The study of uncommon clinical phenotypes can sometimes provide important insights into common genetically influenced traits, such as bone mass. A 50 yo white female was evaluated because of extremely high bone mass (T-scores +6 - +10), radiographically normal appearing bones except for very thick cortices, and chronically and markedly elevated markers of both bone formation and bone resorption. High turnover is usually associated with low bone mass, particularly in adult women, so this patient is very unusual. (It is worth noting that patients with gain-of-function mutations in LRP5 do not have elevated levels of P1NP or CTX.) Whole exome sequencing identified a novel variant in the gene Special AT rich Binding Protein 2 (SATB2) in this individual. A single base pair change, G to C, at position chr2:200193607 (hg19) resulted in an amino acid change, lysine 400 to asparagine (K400N). This allelic variant was not seen in either parent, in 2500 normal individuals or in publicly available databases. SATB2 is a member of the family of proteins that bind to matrix attachment regions (MAR) in DNA and SATB2 has been previously reported to have a critical role in bone formation. Lysine 400 is in the highly conserved CUT1 domain of SATB2 that binds DNA. Lysine 400 is strictly evolutionarily conserved back to flies, indicating a functionally important role for this amino acid. In situ hybridization studies documented high-level specific expression of SABT2 in osteoblasts and osteocytes, but not in cartilage or marrow. Overexpression of the K400N variant in MC3T3-E1 cells led to spontaneous in vitro mineralization. Since at present there are very few targets for bone anabolic therapies, exploring the mechanisms by which the SATB2 K400N mutation results in high bone mass is important. The only way to faithfully recapitulate the genotype of this individual in an experimental model is to replace one wild type SATB2 allele in a mouse with the mutant allele, i.e., by creating a knock-in mouse. To begin to identify the metabolic pathways entrained by this mutation in SATB2, we will pursue the following Specific Aims: 1. Generate knock-in mice bearing the SATB2-K400N mutation using an engineered ES cell clone in which the mutant allele has already been introduced by recombination. 2. Characterize the phenotype of the SATB2- K400N knock-in mice by using densitometric, histomorphometric and biomechanical methodologies and by studying MSCs, osteoblasts and osteoclasts isolated from these animals. 3. Undertake gene profiling studies using RNA isolated from primary calvarial osteoblast cultures from wild type and knock-in mice and Affymetrix whole mouse genome 430 2.0 arrays. High rates of skeletal turnover associated with increasing bone mass are normally only seen during childhood and adolescence, a period of rapid skeletal accrual. The molecular and metabolic pathways by which this occurs are unclear. Whether pathways similar to those are entrained by the SATB2 K400N mutation remains to be determined, but is an intriguing possibility. The proposed studies will help identify new molecular pathways regulating bone anabolism and new targets for drug discovery.
The study of patients with rare disorders can provide important insights into common problems such as osteoporosis. This study will determine how a rare mutation in the SATB2 gene caused a huge increase in bone mass in a 50 year-old woman. This may well lead to new treatments for low bone mass and osteoporosis.
