The aim of this investigation was to better understand the epidemiology and clinical course of disease in pediatric HPP. Here we report characteristics, medical history, laboratory findings including genotypes and treatment history of 50 pediatric patients with HPP that have been followed at the University Children’s Hospital of Wuerzburg over the last 25 years.
Medical history and diagnostic delay: In our cohort more than half of the patients had a so called childhood form with first clinical symptoms after the first 6 months of life. 34% had an infantile form and only 4 out of 50 patients were diagnosed as a perinatal form. Two of the latter died at the age of 3 months. There was no delay in making the diagnosis in perinatal HPP probably due to the severity of the disease in this small cohort with all of them being treated in a neonatal intensive care ward. Interestingly median age at first clinical symptoms in infantile HPP was 2 months and in childhood HPP 9 months. Substantial diagnostic delays between median age at first clinical symptoms and age at diagnosis of HPP was noted in infantile (12 months) and even prolonged in childhood HPP (22.5 months). Unfortunately more differentiated information is lacking: number of doctor contacts, number of diagnostic procedures (laboratory or imaging), exact time from the first clinical symptom until referral to a center, e.g.. Similar findings have been published by Högler et al. with a diagnostic delay of 20.4 months in children (n = 90) and 47.5 months in adults (n = 52) documented in the Global HPP Registry (23). There may be several reasons for the diagnostic delay, including low awareness, heterogeneity of disease manifestations especially in clinically “milder” forms and lack of diagnostic guidelines.
Medical history documented systemic manifestations of HPP in perinatal, infantile and childhood HPP, generally consistent with HPP-related clinical symptoms described in the literature (7–9). First results of the Global HPP Registry documented the importance of considering nonskeletal manifestations as part of the diagnosis of HPP in children and adolescents without differing between age groups. (23).
Interestingly, the most frequent first symptom in our cohort was failure to thrive (34%). Therefor testing for AP activity should be definitely part of a routine diagnostic workup in infants and children with problems in gaining weight, growth retardation and musculoskeletal problems. While suggesting this one has to keep in mind, that children with feeding and growth disorders due to other means than HPP, often have reduced AP levels due to general lack of nutrients, including zink as the catalytic ion of TNAP. As a consequence, careful analysis of AP levels and, in case of suspiciously low levels, subsequent diagnostic measures including the analysis of TNAP levels in leukocytes, substrate analysis and genetic analysis may have to be considered.
Over time impairment of motor skills, impairment of mineralization, premature loss of teeth and failure to thrive were most commonly reported in our cohort. Impairment of mineralisation in infantile and childhood HPP may also be underestimated due to the necessity of radiation exposure. In our cohort we were reluctant to use X-ray diagnostics as they were not performed routinely just in case of acute or prolonged pain problems, suspicion of fractures, e.g. ultrasound and MRI diagnostics were used instead (24). Findings reported in a natural history study of 101 affected children published by Whyte et al. support the clinical impression that the natural history of this disorder is typically one of a chronic but stable condition (25).
Our data also support the fact that HPP in children, regardless of subtype, is not characterized by genuine pathologic fractures, excluding the generalized mineralization deficit in the perinatal form. Other pediatric bone diseases like osteogenesis imperfect are prone to recurrent or poor healing fractures (26). In contrast, adult HPP patiens may well exhibit a significant fracture risk. Musculoskeletal pain has been documented for almost half of the patients being aware that it is difficult to diagnose in small infants probably resulting in an underestimation at least in this age group. On the other hand one cannot completely exclude the possibility of chronically developing myopathies caused by chronically elevated proinflammatory substrate accumulation, which might contribute to explain the higher incidence (55%) of such conditions in the group of childhood HPP, where patients suffered already for some years from this inborn error of metabolism. We found, that in childhood HPP musculoskeletal pain may be the leading symptom and may also be one cause of diagnostic delay if not adequately assigned. In general it appears that a (not exactly quantifiable) threshold of minimum requirement AP activity exists, beyond which important facets of tissue formation and maturation are impaired concerning e.g. the CNS, the lung and bone mineralization, while in milder forms of the disease accumulating substrates can still produce chronic symptoms of dysfunction such as musculoskeletal pain.
