A novel duplication downstream of BMP2 in a Chinese family with Brachydactyly type A2 (BDA2)

A 6.3 Mb maternally derived microduplication of 20p13p12.2 in a fetus with Brachydactyly type D and related literature review

Background

With the introduction of genetic tests such as chromosomal microarray analysis (CMA) and exome sequencing (ES) into fetal medical practices, genotype–phenotype correlations in intrauterine-onset disorders have substantially improved. The BMP2 gene, located on the long arm of chromosome 20 plays a role in bone and cartilage development and is associated with Brachydactyly type A2, an autosomal dominant disease characterized by malformations of the middle phalanx of the index finger and abnormalities of the second toe. However, the BMP2 gene has so far never been reported as a candidate gene for Brachydactyly type D (BDD) affecting only the thumbs.

Methods and results

Here, we report one family possessing a maternally inherited 6.3 Mb microduplication of 20p13p12.2 including the BMP2 gene with discordant phenotypes between the mother and the fetus. The mother was affected with BDD alongside mild facial dysmorphism and learning difficulties, while the female fetus showed BDD, severe symmetric intrauterine growth restriction combined with oligohydramnios. The CMA and Trio ES tests were implemented. Trio ES ruled out other possible monogenic causes for the family. After reviewing cases and literature with duplications within this genomic region, we found that they are extremely rare and most of the cited cases were too small for comparison. The disturbance of the BMP2 gene could explain BDD, but the other clinical presentations in the mother and fetus are not yet fully understood.

Conclusion

This study provides important evidence for the current understanding of genotype–phenotype association of this 6.3 Mb size duplication in the 20p13p12.2 region. This duplication is a unique CNV occurring so far only in this family. Further cases and research are needed to understand the discordance in the phenotypes between the mother and fetus and establish the relationship between BMP2 gene and BDD.

The Role of Bmp2 in the Maturation and Maintenance of the Murine Knee Joint

Bone morphogenetic proteins (BMPs) are key regulators of skeletal development, growth, and repair. Although BMP signaling is required for synovial joint formation and is also involved in preserving joint function after birth, the role of specific BMP ligands in adult joint homeostasis remains unclear. The purpose of this study was to define the role of Bmp2 in the morphogenesis and maintenance of the knee joint. To do this, we first created Bmp2-LacZ and Gdf5-LacZ knock-in mice and compared their expression patterns in the developing and postnatal murine knee joint. We then generated a knockout mouse model using the Gdf5-cre transgene to specifically delete Bmp2 within synovial joint-forming cells. Joint formation, maturation, and homeostasis were analyzed using histology, immunohistochemistry, qRT-PCR, and atomic force microscopy (AFM)-based nanoindentation to assess the cellular, molecular, and biomechanical changes in meniscus and articular cartilage. Bmp2 is expressed in the articular cartilage and meniscus of the embryonic and adult mouse knee in a pattern distinct from Gdf5. The knee joints of the Bmp2 knockout mice form normally but fail to mature properly. In the absence of Bmp2, the extracellular matrix and shape of the meniscus are altered, resulting in functional deficits in the meniscus and articular cartilage that lead to a progressive osteoarthritis (OA) like knee pathology as the animals age. These findings demonstrate that BMP activity provided by Bmp2 is required for the maturation and maintenance of the murine knee joint and reveal a unique role for Bmp2 that is distinct from Gdf5 in knee joint biology. © 2018 American Society for Bone and Mineral Research.