UCLA

Studies performed in rare congenital, primarily mendelian disorders have uncovered critical findings and data that have aided in the understanding and development of treatments for more commonly occurring disorders. Spondylocarpotarsal synostosis (SCT) is an autosomal recessive disorder characterized by progressive fusions of the thoracic and lumbar vertebrae, as well as carpal and tarsal bones, and results from nonsense mutations in the gene Filamin B (FLNB). Utilizing a Flnb global knockout mouse model, we showed that the vertebral fusions are caused by the collapse and ossification of the intervertebral disc (IVD). This collapse is influenced by upregulation of the TGFβ/BMP signaling pathways in the IVD and resembles disc degeneration disorder (DDD), a common condition in the aging population. The mechanism in part results from direct interactions of FLNB with inhibitory Smads; loss of FLNB leads to lack of normal TGFβ/BMP pathway modulation and increased resultant activity. We have shown that increasing TGFβ/BMP signaling in Flnb+/+ in vitro spinal cultures phenocopies the in vivo loss of FLNB. To further understand the genetic mechanisms that underlie progressive vertebral fusions, SCT patients negative for FLNB mutations underwent exome analysis. In three patients, heterozygosity for mutations in the gene Myosin Heavy Chain 3 (MYH3) was identified. All patients presented with vertebral and carpal/tarsal fusions characteristic of SCT. To understand this finding, HEK293 cells were transfected with wild type and mutated MYH3 plasmids. Cells with mutated MYH3 plasmids showed an inhibitory effect on both canonical and noncanonical TGFβ signaling revealing a heretofore unknown regulatory role for MYH3. Furthermore, MYH3 is not only an embryonic myosin; persistent postnatal MYH3 expression (at postnatal day 15) was specifically localized to the muscle tissues joining the neural arches of the spine. In the mouse model of absence of FLNB, matrix and signaling changes within the IVD lead to cell fate changes, transforming IVD cells to bone. However, the newly identified MYH3 mutations suggest that abnormal extraneous mechanical forces stemming from altered TGFβ signaling activity in muscle can also lead to progressive vertebral fusions. Traditional genetic studies now coupled with newer sequence technologies of a rare mendelian disorder have led to new insights into the mechanisms of progressive vertebral fusions and disc generation, expanding our understanding of the cellular functions of both FLNB and MYH3.