Metaphyseal dysplasia with maxillary hypoplasia and brachydactyly in a Finnish woman: first confirmation of a duplication in RUNX2 as pathogenic variant

RUNX2-related metaphyseal dysplasia with maxillary hypoplasia: A rare skeletal disorder resembling SFRP4-related Pyle disease

Metaphyseal dysplasia with maxillary hypoplasia with or without brachydactyly (MDMHB) is an ultra-rare skeletal dysplasia caused by heterozygous intragenic RUNX2 duplications, comprising either exons 3 to 5 or exons 3 to 6 of RUNX2. In this study, we describe a 14-year-old Belgian boy with metaphyseal dysplasia with maxillary hypoplasia but without brachydactyly. Clinical and radiographic examination revealed mild facial dysmorphism, dental anomalies, enlarged clavicles, genua valga and metaphyseal flaring and thin cortices with an osteoporotic skeletal appearance. Exome sequencing led to the identification of a de novo heterozygous tandem duplication within RUNX2, encompassing exons 3 to 7. This duplication is larger than the ones previously reported in MDMHB cases since it extends into the C-terminal activation domain of RUNX2. We review previously reported cases with MDMHB and highlight the resemblance of this disorder with Pyle disease, which may be explained by intersecting molecular pathways between RUNX2 and sFRP4. This study expands our knowledge on the genotypic and phenotypic characteristics of MDMHB and the role of RUNX2 in rare bone disorders.

Brd4 is required for chondrocyte differentiation and endochondral ossification

Differentiation of multi-potent mesenchymal stromal cells (MSCs) is directed by the activities of lineage-specific transcription factors and co-factors. A subset of these proteins controls the accessibility of chromatin by recruiting histone acetyl transferases or deacetylases that regulate acetylation of the N-termini of H3 and H4 histone proteins. Bromodomain (BRD) proteins recognize these acetylation marks and recruit the RNA pol II containing transcriptional machinery. Our previous studies have shown that Brd4 is required for osteoblast differentiation in vitro. Here, we investigated the role of Brd4 on endochondral ossification in C57BL/6 mice and chondrogenic differentiation in cell culture models. Conditional loss of Brd4 in the mesenchyme (Brd4 cKO, Brd4^fl/fl: Prrx1-Cre) yields smaller mice that exhibit alteration in endochondral ossification. Importantly, abnormal growth plate morphology and delayed long bone formation is observed in juvenile Brd4 cKO mice. One week old Brd4 cKO mice have reduced proliferative and hypertrophic zones within the physis and exhibit a delay in the formation of the secondary ossification center. At the cellular level, Brd4 function is required for chondrogenic differentiation and maturation of both ATDC5 cells and immature mouse articular chondrocytes. Mechanistically, Brd4 loss suppresses Sox9 levels and reduces expression of Sox9 and Runx2 responsive endochondral genes (e.g., Col2a1, Acan, Mmp13 and Sp7/Osx). Collectively, our results indicate that Brd4 is a key epigenetic regulator required for normal chondrogenesis and endochondral ossification.

Altered microRNAs in C3H10T1/2 cells induced by p.E95K mutant IHH signaling

Background

Indian Hedgehog (IHH), an important cell signaling protein, plays a key regulatory role in development of cartilage and chondrogenesis. Earlier studies have shown that heterozygous missense mutations in IHH gene may cause brachydactyly type A1 (BDA1), an autosomal dominant inheritance disease characterized by apparent shortness or absence of the middle phalanges of all digits. MicroRNAs (miRNAs) have been found to be significant post-transcriptional regulators of gene expression and significantly influence the process of bone-development. Therefore, it is possible that miRNAs are involved in the mechanism underlying the development of BDA1. However, the relationship between miRNAs and the pathogenesis of BDA1 remains unclear.

Methods

In this study, we used microarray-based miRNA profiling to investigate the role of miRNAs in BDA1 by characterization of differentially expressed miRNAs in C3H10T1/2 cell line induced by wild type (WT) and p.E95K mutant (MT) IHH signaling.

Results

Our results identified 6 differentially expressed miRNAs between WT and control (CT) group and 5 differentially expressed miRNAs between MT and CT groups. In particular, miR-135a-1-3p was found to be a significantly differentially expressed miRNA between WT and CT group. Results of dual-luciferase reporter gene experiment successfully discovered Hoxd10 was one of the target gene of miR-135a-1-3p. Additionally, our pathway analysis revealed that the targets of these miRNAs of interest were highly involved with Runx1/2, Notch and collagen-related pathways.

Conclusions

Taken together, our findings provided important clue for future study of the process of miRNA-regulation in IHH signaling and novel insights into the regulatory role of miRNA in pathogenesis of BDA1.

Supplementary Information

The online version contains supplementary material available at 10.1186/s41065-021-00207-8.

A three-generation family with metaphyseal dysplasia, maxillary hypoplasia and brachydactyly (MDMHB) due to intragenic RUNX2 duplication

Metaphyseal dysplasia with maxillary hypoplasia and brachydactyly (MDMHB) is an autosomal-dominant skeletal dysplasia characterised by metaphyseal flaring of the long bones, enlargement of the medial halves of the clavicles, maxillary hypoplasia, brachydactyly, dental anomalies and mild osteoporosis. To date, only one large French Canadian family and a Finnish woman have been reported with the condition. In both, intragenic duplication encompassing exons 3-5 of the RUNX2 gene was identified. We describe a new, three-generation family with clinical features of MDMHB and an intragenic tandem duplication of RUNX2 exons 3-6. Dental problems were the primary presenting feature in all four affected individuals. We compare the features in our family to those previously reported in MDMHB, review the natural history of this condition and highlight the importance of considering an underlying skeletal dysplasia in patients presenting with significant dental problems and other suggestive features, including disproportionate short stature and/or digital anomalies.

Oro-dental phenotype in patients with RUNX2 duplication

Runt-related transcription factor 2 (RUNX2) is well-known for its role in bone development and tooth morphogenesis. Most RUNX2 mutations described in the literature result in loss-of-function mutations of RUNX2 responsible for cleidocranial dysplasia, an autosomal dominant disorder. We describe here the oro-dental phenotype of four patients of a unique family with a 285 kb duplication including the entire sequence of RUNX2, likely responsible for three functional copies of the gene, leading to an increased RUNX2 dosage. Several dental anomalies of number (hypodontia or oligodontia), morphology (microdontia, radiculomegaly, taurodontism or dens invaginatus) and tooth position (rotation) were found in these patients.