X-linked hypophosphataemia (XLH) is the most common form of inherited rickets that leads to deformities in the lower limbs, poor tooth and skeletal mineralization, and disproportionate short stature in children. In adults, it is often complicated by enthesopathy, an abnormal mineralization of the tendon-bone attachment. Enthesopathy causes pain and stiffness in affected joints, particularly in the knee, hip, and ankle joints. Enthesopathy is reported as one of the most debilitating symptoms in XLH patients. Previous studies showed that entheses from mice with XLH (Hyp) are characterized by enhanced Bone morphogenic protein (BMP) and Indian hedgehog (IHH) signaling. This study aims to investigate the role of GDF5 in the development of enthesopathy in the Hyp mice. The study has two specific aims: (1) to determine if deleting GDF5 in enthesis (scleraxis-expressing (Scx+)) cells affects BMP/IHH signaling in entheses and (2) to determine if deleting GDF5 in Scx+ cells of Hyp mice attenuates XLH enthesopathy. The study hopes to gain a better understanding of role of GDF5 in enthesis maturation and XLH enthesopathy development. This study finds that deleting GDF5 in wild-type mice does not change normal enthesis maturation. However, deleting GDF5 in Hyp mice attenuates enthesopathy as indicated by decreased BMP/IHH signaling in Hyp entheses. / 2026-03-19T00:00:00Z

Brachydactyly type A1 (BDA1) is a rare autosomal dominant trait characterized by the shortening of the middle phalanges of digits 2-5 and of the proximal phalange of digit 1 in both hands and feet. Many of the brachymesophalangies including BDA1 have been associated with genetic perturbations along the BMP-SMAD signaling pathway. The goal of this thesis is to identify the molecular pathways that are associated with the BDA1 phenotype through the genetic assessment of BDA1-affected families. We identified four missense mutations that are clustered with other reported BDA1 mutations in the central region of the N-terminal signaling peptide of IHH. We also identified a missense mutation in GDF5 cosegregating with a semi-dominant form of BDA1. In two families we reported two novel BDA1-associated sequence variants in BMPR1B, the gene which codes for the receptor of GDF5. In 2002, we reported a BDA1 trait linked to chromosome 5p13.3 in a Canadian kindred (BDA1B; MIM %607004) but we did not discover a BDA1-causal variant in any of the protein coding genes within the 2.8 Mb critical region. To provide a higher sensitivity of detection, we performed a targeted enrichment of the BDA1B locus followed by high-throughput sequencing. We report the identification of a novel 9.5 Kb intergenic tandem duplication in two unrelated BDA1-affected families. In-vitro and in-vivo reporter assays demonstrated the enhancer activity of noncoding conserved sequence elements found within the microduplication. We also show an upregulation of the neighboring genes, NPR3 and PDZD2, in the patients' fibroblasts that suggests a gain-of-function through the duplication of cis-regulatory elements on dose sensitive genes. By expanding the repertoire of BDA1-causing mutations in IHH, GDF5, BMPR1B and at the BDA1B locus, we have begun to elucidate a common genetic pathway underlying phalangeal formation and elongation.