Synergistic interactions between two skeletal mutations in mice: individual and combined effects of the semidominants cleidocranial dysplasia (Ccd) and short digits (Dsh)

An Exploration of Mutagenesis in a Family with Cleidocranial Dysplasia without RUNX2 Mutation

Cleidocranial dysplasia (CCD) is an autosomal dominant inheritable skeletal disorder characterized by cranial dysplasia, clavicle hypoplasia, and dental abnormalities. Mutations involving Runt-related transcription factor 2 (RUNX2) are currently the only known molecular etiology for CCD but are not identified in all CCD patients. No RUNX2 abnormality can be detected in about 20–30% of patients, and the molecular cause remains unknown. The present study includes a family case with typical features of CCD. RUNX2 mutation was first screened by sequencing analysis, and no mutation was detected. Copy number alterations of the RUNX2 gene were then measured by quantitative PCR and multiplex ligation-dependent probe amplification (MLPA). No copy number variation in RUNX2 could be detected. We performed whole-exome sequencing (WES) to identify the underlying genetic mutations. Unexpectedly, no abnormalities could be detected in genes related to the RUNX2 signaling pathway. Therefore, it was supposed that other new unknown gene variations might contribute to the CCD phenotype. We focused on Immunoglobulin superfamily member 10 (IGSF10), a gene related to bone development. An IGSF10 frameshift mutation (c.6001_6002delCT, p.Leu2001Valfs*24) was detected by WES. Sanger sequencing verified that this mutation was only detected in the patient and her affected mother but not in her unaffected father. Bioinformatics studies demonstrated that this mutation could change the 3D structure of the IGSF10 protein and severely damage its function. In addition, alkaline phosphatase (ALP) activity and the ability to form mineralized nodules were inhibited by IGSF10 knockdown compared with normal controls. The expression of bone sialoprotein (BSP) was significantly reduced by IGSF10 knockdown, but not that of other osteogenic markers. Our results provide new genetic evidence that IGSF10 mutation might contribute to CCD.

An inversion involving the mouse Shh locus results in brachydactyly through dysregulation of Shh expression

Short digits (Dsh) is a radiation-induced mouse mutant. Homozygous mice are characterized by multiple defects strongly resembling those resulting from Sonic hedgehog (Shh) inactivation. Heterozygous mice show a limb reduction phenotype with fusion and shortening of the proximal and middle phalanges in all digits, similar to human brachydactyly type A1, a condition caused by mutations in Indian hedgehog (IHH). We mapped Dsh to chromosome 5 in a region containing Shh and were able to demonstrate an inversion comprising 11.7 Mb. The distal breakpoint is 13.298 kb upstream of Shh, separating the coding sequence from several putative regulatory elements identified by interspecies comparison. The inversion results in almost complete downregulation of Shh expression during E9.5-E12.5, explaining the homozygous phenotype. At E13.5 and E14.5, however, Shh is upregulated in the phalangeal anlagen of Dsh/+ mice, at a time point and in a region where WT Shh is never expressed. The dysregulation of Shh expression causes the local upregulation of hedgehog target genes such as Gli1-3, patched, and Pthlh, as well as the downregulation of Ihh and Gdf5. This results in shortening of the digits through an arrest of chondrocyte differentiation and the disruption of joint development.

An inversion involving the mouse Shh locus results in brachydactyly through dysregulation of Shh expression

Short digits (Dsh) is a radiation-induced mouse mutant. Homozygous mice are characterized by multiple defects strongly resembling those resulting from Sonic hedgehog (Shh) inactivation. Heterozygous mice show a limb reduction phenotype with fusion and shortening of the proximal and middle phalanges in all digits, similar to human brachydactyly type A1, a condition caused by mutations in Indian hedgehog (IHH). We mapped Dsh to chromosome 5 in a region containing Shh and were able to demonstrate an inversion comprising 11.7 Mb. The distal breakpoint is 13.298 kb upstream of Shh, separating the coding sequence from several putative regulatory elements identified by interspecies comparison. The inversion results in almost complete downregulation of Shh expression during E9.5-E12.5, explaining the homozygous phenotype. At E13.5 and E14.5, however, Shh is upregulated in the phalangeal anlagen of Dsh/+ mice, at a time point and in a region where WT Shh is never expressed. The dysregulation of Shh expression causes the local upregulation of hedgehog target genes such as Gli1-3, patched, and Pthlh, as well as the downregulation of Ihh and Gdf5. This results in shortening of the digits through an arrest of chondrocyte differentiation and the disruption of joint development.

Identification of novel CBFA1/RUNX2 mutations causing cleidocranial dysplasia

Core binding factor A1 (CBFA1/RUNX2) is a runt-like transcription factor essential for osteoblast differentiation. Haplotype insufficiency causes cleidocranial dysplasia (CCD), a syndrome featuring supernumerary tooth buds, delayed tooth eruption, patent fontanels, Wormian bones, short stature, dysplasia of the clavicles, growth retardation and hypoplasia of the distal phalanges. We identified novel CBFAI/RUNX2 mutations after PCR and direct sequencing of patient leukocyte DNA. In family 1 mother and son are affected by CCD. Both carry the missense mutation R190W (CGG > TGG). This nucleotide change introduced a BsmI restriction site, which was used to independently confirm the mutation. It was absent in healthy members of the family. Family 2, in which father and daughter are affected by CCD, shows a deletion of nucleotide C821. This deletion causes a frameshift mutation with premature stop after the insertion of 18 aberrant amino acids. Healthy family members did not have this mutation. The clavicular dysplasia was more pronounced with the R19OW mutation, while the bone density was markedly reduced in individuals with either mutation, suggesting a previously underemphasized increased risk for osteoporosis in CCD.

Sonic hedgehog is a survival factor for hypaxial muscles during mouse development

Sonic hedgehog (Shh) has been proposed to function as an inductive and trophic signal that controls development of epaxial musculature in vertebrate embryos. In contrast, development of hypaxial muscles was assumed to occur independently of Shh. We here show that formation of limb muscles was severely affected in two different mouse strains with inactivating mutations of the Shh gene. The limb muscle defect became apparent relatively late and initial stages of hypaxial muscle development were unaffected or only slightly delayed. Micromass cultures and cultures of tissue fragments derived from limbs under different conditions with or without the overlaying ectoderm indicated that Shh is required for the maintenance of the expression of myogenic regulatory factors (MRFs) and, consecutively, for the formation of differentiated limb muscle myotubes. We propose that Shh acts as a survival and proliferation factor for myogenic precursor cells during hypaxial muscle development. Detection of a reduced but significant level of Myf5 expression in the epaxial compartment of somites of Shh homozygous mutant embryos at E9.5 indicated that Shh might be dispensable for the initiation of myogenesis both in hypaxial and epaxial muscles. Our data suggest that Shh acts similarly in both somitic compartments as a survival and proliferation factor and not as a primary inducer of myogenesis.