Background
Hypophosphatasia (HP) is an inherited disorder characterized by defective bone and tooth mineralization. HP is due to mutations in the liver/bone/kidney alkaline phosphatase gene (
ALPL, MIM 171760) encoding the tissue-nonspecific alkaline phosphatase (TNAP) [
1,
2].
The disease is highly variable in its clinical expression, due to strong allelic heterogeneity in the
ALPL gene. More than 190 mutations have been described and most of them (79%) are missense mutations. This diversity results in highly variable clinical expressivity and in a great number of compound heterozygous genotypes [
3].
Clinical expression ranges from the extremes of stillbirth without mineralized bone to the isolated premature loss of primary teeth [
2,
4]. Six clinical forms of HP are currently recognized depending upon the age of onset: lethal perinatal (MIM 146300), prenatal (benign), infantile (MIM 241500), childhood (MIM 241510), adult and odontohypophosphatasia [
5].
Severe forms of the disease (perinatal and infantile) are transmitted as an autosomal recessive trait, whereas both autosomal recessive and autosomal dominant transmission have been shown in moderate forms (childhood, adult and odontohypophosphatasia) [
6]. Moreover, one case of
de novo mutations was reported in 2005 [
7].
The birth prevalence of severe HP has been estimated to be 1/100000 [
4]. The incidence of moderate forms has never been estimated but is expected to be much higher [
5].
In addition to clinical and radiographical examinations, laboratory assays are performed in case of suspicion of HP: total serum alkaline phosphatatase activity is markedly reduced. Molecular biology alone can establish the diagnosis. The detection rate is about 95% in the severe forms [
5].
Currently, no established treatment exists for HP. Enzyme replacement using a substitutive enzyme targeting mineralized tissue seems to be a promising way of treatment which may become available in the next few years [
8].
-
In the lethal perinatal subtype, the patients show markedly impaired mineralization
in utero. Some infants survive a few days but have respiratory complications due to hypoplastic lungs and rachitic deformities of the chest [
5].
-
In the prenatal benign form, despite prenatal symptoms, there is a spontaneous improvement of skeletal defect [
5].
-
Infantile HP may appear normal at birth; however, the clinical signs start during the first 6 months: hypercalcemia, rachitic deformities of the chest, premature craniosynostosis, widespread demineralization and rachitic changes in the metaphyses are classical features. In infants who survive (50%), there is often spontaneous improvement in mineralization and remission of clinical problems. Short stature in adulthood and premature shed of deciduous teeth are also common [
1].
-
The childhood form is heterogeneous. Clinical signs appear after 6 months of age. Skeletal deformities, a delay in walking, a short stature and a waddling are common features. Signs of intracranial hypertension or failure to thrive are typical. A history of fractures and bone pain usually exists as well. Premature loss of primary teeth is a trigger sign predicting the diagnosis [
2]. Spontaneous remission of bone symptoms is well known, but the disease may re-appear in middle or late adulthood.
-
The adult subtype develops during middle age. The first complaint may be foot pain, related to stress fractures of the metatarsals. Many of the patients present premature loss of permanent teeth [
6].
-
Odontohypophosphatasia seems to represent a form of the disease with only a dental phenotype. Indeed, this form is characterized by spontaneous exfoliation of fully rooted deciduous teeth, enlarged pulp chambers and root canals, not associated with abnormalities of the skeleton. Biochemical findings are generally indistinguishable from those of patients presenting mild forms of HP [
5,
9,
10]. Odontohypophosphatasia should be considered in any patient with a history of early unexplained loss of teeth [
1].
Oral findings may represent the main clinical manifestations in the moderate subtypes (childhood, adult and odontoHP), which are rarely diagnosed
per se. Furthermore premature exfoliation of primary teeth is a "trigger sign" which can lead to a possible diagnosis. The most frequently reported oral symptoms are premature loss of anterior primary teeth without root resorption, reduced alveolar bone height. Hypoplasia of the cementum and enlargement of the pulp chambers and root canals are other findings [
11]. Others features may exist, such as enamel hypoplasia, delayed dentine formation or delayed eruption [
4,
10,
12‐
17].
The objectives of this study were to highlight dental aspects of HP in all clinical forms, including abnormalities of the permanent dentition and to relate the dental anomalies to the various subtypes of HP. Genotype-phenotype correlations were done in order to orientate the paediatrician and the paediatric dentist towards an earlier detection of the disease, thus leading to a better management of the patient.
Discussion
For the first time, oral findings in five patients affected by several forms of HP are described in the same study. Our purpose was to highlight the differences between all the clinical forms. The classic features previously described in the literature were present in all the subtypes [
10‐
13,
19]. The time point of clinical manifestations is currently used to name the subtype of the disease, like perinatal, infantile (before 6 months) and childhood (after 6 months) forms. Indeed in our patients' sample such a classification is subject to questioning. For example even so affected by a clinically recognised severe infantile form, patient 1 was only diagnosed at 2 years of age. Patient 3 diagnosed at 10 months of age could be considered as a milder form of infantile with delay in diagnosis or as a childhood form. These considerations clearly show that especially the distinction of infantile and childhood hypophosphatasia forms is somewhat arbitrary and the phenotype is more a continuum than a precise subtype, even though, in the literature, these subtypes are usually cited.
