Multicentric Carpotarsal Osteolysis: a Contemporary Perspective on the Unique Skeletal Phenotype

Defining unique structural features in the MAFA and MAFB transcription factors that control Insulin gene activity

MAFA and MAFB are related basic-leucine-zipper domain containing transcription factors which have important overlapping and distinct regulatory roles in a variety of cellular contexts, including hormone production in pancreatic islet α and β cells. Here we first examined how mutating conserved MAF protein-DNA contacts obtained from X-ray crystal structure analysis impacted their DNA-binding and Insulin enhancer-driven activity. While most of these interactions were essential and their disruption severely compromised activity, we identified that regions outside of the contact areas also contributed to activity. AlphaFold 2, an artificial intelligence-based structural prediction program, was next used to determine if there were also differences in the three-dimensional organization of the non-DNA binding/dimerization sequences of MAFA and MAFB. This analysis was conducted on the wildtype (WT) proteins as well as the pathogenic MAFA Ser64Phe and MAFB Ser70Ala trans -activation domain mutants, with differences revealed between MAFA WT and MAFB WT as well as between MAFA Ser64Phe and MAFA WT , but not between MAFB Ser70Ala and MAFB WT . Moreover, dissimilarities between these proteins were also observed in their ability to cooperatively stimulate Insulin enhancer-driven activity in the presence of other islet-enriched transcription factors. Analysis of MAFA and MAFB chimeras disclosed that these properties were greatly influenced by unique C-terminal region structural differences predicted by AlphaFold 2. Importantly, these results have revealed features of these closely related proteins that are functionally significant in islet biology.

Multicentric Carpo-Tarsal Osteolysis Syndrome (MCTO) and "Function Profile": a rehabilitative approach

BACKGROUND

Multicentric Carpo-Tarsal Osteolysis Syndrome (MCTO) is an autosomal dominant disease with increased bone reabsorption in the carpus and tarsus and the elbows, knees and spine. The disease is extremely heterogeneous and secondary and tertiary injuries vary widely and can lead to progressive disability and severe functional limitations. In addition to the available and upcoming drug therapies, physical medicine and rehabilitation are important treatment options. Currently, the indication and plan are overlooked, nonspecific and reported only for one patient.

METHODS

We describe a case series of MCTO patients diagnosed and followed by a centre to identify functional deficit as a potential clinical marker of disease progression for future etiological therapies. In addition, we define a symptomatic treatment approach and specific clinical management, including a patient-centred rehabilitation approach. Functional assessments are performed independently by a multidisciplinary group to establish the functional abilities of patients and the relationship between residual motor skills and their degree of autonomy and participation. We suggest a way to identify a rehabilitation plan based on a specific disease using the International Classification of Functioning, Disability and Health Children and Youth (ICF-CY).

RESULTS

To define a reliable and reproducible "Function Profile", through age and over time, we used to value the disease status according to the ICF-CY domains. It could be used to determine the complexity of the illness, its overall impact on the complexity of the person and the burden on the caregiver, and an eventual short- and long-term rehabilitation plan for MCTO and other ultra-rare diseases.

CONCLUSION

Based on the MCTO experience, we suggest a way to determine a rehabilitation plan based on a specific disease and patient needs, keeping in mind that often the final point is not recovering the full function but improving or maintaining the starting point. In all cases, each patient at the time of diagnosis requires a functional assessment that must be repeated over time to adjust the course of rehabilitation. The evaluations revealed the importance of early rehabilitation management in enhancing independence, participation and control of stress deconditioning, shrinking of muscle tendons and loss of movement to immobility.

Treatment of a young child with multicentric carpotarsal osteolysis exhibiting joint inflammation and dysfunctional bone formation

Multicentric carpotarsal osteolysis (MCTO) is a rare skeletal dysplasia characterized by osteolysis of the carpal and tarsal bones. Antiresorptive agents have proven ineffective and the pathogenesis of MCTO remains poorly understood. We report a young child with a novel variant in MAFB who demonstrated clinical improvement of joint symptoms following anti-rheumatic therapies. Also, radiographs from a young age suggest that dysfunctional bone formation may play a role in the skeletal phenotype of MCTO.

Denosumab Treatment Does Not Halt Progression of Bone Lesions in Multicentric Carpotarsal Osteolysis Syndrome

Here we report the use of denosumab, a monoclonal antibody against receptor activator of nuclear factor κB ligand (RANKL), as monotherapy for multicentric carpotarsal osteolysis syndrome (MCTO) in an 11.5-year-old male with a heterozygous missense mutation in MAFB (c.206C>T; p.Ser69Leu). We treated the subject with 0.5 mg/kg denosumab every 60-90 days for 47 months and monitored bone and mineral metabolism, kidney function, joint range of motion (ROM), and bone and joint morphology. Serum markers of bone turnover reduced rapidly, bone density increased, and renal function remained normal. Nevertheless, MCTO-related osteolysis and joint immobility progressed during denosumab treatment. Symptomatic hypercalcemia and protracted hypercalciuria occurred during weaning and after discontinuation of denosumab and required treatment with zoledronate. When expressed in vitro, the c.206C>T; p.Ser69Leu variant had increased protein stability and produced greater transactivation of a luciferase reporter under the control of the PTH gene promoter than did wild-type MafB. Based on our experience and that of others, denosumab does not appear to be efficacious for MCTO and carries a high risk of rebound hypercalcemia and/or hypercalciuria after drug discontinuation. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.