The aim of the study is to accomplish a complete bone status of patients with HPP using new approaches to assess bone quality.

Hypophosphatasia (HPP) is a hereditary disease of bone metabolism that is not yet curable. Clinical phenotype is variable and reaches from demineralization of bone, deformation of the skeleton, microsomia and gait abnormality to breathing difficulties. Symptoms of the adult form are low-traumatic fractures, hip or thigh pain and arthropathy. Cause of the disease is a mutation in the ALPL-gene (1p36.1-p34) coding for the tissue-nonspecific isoenzyme of alkaline phosphatase (TNAP) in liver, bone and kidney. This leads to a low activity of alkaline phosphatase (AP) and elevated levels of phosphoethanolamine (PEA) in urine.

HPP is a very rare disease with a prevalence of ~1/100 000. The Medical Department II of the St. Vincent Hospital Vienna, Department of the Medical University of Vienna and the Sigmund Freud University Vienna is a department that is specialized on bone diseases and, as a member of "Orphanet", also on In particular, (i) bone microstructure as a main component of bone strength and (ii) circulating microRNAs (miRNAs) as promising biomarkers for bone diseases will be analyzed in patients with HPP and age-, and gender-matched healthy controls.

Microstructural deteriorations of cortical and trabecular bone as well as volumetric bone density (vBMD) in radius and tibia in patients with HPP will be compared to healthy individuals using HR-pQCT (High resolution peripheral quantitative computer tomography, Scanco Medical, Brütisellen). HR-pQCT is a high-resolution, non-invasive technique to measure cortical and trabecular bone mircostructures as well as vBMD at a high resolution level (82µm).

Micro-RNAs (miRNAs) are short, non-coding RNA molecules of which some have been identified as bone specific (e.g. miR-31, miR-335, miR-155, miR-29b, miR-188, miR-550a). They play a significant role in bone metabolism controlling synthesis and function of osteoblasts as well as osteoclasts.

In recent studies we could show that these microRNAs can be detected in serum and that their serum concentration correlates with the risk for osteoporotic fractures. Data for patients with HPP do not exist yet. miRNAs will be measured by qPCR (quantitative polymerase chain reaction) in serum of patients with HPP and respective controls.

In addition, measurements of areal BMD (aBMD) by DXA (Dual Energy X-ray Absorptiometry) and DXL (Dual X-ray and Laser) will be performed. Vitamin D and established bone turnover markers including PINP (N-terminal propeptide of type I collagen), CTX (collagen type 1 cross-linked C-telopeptid) and sclerostin will be analyzed. Moreover, body composition will be determined.

Inclusion Criteria for Hypophosphatasia (HPP)

• genetically verified hypophosphatasia
• age >18 years
• written informed consent
• complete serological and radiological examinations

Inclusion Criteria für Controls:

• healthy men and women without any history of musculoskeletal diseases
• written informed consent
• Alkaline phosphatase (AP) in reference range
• complete serological and radiological examinations

Exclusion Criteria for both Groups:

• inflammatory diseases
• other genetic disorders affecting bone such as osteogenesis imperfecta, Ehlers-Danlos-syndrome and fibrous dysplasia
• diabetes mellitus type 1 and 2
• COPD
• chronic kidney and liver dysfunction
• systemic glucocorticoid use and glucocorticoid induced osteoporosis
• eating disorders
• HIV-infections and any malignancy including plasmacytosis and lymphoma.

30 adult patients with genetical verified childhood-onset hypophophatasia. All data will be compared to a healthy, age- and gender-matched control Group (CTRL, n=30), recruited from the general population.

Healthy controls will be matched by age and gender. All subjects will be recruited from the general population to avoid bias. The number of female and male subjects will be equal in both groups. Postmenopausal status will be taken into account.