Inactivation of spermine synthase in mice causes osteopenia due to reduced osteoblast activity

Spermine synthase, encoded by the SMS gene, is involved in polyamine metabolism, as it is required for the synthesis of spermine from its precursor molecule spermidine. Pathogenic variants of SMS are known to cause Snyder-Robinson syndrome (SRS), an X-linked recessive disorder causing various symptoms, including intellectual disability, muscular hypotonia, infertility, but also skeletal abnormalities, such as facial dysmorphisms and osteoporosis. Since the impact of a murine SMS deficiency has so far only been analyzed in Gy mice, where a large genomic deletion also includes the neighboring Phex gene, there is only limited knowledge about the potential role of SMS in bone cell regulation. In the present manuscript, we describe 2 patients carrying distinct SMS variants, both diagnosed with osteoporosis. Whereas the first patient displayed all characteristic hallmarks of SRS, the second patient was initially diagnosed, based on laboratory findings, as a case of adult-onset hypophosphatasia. To study the impact of SMS inactivation on bone remodeling, we took advantage of a newly developed mouse model carrying a pathogenic SMS variant (p.G56S). Compared to their wildtype littermates, 12-wk-old male SMSG56S/0 mice displayed reduced trabecular bone mass and cortical thickness, as assessed by μCT analysis of the femur. This phenotype was histologically confirmed by the analysis of spine and tibia sections, where we also observed a moderate enrichment of non-mineralized osteoid in SMSG56S/0 mice. Cellular and dynamic histomorphometry further identified a reduced bone formation rate as a main cause of the low bone mass phenotype. Likewise, primary bone marrow cells from SMSG56S/0 mice displayed reduced capacity to form a mineralized matrix ex vivo, thereby suggesting a cell-autonomous mechanism. Taken together, our data identify SMS as an enzyme with physiological relevance for osteoblast activity, thereby demonstrating an important role of polyamine metabolism in the control of bone remodeling.