# CRTAP-Null Osteoblasts Have Increased Proliferation, Protein Secretion, and Skeletal Morphogenesis Gene Expression with Downregulation of Cellular Adhesion

## TL;DR

This study explores how a genetic mutation in CRTAP causes skeletal issues by increasing bone cell growth and altering signaling pathways.

## Contribution

The novel contribution is identifying increased proliferation and BMP2 signaling in CRTAP-null osteoblasts linked to type VII osteogenesis imperfecta.

## Key findings

- CRTAP-null osteoblasts show increased proliferation and DNA replication gene activity.
- BMP2 signaling is upregulated in CRTAP-null osteoblasts, with increased BMP2 and MSX2 expression.
- Cell adhesion and extracellular matrix pathways are downregulated in CRTAP-null osteoblasts.

## Abstract

Type VII osteogenesis imperfecta (OI), caused by recessive CRTAP mutations, is predominantly lethal in the first year of life. Due to its early lethality, little is known about bone dysplasia mechanism. RNA-seq analysis of differentiated osteoblasts of siblings with a non-lethal homozygous CRTAP-null variant showed an enrichment of gene ontology terms involved in DNA replication and cell cycle compared to control. BrdU incorporation confirmed a ≈2-fold increase in proliferation in non-lethal proband osteoblasts in comparison to control cells. In addition, the expression of cyclin dependent kinase inhibitor 2A (CDKN2A), encoding a protein involved in cell cycle inhibition, was significantly reduced (>50%) in CRTAP-null osteoblasts, while cyclin B1 (CCNB1), encoding a promoter of the cell cycle, was enhanced. Ossification and bone and cartilage development gene ontology pathways were enriched among upregulated genes throughout osteoblast differentiation, as was protein secretion. Ingenuity pathway analysis indicated an upregulation of BMP2 signaling, supported by increase in both BMP2 and MSX2, an early BMP2-responsive gene, by qPCR. Throughout differentiation, CRTAP-null osteoblasts showed a decrease in transcripts related to cell adhesion and extracellular matrix organization pathways. We propose that increased proliferation and osteogenesis of type VII OI osteoblasts may be stimulated through upregulation of BMP2 signaling, altering bone homeostasis, and leading to weaker bones.