ENPP1 deficiency: A clinical update on the relevance of individual variants using a locus-specific patient database

Loss-of-function variants in the ectonucleotide pyrophosphatase/phosphodiesterase family member 1 (ENPP1) cause ENPP1 Deficiency, a rare disorder characterized by pathological calcification, neointimal proliferation, and impaired bone mineralization. The consequence of ENPP1 Deficiency is a broad range of age dependent symptoms and morbidities including cardiovascular complications and 50% mortality in infants, autosomal recessive hypophosphatemic rickets type 2 (ARHR2) in children, and joint pain, osteomalacia and enthesopathies in adults. Recent research continues to add to the growing clinical presentation profile as well as expanding the role of ENPP1 itself. Here we review the current knowledge on the spectrum of clinical and genetic findings of ENPP1 Deficiency reported in patients diagnosed with GACI or ARHR2 phenotypes using a comprehensive database of known ENPP1 variants with associated clinical data. A total of 108 genotypes were identified from 154 patients. Of the 109 ENPP1 variants reviewed, 72.5% were demonstrably disease-causing, a threefold increase in pathogenic/likely pathogenic variants over other databases. There is substantial heterogeneity in disease severity, even among patients with the same variant. The approach to creating a continuously curated database of ENPP1 variants accessible to clinicians is necessary to increase the diagnostic yield of clinical genetic testing and accelerate diagnosis of ENPP1 Deficiency.

Lifelong impact of ENPP1 Deficiency and the early onset form of ABCC6 Deficiency from patient or caregiver perspective

The ectonucleotide pyrophosphatase/phosphodiesterase family member 1 (ENPP1) and ATP-binding cassette subfamily C member 6 (ABCC6) proteins play a prominent role in inhibiting ectopic calcification and arterial stenosis. Patients with ENPP1 Deficiency or infant onset ABCC6 Deficiency often present with pathological calcification, narrowed blood vessels, multiorgan dysfunction and high infant mortality. The heterogenous presentation and progression is well documented. Our objective was to characterize how these morbidities lead to burden of illness and poor quality of life across ages from the patient/caregiver perspective. Patients/caregivers were interviewed via phone using Institutional Review Board–approved questionnaires. Patient-reported outcomes were collected via validated instruments. Thirty-one caregivers and 7 patients participated: infant onset ABCC6 Deficiency, n = 6 (infants/children); ENPP1 Deficiency, n = 32 (13 infants, 12 children, 7 adults). ENPP1 and ABCC6-deficient children aged <8 years and aged 8–18 years reported poor school functioning (0.69 vs 0.72 effect size, respectively) and poor physical health (0.88 vs 1, respectively). In the total ENPP1 cohort, 72% (23/32) reported bone/joint pain and/or mobility/fatigue issues. Three of seven ENPP1-deficient adults reported moderate to severe pain (>4), as measured by the Brief Pain Inventory (BPI), that interfered with daily activities despite pain medication. Top reported burdens for caregivers of infants with ABCC6/ENPP1 Deficiencies included heart-related issues and hospitalizations. Treatment/medications, and hearing loss were the highest burdens reported by caregivers/families of the pediatric ENPP1 Deficiency cohort, whereas adults reported bone/joint pain and mobility impairment as the greatest burdens. Individuals with ENPP1 Deficiency or infant onset ABCC6 Deficiency experience lifelong morbidity causing substantial physical and emotional burden to patients/caregivers.

