Many types of gene mutations are known to negatively affect a child's growth. Although the result is deformed or underdeveloped bones, many of these skeletal abnormalities are due to disorders of the cartilage. A new study by Prof. Junya Toguchida and colleagues reports a system that uses patient iPS cells to investigate the cellular events involved in two such disorders, multiple epiphyseal dysplasia and metaphyseal chondrodysplasia. The study further shows the potential of this system for finding drug candidates.

Chondrodysplasia is a general term for hereditary cartilage disorders that lead to skeletal malformations. They are apparent in childhood and besides causing unusual sized limbs, they can result in debilitating pain and in the most severe cases lethal disabilities.

"Multiple epiphyseal dysplasia and metaphyseal chondrodysplasia patients show short-limbed dwarfism and deformities of the hips and knees. Although the same genes are mutated, the symptoms widely range with the patient, making study and treatment difficult," explained Toguchida, who is also an orthopedic surgeon using iPS cell technology to study many bone and cartilage diseases.

One of the advantages of iPS cell technology is that it gives researchers a human model to observe how cells deviate from normal development into a disease state. While children may still be growing, the evidence of symptoms indicates that cartilage cells, or chondrocytes, have already become pathological. Preferably, researchers will access cells before they take the pathological state.

In the reported study, Toguchida and his team reprogrammed cells from one multiple epiphyseal dysplasia patient and two metaphyseal chondrodysplasia patients. Using gene editing technology, they also mutated iPS cells from healthy donors to express one additional mutation for each disease, providing an analysis of five mutations in total in two genes: MATN3 for multiple epiphyseal dysplasia and COL10A1 for metaphyseal chondrodysplasia.

Chondrocytes prepared from the mutant iPS cells showed irregularities in their unfolded protein response, an intracellular system that disposes of dysfunctional proteins and without which the cell will die.

"We found cells retained abnormal aggregates in their endoplasmic reticulum, which were severely enlarged depending on the mutation. We also found evidence of a hyperactivated unfolded protein response," explained Yann Pretemer, who will be earning his Ph.D. from the study.

Further, the degree of the changes in the cell morphology correlated well with the amount of change in the gene expressions of the cells, which could help explain why the severity is so variable for the two diseases. For the multiple epiphyseal dysplasia case with harsher symptoms, genes related to the unfolded protein response showed more expression change. On those cells, Pretemer and colleagues tested several drugs, finding one that could reduce the size of the endoplasmic reticulum and aggregate retention.

Toguchida is encouraged by the promise of using their system to study chondrodysplasias.

"Our findings suggest we can recapitulate the phenotype severity of the disease and explore drug targets. Chondrodysplasias are normally studied with mouse models. We are trying to create a reliable human model," he said.

• doi: 10.1016/j.stemcr.2021.01.014