Genetic causes and molecular mechanisms underlying rare metabolic bone diseases

The skeletal system provides support for the body, enables movement and protects inner organs. Moreover, it supplies blood cells and acts as a reservoir for minerals and fat. Several external factors, including nutrition and long-term illness, influence bone health but genetic factors also play an important role. More than 400 different rare skeletal diseases, collectively called skeletal dysplasias, have thus far been delineated and mutations in over 350 genes have been identified as underlying causes in these conditions. Although the recent evolution of the sequencing technologies and molecular methods has increased diagnostic yield of rare skeletal diseases, knowledge on the genetic and phenotypic features in some of these conditions is still limited and novel forms of skeletal dysplasia still remain to be characterized.

This thesis focused on rare skeletal diseases primarily affecting the major component of the skeleton, the bone. In paper I and III Sanger sequencing was used. In paper I this method excluded the presence of rare variants in CRTAP, encoding the cartilage associated protein, in patients with mild-to-severe skeletal fragility. In paper III two novel mutations in two components of the WNT signaling pathway, LRP5 and AMER1, were identified in two patients affected by high bone mass. In paper II a custom designed highresolution array-CGH, targeting all the genes thus far linked to skeletal diseases and the cilia genes, enabled the identification of two novel copy number variants (CNVs) affecting COL1A2 and PLS3 in two index patients with primary osteoporosis. Other rare CNVs in genes not yet related to bone homeostasis were detected and regarded as variants of unknown significance. In papers IV and V massively-parallel sequencing was applied. In paper IV five novel variants in the fibronectin gene (FN1), which was recently linked to spondylometaphyseal dysplasia with “corner fractures”, were revealed in five patients affected by this disease. Finally, in paper V two novel variants in the gene encoding the ribosomal protein L13, RPL13, were for the first time associated with a novel form of spondyloepimetaphyseal dysplasia.

Our findings expand the genetic and phenotypic spectrum of some known rare skeletal diseases. Moreover, a novel gene-disease association was identified but further studies are required to explore the pathomolecular mechanisms underlying this condition. Studying rare metabolic bone diseases is important not only for arriving at a specific diagnosis but also for understanding the pathogenesis of these conditions - only an increased understanding of the molecular mechanisms will enable the development of targeted therapies.

List of scientific papers

I. CRTAP variants in early-onset osteoporosis and recurrent fractures. Costantini A, Vuorimies I, Makitie R, Mayranpaa MK, Becker J, Pekkinen M, Valta H, Netzer C, Kampe A, Taylan F, Jiao H, Makitie O. Am J Med Genet A. 173(3) (2017), 806-808.
https://doi.org/10.1002/ajmg.a.38065

II. Rare Copy Number Variants in Array-Based Comparative Genomic Hybridization in Early-Onset Skeletal Fragility. Costantini A, Skarp S, Kämpe A, Mäkitie RE, Pettersson M, Männikko M, Jiao H, Taylan F, Lindstrand A, Mäkitie O. Front Endocrinol. 9 (2018) 380.
https://doi.org/10.3389/fendo.2018.00380

III. High bone mass due to novel LRP5 and AMER1 mutations. Costantini A, Kekäläinen P, Mäkitie RE, Mäkitie O. Eur J Med Genet. 60(12) (2017), 675-679.
https://doi.org/10.1016/j.ejmg.2017.09.001

IV. Novel fibronectin mutations and expansion of the phenotype in spondylometaphyseal dysplasia with "corner fractures". Costantini A, Valta H, Baratang NV, Yap P, Bertola DR, Yamamoto GL, Kim CA, Chen J, Wierenga KJ, Fanning EA, Escobar L, McWalter K, McLaughlin H, Willaert R, Begtrup A, Alm JJ, Reinhardt DP, Mäkitie O, Campeau PM. Bone. 121 (2019), 163-171.
https://doi.org/10.1016/j.bone.2018.12.020

V. RPL13 variants in spondyloepimetaphyseal dysplasia. Costantini A, Alm JJ, Tonelli F, Valta H, Tran A, Chen S, Chagin A, Newton P, Daponte V, Kwon YU, Bae JY, Chung WY, Larsson O, Nishimura G, Näreoja T, Kim OH, Forlino A, Cho TJ, Mäkitie O. [Manuscript]