Dental manifestations especially premature loss of teeth are common HPP-related symptoms that can be found in all ages and should always prompt a dentist to advise patients and their families for further diagnostic procedures including AP testing.
HPP is associated with an increased incidence of craniosynostosis, hydrocephalus, syringomyelia and chiari malformation (27). In our cohort craniosynostosis has been diagnosed in almost half of the patients predominantly in perinatal and infantile forms. Half of them developed raised intracranial pressure (diagnosed by cranial X-rays, eye examination - papilledema, lumbar punction with measurement of cerebospinal fluid opening pressure) and followed by neurosurgical intervention (skull remodelling). This highlights the importance to carefully look for craniosynostosis in patients with pediatric HPP in particular in more severely affected individuals with early disease manifestations and to follow them very closely in an experienced center for pediatric neurosurgery. The more sutures fuse prematurely the more likely it results in raised pressure due to imbalance between the space requirement of the growing brain and the limited intracranial space (26). This has also been found in our cohort of HPP patients. But interestingly radiological signs of raised intracranial pressure can also be observed in patients with only one single suture being involved; e. g. in a girl (infantile HPP) with premature fusion of the left coronal suture (Fig. 1). This girl did not show clinical symptoms of elevated pressure, no papilledema and an unremarkable MRI like chiari malformation or syringomyelia.
As craniosynostosis usually is a very slow and chronic process it mostly does not result in obvious acute neurological symptoms based on elevated intracranial pressure, like headache, nausea, vomiting or altered consciousness. It must also be taken into account that after a neurosurgical therapy the affected suture may fuse again prematurely. The reason for rapid desmal reossification or sclerosis is not understood. In addition, premature closing of the sutures may come along with ossification/calcification of the meningeal membranes, a particular aggravation of the complexity of the neurosurgical procedure. In our cohort 3 patients needed a second operation. Therefore, patients should be monitored for intracranial pressure at least until the end of brain/skull growth which is around 12 years of age. As the reported incidence of craniosynostosis and even craniosystenosis in pediatric HPP seems to be remarkably lower (15–19) this fact adds an important feature to the protocol/algorithm that we have to be aware of in diagnosis and guidance of these patients.
Laboratory findings and genotypes: AP activity is almost absent in our perinatal forms, very low in infantile HPP (30, reference level 110–590 U/l) and low in childhood HPP (45, reference level 110–550 U/l). From the literature it is known for pediatric HPP that AP levels seem to correlate with age of onset and negatively correlate with disease severity (7–9).
In two HPP-patients with a positive family history AP measurement was in the lower normal range at time of diagnosis (both heterozygous genotype). Both showed suggestive symptoms during the course of the disease and AP activity fell below the lower limit later on. These findings highlight the need for careful clinical assessment, which includes measurement of AP activity also over time (several times if applicable) in children with a positive family history or/and HPP related symptoms. Especially heterozygous carriers harbouring a dominant negative mutation may show a very mild phenotype. Making the diagnosis in these individuals is a great challenge. The “carrier” status of a single mutation should eventually be clinically unremarkable, almost. The physician has to be aware of the lower limits of age-, and sex adjusted AP reference intervals and clinical conditions also resulting in reduced levels.
PLP values were increased in our infantile and childhood patients and may be helpful in establishing the diagnosis of HPP especially if clinical findings and AP measurements are somehow not clear or borderline. One must also be aware that PLP measurement may not be helpful in case of vitamin B6 or multivitamin intake (treatment of seizures, dietary supplement e.g.).
Our findings support the importance to check calcium phosphate metabolism in all patients in general over time (with or without ERT). Especially in more severe forms a low calcium diet may be necessary, at least before ERT. During ERT calcium levels tend to go down, presumably caused by an uptake into bone (“hungry bones”) and need to be monitored even more closely.