Musculoskeletal Comorbidities and Quality of Life in ENPP1-Deficient Adults and the Response of Enthesopathy to Enzyme Replacement Therapy in Murine Models

Ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) deficiency leads to cardiovascular calcification in infancy, fibroblast growth factor 23 (FGF23)-mediated hypophosphatemic rickets in childhood, and osteomalacia in adulthood. Excessive enthesis mineralization and cervical spine fusion have been previously reported in patients with biallelic ENPP1 deficiency, but their effect on quality of life is unknown. We describe additional musculoskeletal complications in patients with ENPP1 deficiency, namely osteoarthritis and interosseous membrane ossification, and for the first time evaluate health-related quality of life (HRQoL) in patients with this disease, both subjectively via narrative report, and objectively via the Brief Pain Inventory-Short Form, and a Patient Reported Outcome Measurement Information System Physical Function (PROMIS PF) short form. Residual pain, similar in magnitude to that identified in adult patients with X-linked hypophosphatemia, was experienced by the majority of patients despite use of analgesic medications. Impairment in physical function varied from mild to severe. To assess murine ENPP1 deficiency for the presence of enthesopathy, and for the potential response to enzyme replacement therapy, we maintained Enpp1^asj/asj mice on regular chow for 23 weeks and treated cohorts with either vehicle or a long-acting form of recombinant ENPP1. Enpp1^asj/asj mice treated with vehicle exhibited robust calcification throughout their Achilles tendons, whereas two-thirds of those treated with ENPP1 enzyme replacement exhibited complete or partial suppression of the Achilles tendon calcification. Our combined results document that musculoskeletal complications are a significant source of morbidity in biallelic ENPP1 deficiency, a phenotype which is closely recapitulated in Enpp1^asj/asj mice. Finally, we show that a long-acting form of recombinant ENPP1 prevents the development of enthesis calcification at the relatively modest dose of 0.3 mg/kg per week, suggesting that suppression of enthesopathy may be attainable upon dose escalation. © 2021 American Society for Bone and Mineral Research (ASBMR). This article has been contributed to by US Government employees and their work is in the public domain in the USA.

Non-memory subjective cognitive concerns predict incident motoric cognitive risk syndrome

BACKGROUND AND PURPOSE

Motoric cognitive risk syndrome (MCR) is a predementia condition that combines slow gait and subjective cognitive concerns. As the earliest markers of MCR are relatively unknown, the role of subjective cognitive concerns was investigated to predict incident MCR in a well-characterized prospective cohort of non-demented older adults.

METHODS

Non-demented MCR-free older adults (n = 476) from the Central Control of Mobility in Aging cohort completed gait, subjective cognition and neuropsychological assessment at baseline and follow-up. Subjective concerns were analyzed via responses to 12 items from three validated measures, the Late-Life Function and Disability Instrument - Disability Component, the Activities of Daily Living Prevention Instrument and the Geriatric Depression Scale, and were independent of items utilized to diagnose MCR. Cox proportional hazard models examined the association between cognitive concerns and incident MCR.

RESULTS

After 2.36 ± 1.4 years, 28 participants developed MCR. Executive functioning (adjusted hazard ratio 2.458, 95% confidence interval 1.094-5.524, P = 0.029) and mental clarity concerns (adjusted hazard ratio 3.917, 95% confidence interval 1.690-9.077, P = 0.001) were associated with incident MCR, controlling for age, sex, education and gait speed.

CONCLUSIONS

Subjective cognitive concerns in non-memory cognitive domains predict incident MCR. Although most MCR studies assess cognitive concerns about memory, our findings suggest the need to broaden the scope of subjective cognitive assessment to enhance the accuracy of diagnosis and prediction of future cognitive decline.

A randomised controlled trial and cost-consequence analysis of traditional and digital foot orthoses supply chains in a National Health Service setting: application to feet at risk of diabetic plantar ulceration

BACKGROUND

Diabetic foot ulceration is a considerable cost to the NHS and foot orthotic provision is a core strategy for the management of the people with diabetes and a moderate to high risk of foot ulceration. The traditional process to produce a custom-made foot orthotic device is to use manual casting of foot shape and physical moulding of orthoses materials. Parts of this process can be undertaken using digital tools rather than manual processes with potential advantages. The aim of this trial was to provide the first comparison of a traditional orthoses supply chain to a digital supply chain over a 6 month period. The trial used plantar pressure, health status, and health service time and cost data to compare the two supply chains.

