Diverging pathomechanisms underlying collagen I-related and MBTPS2-related osteogenesis imperfecta: insights from patient-derived fibroblasts and iPSC-based modelling of bone.

Osteogenesis imperfecta (OI), characterised by low bone mass and bone fragility, is a heritable disorder with a heterogeneous genetic cause. Type I collagen is the most predominant type of collagen in the bone. Hence, the vast majority of patients with OI carry genetic variants in the genes that encode for type I collagen. However, a fraction of patients have defects in genes that either participate in collagen synthesis and maturation, in osteoblast maturation and functions including bone mineralisation, or with yet fully-understood mechanisms. An intriguing example is MBTPS2, a gene in which missense variants cause two non-overlapping clinical spectrums - either OI or a dermatological spectrum condition (IFAP/KFSD). Our work in the past decade aimed at molecular profiling of MBTPS2-OI using patient-derived fibroblasts. Here, we expand on this by generating induced pluripotent stem cells (iPSCs) from patient-derived fibroblasts and subsequently differentiated step-wise through the sclerotome and into osteoblasts. We also developed a Fiji-based image analysis pipeline to examine extracellular collagen misfolding and fibril organization in 2-dimensional (2D) fibroblast cultures in vitro, which will complement existing methods to qualitatively assess collagen. Together, qualitative assessment of type I collagen and iPSC-based in vitro bone modelling revealed differences in pathomechanisms underlying MBTPS2-OI and classical COL1-OI This showcases diverging pathologies underlying different genetic forms of OI and highlights the need for better molecular characterization of each genetic form to optimise approaches for patient management and treatment.