. Cerebro‐Costo‐Mandibular syndrome (CCMS) is one such spliceosomal disease, which presents as congenital skeletal dysmorphism and is caused by mutations of SNRPB gene encoding Small Nuclear Ribonucleoprotein Polypeptides B/B’ (SmB/B’). This study employed in vitro cell cultures to monitor osteo‐ and chondro‐differentiation and examined the role of SmB/B’ in the differentiation process. We found that low levels of SmB/B’ by knockdown or mutations of SNRPB led to suppressed osteodifferentiation in Saos‐2 osteoprogenitor‐like cells, which was accompanied by affected splicing of Dlx5. On the other hand, low SmB/B’ led to promoted chondrogenesis in HEPM mesenchymal stem cells. Consistent with other reports, osteogenesis was promoted by the Wnt/β‐catenin pathway activator and suppressed by Wnt and BMP blockers, whereas chondrogenesis was promoted by Wnt inhibitors. Suppressed osteogenic markers by SNRPB knockdown were partly rescued by Wnt/β‐catenin pathway activation. Reporter analysis revealed that suppression of SNRPB results in attenuated Wnt pathway and/or enhanced BMP pathway activities. SNRPB knockdown altered splicing of TCF7L2 which impacts Wnt/β‐catenin pathway activities. This work helps unravel the mechanism underlying CCMS whereby reduced expression of spliceosomal proteins causes skeletal phenotypes.