The PORTRAIT Study

Hypophosphatasia (HPP) is caused by mutations in the tissue non-specific alkaline phosphatase (TNSALP) gene with deficiency in TNSALP activity. This leads to accumulation of inorganic pyrophosphate, a potent inhibitor of bone mineralization, which causes rickets and osteomalacia, and of pyridoxal 5-phosphate (PLP) which causes seizures in neonates but no harm in adults. HPP is also associated with muscle weakness (which is very severe in perinatal and infantile forms) and early tooth loss. The most severe forms of HPP present in infancy, and were fatal until the recent development of TNSALP enzyme replacement therapy, Asfotase Alpha. Adult-onset HPP is less severe. It often presents with fractures of the femur or metatarsals, and so is likely to be misdiagnosed as osteoporosis. Correct diagnosis of HPP is important as the usual osteoporosis treatments, for example bisphosphonates, may be harmful and increase the risk of fracture.

High resolution peripheral quantitative computed tomography (HR-pQCT) is an imaging technique that allows the detailed assessment of BMD, microstructure and strength. HR-pQCT could reveal important information about the effects of HPP on bone microstructural properties.

HPP can also have a significant effect on physical functioning. In infantile HPP, motor function delay is usual but in adult-onset HPP effects on physical functioning are less apparent. These effects can be explored through the use of physical function testing and clinical gait analysis. Cognitive function may be affected by low levels of or inactivity of ALP in the neural tissue (TNALP). Patients with HPP have reported forgetfulness and 'fogginess'.

The aims of the study are to examine the effects of HPP on (i) bone structure and strength, (ii) physical functioning, (iii) cognition and (iv) quality of life (QoL). Researchers will study adults with HPP and healthy individuals. The results will help to determine if there should be a trial of a new drug treatment for adults with HPP. This study forms part of our wider programme of research into HPP.

Researchers will recruit 7 patients with childhood- or adult-presenting HPP (cases), who are known to the Metabolic Bone Clinic at the Metabolic Bone Centre (MBC), Northern General Hospital (NGH), Sheffield. Researchers will also recruit 7 healthy controls matched to the cases by gender and age. Researchers will approach first degree relatives (with their permission) of patients with HPP to offer them genetic testing for HPP. If HPP is confirmed, approach these first degree relatives to invite them to participate in the study, Fasting blood samples will be obtained for measurement of serum or plasma bone alkaline phosphatase (bone ALP), PLP, procollagen type I N-propeptide (PINP), C-terminal telopeptide (CTX) and genetic testing (if not already performed).

Medical history and lifestyle, quality of life and cognitive function will be assessed using questionnaires.

Bone mineral density (BMD) of the lumbar spine, proximal femur and whole body will be measured using dual energy x-ray absorptiometry (DXA). Whole body composition will also be determined by DXA. Trabecular and cortical BMD and bone microstructure and strength will be examining using HR-pQCT of the distal radius and tibia.

Physical functioning will also be assessed. Participants will undergo a series of physical performance tests/gait analysis tests. Chair rises (sit to stand), a timed up and go test. A 10 m walking test and a 4 stair climb will be used to assess muscle strength. A modified performance oriented mobility assessment-gait (MPOMA-G) will also be performed. Recordings of the kinematics, ground reaction forces, electromyography (EMG) data and muscle function will be made using a motion capture system. This will allow the quantification of joint kinematics and kinetics and of muscular pattern activation during the execution of a number of different motor skills. Lower limb magnetic resonance imaging (MRI), with the participants dressed in a set of MRI visible markers, will allow the registration of the MRI and physical functioning/gait data enhancing the participant's specific musculoskeletal modelling.

Cognitive function tests (a Montreal Cognitive Assessment) will be performed to assess the burden of disease of HPP on cognitive and mental health as forgetfulness and 'fogginess' have been reported by patients.

The results from the various assessments performed will be compared between the age-gender matched cases and controls using paired samples t-tests. For each test, the mean difference, 95% confidence interval and associated p-value will be reported. In the event of the normality assumption of the paired samples t-test being violated comparisons will be made using the non-parametric Wilcoxon signed rank test. The burden of disease of HPP on bone quality (i.e. BMD, bone structure and strength), mental health, cognition, QoL, pain, physical functioning and gait will be examined using Spearman Rank Correlation.