METHODS

Fifty-seven participants with diabetes were randomly allocated to each supply chain. Plantar pressure data and health status (EQ5D, ICECAP) was assessed at point of supply and at six-months. The costs for orthoses and clinical services accessed by participants were assessed over the 6 months of the trial. Primary outcomes were: reduction in peak plantar pressure at the site of highest pressure, assessed for non-inferiority to current care. Secondary outcomes were: reduction in plantar pressure at foot regions identified as at risk (> 200 kPa), cost-consequence analysis (supply chain, clinician time, service use) and health status.

RESULTS

At point of supply pressure reduction for the digital supply chain was non-inferior to a predefined margin and superior (p < 0.1) to the traditional supply chain, but both supply chains were inferior to the margin after 6 months. Custom-made orthoses significantly reduced pressure for at risk regions compared to a flat control (traditional - 13.85%, digital - 20.52%). The digital supply chain was more expensive (+£13.17) and required more clinician time (+ 35 min). There were no significant differences in health status or service use between supply chains.

CONCLUSIONS

Custom made foot orthoses reduce pressure as expected. Given some assumptions about the cost models we used, the supply chain process adopted to produce the orthoses seems to have marginal impact on overall costs and health status.

TRIAL REGISTRATION

Retrospectively registered on ISRCTN registry (ISRCTN10978940, 04/11/2015).

A device for characterising the mechanical properties of the plantar soft tissue of the foot

The plantar soft tissue is a highly functional viscoelastic structure involved in transferring load to the human body during walking. A Soft Tissue Response Imaging Device was developed to apply a vertical compression to the plantar soft tissue whilst measuring the mechanical response via a combined load cell and ultrasound imaging arrangement. Accuracy of motion compared to input profiles; validation of the response measured for standard materials in compression; variability of force and displacement measures for consecutive compressive cycles; and implementation in vivo with five healthy participants. Static displacement displayed average error of 0.04 mm (range of 15 mm), and static load displayed average error of 0.15 N (range of 250 N). Validation tests showed acceptable agreement compared to a Houndsfield tensometer for both displacement (CMC > 0.99 RMSE > 0.18 mm) and load (CMC > 0.95 RMSE < 4.86 N). Device motion was highly repeatable for bench-top tests (ICC = 0.99) and participant trials (CMC = 1.00). Soft tissue response was found repeatable for intra (CMC > 0.98) and inter trials (CMC > 0.70). The device has been shown to be capable of implementing complex loading patterns similar to gait, and of capturing the compressive response of the plantar soft tissue for a range of loading conditions in vivo.

Invasive in vivo measurement of rear-, mid- and forefoot motion during walking

The aim of this work was to use bone anchored external markers to describe the kinematics of the tibia, fibula, talus, calcaneus, navicular, cuboid, medial cuneiform, first and fifth metatarsals during gait. Data were collected from six subjects. There was motion at all the joints studied. Movement between the talus and the tibia showed the expected predominance of sagittal plane motion, but the talocalcaneal joint displayed greater variability than expected in its motion. Movement at the talonavicular joint was greater than at the talocalcaneal joint and motion between the medial cuneiform and navicular was far greater than expected. Motion between the first metatarsal and the medial cuneiform was less than motion between the fifth metatarsal and cuboid. Overall the data demonstrated the complexity of the foot and the importance of the joints distal to the rearfoot in its overall dynamic function.

