Mouse Model May Help Test Therapies Targeting Bone, Joint Issues in XLH

A mouse model of X-linked hypophosphatemia (XLH) that mimics the skeletal alterations seen in human patients may be a valuable tool to test new therapies that aim to prevent bone and joint complications in XLH, scientists say.

The study, “Development of Enthesopathies and Joint Structural Damage in a Murine Model of X-Linked Hypophosphatemia,” was published in the journal Frontiers in Cell and Development Biology.

XLH is caused by mutations in a gene called PHEX and is characterized by low levels of phosphate in the blood. As phosphorus, the mineral found in phosphate compounds, is important for healthy bones and teeth, people with XLH typically have weakened bones.

As a result of poor mineralization, children with XLH develop rickets and osteomalacia — the abnormal softening of bones — skeletal deformities, and short stature.

In adulthood, XLH patients also show ossification, or bone formation in entheses (tissue between tendons or ligaments and bone) called enthesophytes, which can occur in the spine, joints, and the Achilles tendon. Enthesophytes cause chronic pain, stiffness, and disability.

Currently, phosphate supplements and calcitriol (the active form of vitamin D) are used to halt the bone deformities in XLH patients. However, whether this approach impacts enthesophytes formation remains unclear.

To better understand enthesophytes and joint alterations in XLH, researchers at the Université de Paris and colleagues used a mouse model of XLH, named hypophosphatemic (Hyp), which carries a spontaneous mutation in the PHEX gene and recapitulates disease features.

They evaluated joints and enthesophytes formation in the mice with micro-CT scans taken every three months, up to one year of age.

Two rheumatologists scored the severity of spinal enthesopathies and hip joint alterations from zero to three (normal to worst severity), and from zero (absent) to two (present) for peripheral calcifications — or calcium deposits — and enthesophytes.

Hyp mice showed signs of enthesopathies, bone erosions, and osteophytes (bone spurs) as early as 3 months of age, while no abnormalities were observed in healthy control mice.

Overall scoring indicated that bone alterations worsened with time. All mice showed high (severe) scores for bone erosion of the sacroiliac joint, which connects the lower spine and pelvis, from 3 months to 1 year of age. Four of five mice had hip osteoarthritis and/or calcifications in the tarsals, a group of bones in the ankle.

Micro-CT scans revealed that enthesophytes formed early and worsened over time. Two of five mice had visible calcaneal (heal) enthesophytes at 3 months of age, and all had expanding calcifications by one year.

No ossifications were observed in the animals’ spines, but a greater spine curvature was seen at 9 months of age in Hyp mice compared to controls.

A delay in ossification in the thigh bone (femur) head was observed at 3 and 6 months in Hyp mice, and further analysis revealed enlargement of chondrocytes, the cells that form cartilage. Hyp mice also showed a progressive erosion of the sacroiliac joint compared to controls.

New bone formation was seen in the shinbone (tibia) of Hyp mice, which worsened with time, as well as calcifications in various regions, including the hind paw. This abnormal bone formation was linked to altered levels of osteoponin and sclerostin, two proteins involved in mineralization, in the heel and sacroiliac joints.

Overall, these findings suggest that Hyp mice develop progressive enthesopathies and joint abnormalities, which mimic the “skeletal features of adult patients with XLH,” the researchers wrote.

The results “highlight the relevance of this preclinical model to test new therapies aiming to prevent bone and joint complications in XLH,” the team concluded.