In our cohort for all but two patients results of genetic testing were available and a high number of different mutations could be found. Most of our patients (35/48) had a compound heterozygous genotype, 2 had a homozygous genotype. All patients with a single heterozygous mutation (two of them are known to have a dominant negative effect) were not severely affected and characterized as childhood HPP. Interestingly p.Glu191Lys has been found in 18, p.Gly334Asp in 8 and the combination of both most common mutations in 7 patients (2 infantile, 5 childhood). P.Glu191Lys is known to occur with a high frequency (up to 55%) in HPP patients with European ancestry and revealed a moderately reduced AP activity in our in vitro testing (68% wild type AP enzyme activity) and it has no dominant-negative effect (21). P.Gly334Asp has been found previously in homozygous Mennonite patients (founder effect) and in vitro testing showed very low residual activity (1.2%) of this severe mutation and a clear dominant negative effect (21).
The very high number of different mutations in the ALPL gene with various effects on the enzymatic activity in in vitro studies has been correlated with the high variability of phenotypes observed in patients with HPP (28). According to our clinical observations, however, intra- and interfamilial variability of phenotypes can also be observed in patients with identical genotypes suggesting that additional genetic confounders, as well as epigenetic or environmental factors, may also be involved individually or even at a tissue level (21).
Diagnosis of the disease: In our cohort in most cases typical HPP-associated symptoms and subsequently a low AP activity level led to the diagnosis of HPP. To a minor extend high substrate levels were available at time of diagnosis in addition. Substrate analysis was performed only in AP borderline cases or within a clinical trial. Furthermore, a positive family history may lead to further diagnostic procedures. Radiological abnormalities at the time of diagnosis were only documented in less than half of our patients. This may be due to restrictive usage of X-rays with regard to radiation exposure for children. X-ray diagnostics were performed in case of suspicion of fracture or elevated intracranial pressure. Currently X-rays are performed more often due to low exposure doses in modern X-ray equipments and to assess the individual bone phenotype in the light of an approved bone-targeted therapy
It still remains a challenge to establish diagnosis of HPP in heterozygous carriers who might present with non-typical or mild symptoms, nevertheless of relevance for the patient (“carrier” status versus HPP patient, as discussed previously).
Treatment of pediatric HPP: In our cohort nobody was treated with bisphosphonates as these drugs should be avoided because of their similarity with inorganic pyrophosphate and because they limit bone turnover resulting in reduced activity levels of bone-specific alkaline phosphatase. Supplementation with vitamin D seems reasonable due to low serum levels (Table 3). Treatment with NSAIDs was performed in almost half of our patients mainly during childhood when musculoskeletal pain and /or inflammation has been documented (on demand or 4 to 8 weeks).
The high percentage of patients receiving Asfotase alfa (AA) may be explained as our center was a study site in clinical studies of AA before the approval by the EMEA. All patients treated with AA showed compound heterozygous or homozygous mutations in the ALPL gene. All benefited from treatment with regard to bone, muscle, growth, e.g. Further treatment outcome details will be reported elsewhere.
Conclusion: Reported findings support our clinical impression of HPP as a chronic musculoskeletal disease with multi-systemic manifestations. Diagnosis is often delayed in particular in children with not life-threatening disease manifestations like impaired motor skills, ricket-like changes, musculoskeletal pain and inflammation for example.
Our natural history information provides detailed insights into the prevalence of different symptoms, which can help to improve and to shorten the time to diagnosis. As pediatric HPP may mimic rheumatologic conditions one has to keep in mind that simple laboratory tests (AP activity in the serum, PLP in the plasma) can substantiate the clinical differential diagnosis. Genetic testing for mutations in the ALPL gene may add additional information for making the diagnosis of HPP.
The very mild end of the spectrum of even unspecific symptoms poses a particular problem, not to “overdiagnose” HPP. Especially in case of mild or unspecific complaints and a moderately reduced AP, diagnosis should be evaluated very carefully if only one single mutation is found in the ALPL gene. Promising therapeutic options like ERT might help to optimise medical care when an early diagnosis is made.