Predicting lower limb joint kinematics using wearable motion sensors

The aim of this study was to estimate sagittal plane ankle, knee and hip gait kinematics using 3D angular velocity and linear acceleration data from motion sensors on the foot and shank. We explored the accuracy of intra-subject predictions (i.e., where training and testing uses trials from the same subject) and inter-subject (where testing uses subjects different from the ones used for training) predictions, and the effect of loss of sensor data on prediction accuracy. Hip, knee and ankle kinematic data were collected using reflective markers. Simultaneously, foot and shank angular velocity and linear acceleration data were collected using small integrated accelerometers/gyroscope units. A generalised regression networks algorithm was used to predict the former from the latter. The best results were from intra-subject predictions, with very high correlations (0.93-0.99) and low mean absolute deviation (< or =2.3 degrees ) between measured kinematic joint angles and predicted angles. The inter-subject case produced poorer correlations (0.70-0.89) and larger absolute differences between measured and predicted angles, ranging from 4.91 degrees (left ankle) to 9.06 degrees (right hip). The angular velocity data added little to the accuracy of predictions and there was also minimal benefit to using sensor data from the shank. Thus, a wearable system based only on footwear mounted sensors and a simpler sensor set providing only acceleration data shows potential. Whilst predictions were generally stable when sensor data was lost, it remains to be seen whether the generalised regression networks algorithm is robust for other activities such as stair climbing.

Intrinsic foot kinematics measured in vivo during the stance phase of slow running

An accurate kinematic description of the intrinsic articulations of the foot during running has not previously been presented, primarily due to methodological limitations. An invasive method based upon reflective marker arrays mounted on intracortical pins drilled into the bones was used in this study. Four male volunteers participated as subjects. Pins (1.6mm diameter) were inserted under local anaesthetic in the tibia, fibula, calcaneus, talus, navicular, cuboid, medial cuneiform and metatarsals I and V. A 10 camera motion analysis system was used for kinematic data capture and the ground reaction force was simultaneously measured. Segment motion relative to adjacent proximal segments was determined using helical axes projected into the coordinate system of the proximal segment. Coefficients of multiple correlation calculated to determine the strength of association between running style with and without the pins inserted indicated that the subjects had little restriction due to the inserted pins. Individual and mean results were presented for rotations defined in the planes of the proximal segment's coordinate system and showed frontal plane rotation of the talocrural joint (12.2+/-7.1 degrees ), which exceeded that of the subtalar joint (8.9+/-3.2 degrees ). Considerable mobility of the talonavicular joint was found (6.5+/-2.9 degrees , 13.5+/-4.1 degrees and 8.7+/-1.4 degrees in the sagittal, frontal and transverse planes, respectively). Furthermore, little, but non-negligible motion between the fibula and tibia was found (3.3+/-2.4 degrees in the sagittal plane). The presented data are of interest as input for future biomechanical modelling and clinical decision making in particular, concerning joint fusion.

Foot kinematics during walking measured using bone and surface mounted markers

The aim was to compare kinematic data from an experimental foot model comprising four segments ((i) heel, (ii) navicular/cuboid (iii) medial forefoot, (iv) lateral forefoot), to the kinematics of the individual bones comprising each segment. The foot model was represented using two different marker attachment protocols: (a) markers attached directly to the skin; (b) markers attached to rigid plates mounted on the skin. Bone data were collected for the tibia, talus, calcaneus, navicular, cuboid, medial cuneiform and first and fifth metatarsals (n=6). Based on the mean differences between the three data sets during stance, the differences between any two of the three kinematic protocols (i.e. bone vs skin, bone vs plate, skin vs plate) were >3 degrees in only 35% of the data and >5 degrees in only 3.5% of the data. However, the maximum difference between any two of the three protocols during stance was >3 degrees in 100% of the data, >5 degrees in 73% of the data and >8 degrees in 23% of the data. Differences were greatest for motion of the combined navicular/cuboid relative to the calcaneus and the medial forefoot segment relative to the navicular/cuboid. The differences between the data from the skin and plate protocols were consistently smaller than differences between either protocol and the kinematic data for each bone comprising the segment. The pattern of differences between skin and plate protocols and the actual bone motion showed no systematic pattern. It is unlikely that one rigid body foot model and marker attachment approach is always preferable over another.

Interclinician variation in diabetes foot assessment--a national lottery?

AIM

The aim was to evaluate variation among clinicians in the outcome of assessments of foot health status and risk status in patients with diabetes.

METHODS

Seventeen clinicians assessed three patients with diabetes using a standardized assessment form and risk classification system.

RESULTS

There was variation among clinicians in all aspects of the assessment; recording basic demographic information; taking a medical history; vascular and neurological assessments. Variation was also evident in the risk categories allocated to each of the three patients.

CONCLUSIONS

As a consequence of the variation among clinicians in the foot assessment the same patient would have received different care pathways to monitor and manage their foot health depending upon which clinician undertook their initial assessment. We therefore recommend that more attention is placed on training for objective clinical testing, at both pre- and postgraduate levels.

Effect of errors in the identification of anatomical landmarks on the accuracy of Q angle values

OBJECTIVE

To determine the effect of error in the location of the anterior superior iliac spine, and the centres of the patella and tibial tuberosity, on the measured value of the quadriceps angle.

BACKGROUND

The quadriceps angle is said to be relevant in the etiology and management of patello-femoral pain. However, the issues around measurement accuracy have not been reported.

METHODS

Errors between 1 and 5 mm were introduced to the medial/lateral and vertical co-ordinate data describing the position of the anterior superior iliac spine, the centre of the patella and the centre of the tibial tuberosity, and the effect on the quadriceps angle determined.

RESULTS

Errors between 1 and 5 mm in the medial/lateral location of the centre of the patella produced changes in the quadriceps angle between 1.13 degrees and 5.53 degrees. Errors between 1 and 5 mm in the medial/lateral location of the tibial tuberosity produced changes in the quadriceps angle between 1.02 degrees and 5.18 degrees.

CONCLUSIONS

The quadriceps angle is highly sensitive to error in the definition of the centre of the patella and tibial tuberosity. As an approximation, these centres need to be defined with an accuracy of less than 2 mm if the error in the quadriceps angle is to remain below 5 degrees.

RELEVANCE

Until a clinical technique for measuring the quadriceps angle with a high level of accuracy is developed, the clinical use of the quadriceps angle is questionable.

A study of primary care teaching comparing academic and community-based settings

OBJECTIVE

To compare teaching activity and content between academic and community-based practices used in third year medical student primary care training.

SETTING

Academic and community-based primary care practices participating in third-year internal medicine, family medicine, and primary care core clerkships.

PARTICIPANTS

Five-hundred thirteen preceptor-student encounters involving 32 preceptors and 26 third-year medical students were evaluated.

DESIGN

Student-preceptor pairs collected a convenience sample of data from shared patient encounters. Preceptors recorded the content of teaching interventions, and students independently documented learning points received for each clinical encounter.

MEASUREMENTS AND MAIN RESULTS

Comparison of problem exposure, frequency and content of teaching interventions, and the effect of patient complexity and patient care workload on teaching frequency was made between the academic and community-based practices. Several small differences were found in the frequency of clinical problem exposure between the 2 settings. The frequency and focus of teaching interventions did not differ by practice type. Teaching by community-based preceptors tended to decrease with increased patient care workload, but increased in academically based practices.

CONCLUSIONS

Although several differences exist between educational experiences in community- and academically based primary care practices, they appear to be minor and of minimal educational significance.

The relationship between transverse plane leg rotation and transverse plane motion at the knee and hip during normal walking

The purpose of this study was to test the clinical hypothesis that the magnitude and temporal characteristics of rearfoot complex motion are closely correlated with those of the transverse plane motion at the knee and hip. Twenty subjects underwent kinematic assessment during walking at 108 steps/minute. The transverse plane rotation of the leg relative to the foot was used to indicate rearfoot complex pronation and supination. Taking into account errors inherent in kinematic assessment involving skin mounted markers, it is unlikely that a correlation exists between the range of internal leg rotation during the contact phase and the total range of transverse plane leg rotation during gait and the corresponding values for the transverse plane motion at the knee and hip. Correlation tests were performed to assess the temporal characteristics of the motions at the joints that showed that there was no correlation between the transverse plane motion in the rearfoot complex, knee and hip. Thus the hypothesis that the magnitude and temporal characteristics of rearfoot complex motion are closely correlated with the transverse plane motion at the knee and hip was rejected.