Severe forms demonstrated premature loss of anterior teeth before one year of age and exfoliation of posterior teeth also. In the moderate forms, only primary anterior teeth exfoliated. Furthermore, others dental defects such as abnormalities of tooth shape, structure or eruption preferentially occured in the infantile form. We have shown that anomalies of the shape of the crowns and dentin defects, could also be a "trigger feature" pointing to a diagnosis of HP in the severe forms. In addition, we reported bilateral impaction associated with intra-coronal pulp resorption in HP a feature never described in HP before. Nevertheless, the submerged impacted second primary molars can also be seen in the normal child population where permanent second premolars are congenitally absent.
We did not find any patient with inflammation of the gingiva or periodontitis. The presence of periodontitis in HP is controversial in the literature: Baab
et al., Plagmann
et al., and Watanabe
et al. described an association between HP and periodontitis [
16,
20,
21], while Valenza
et al. found no signicative differences in subgingival microbiota of healthy patients and patients suffering from HP [
22]. Baab
et al. and El-Labban
et al. discussed the possibility that the weakened periodontal ligament was considerably more susceptible to bacterial invasion and could promote periodontitis [
20,
23]. Attachment loss in HP may be the result of a cemental or alveolar bone defects rather than the outcome of inflammation induced by bacterial growth. Longitudinal studies will be of interest to follow changes of the microbiota over time [
22].
The eruption of permanent teeth must be considered carefully. As in patient 1, some patients could have lost all their primary teeth prematurely and later present in adulthood with a complete permanent dentition and few or no dental defects [
14]. This may be attributable to a progressive general improvement of affected patients from infancy to adulthood [
6]. Lepe
et al. described a case of childhood HP without dental manifestations in adulthood [
14]. This hypothesis could explain, in addition to incomplete penetrance of the disease, the reason why most of the parents have no detectable dental defects in their permanent dentition. Some may however have a history of early loss of primary teeth and this should be investigated during the medical and familial history. A precise dental status of the family would indeed be also very instructive especially in the cases of compound heterozygosity where parents are affected by one mutation. In another cohort [
9] especially these parents were also shown to have clinical features of dental disease. In our group only the father of patient 2 had a history of early loss of primary teeth.
Moreover, there is no previous case report in the literature which describes patients with more defects in the permanent than in the primary dentition. Another point to be considered is that: at the time of the occurrence of the disease in the adult form, odontogenesis has already been completed. Thus, premature loss of teeth alone should be looked for in patients affected by the adult form without the presence of other dental anomalies. Nevertheless, if some dental anomalies are present, it could be possible that the patient has actually a childhood subtype undiagnosed at the time.
Of interest, are the strong differences between the panoramic views at 12 and 36 years of age for the patient 1. At 13 years old, the patient presented with enlargement of the pulp chambers, a slight reduced alveolar bone height and small bulbous crowns, covered by thin enamel that does not seem to reach the cervical area. Twenty years later, all the teeth are still present, the level of the alveolar bone is almost the same the pulp chambers have secondary dentine apposition. Enamel hypoplasia and a reduction of the enamel thickness were noted in the initial radiograph. A hypothesis of a delay in dentin formation could explain these changes.
In some reported cases, the same defects were observed in both dentitions [
13,
15,
23] and in other cases the permanent dentition appears healthy [
14]. Thus, we can not establish a prognosis for the permanent dentition in the light of the changes in the primary one. This indicates, that carefull monitoring is necessary. Suitable prophylactic programs should be launched, aiming at preserving general health and a healthy oral cavity avoiding any bacterial invasion which could trigger periodontal disease [
11].
Alkaline phosphatase (AP) participates in tooth formation and is seen in dental and peridental cells – ameloblasts, odontoblasts, cementoblasts and osteoclasts [
24]. Moreover, the enzyme is also expressed very intensively in "satellite" cells: osteoprogenitor, supra-ameloblastic, and subodontoblastic cells. This enzyme plays a significant role during two critical phases, initiation and completion of dental biomineralization. AP is intensely expressed during terminal differentiation of osteoblasts and in initial biomineralization and decreases when bone mineralization has been achieved [
24]. These findings and the role of the enzyme during these two essential phases could explain why different types of dental anomalies could be present in HP patients. Van den Bos
et al., observed that both acellular and cellular cementum formation was affected. They also showed that mineralization of dentin is less likely to be under the influence of the inhibitory action of inorganic pyrophosphates (one of the substrates of TNAP) than mineralization of cementum. This may suggest that different regulatory mechanisms operate for mineralization in these two tissues [
25].
More than 190 mutations are currently described worldwide in HP patients. This diversity of mutations results in variable clinical expressions even among the severe or moderate types [
9]. This may explain the heterogeneity of the phenotype and the overlap of the clinical subtypes: for instance, infantile and childhood HP share some clinical symptoms, and patients with adult HP often recall childhood rickets or premature exfoliation of primary teeth [
5,
15,
26]. Depending upon the impact of the mutation on enzyme function, the clinical manifestations are highly variable, which is reflected by the age of onset [
27].
Here patients 1, 2 and 3 carry two heterozygous mutations, one moderate (c.571G>A or c.526G>A), the other one severe (c.1361A>G, c.550C>T or c.648+1G>A) [
18]. This is a common situation in non-lethal forms of HP, the moderate allele allowing the production of AP activity from which depends the severity of the disease. The most midly affected patients of our series were found heterozygotes for the missense mutation c.1250A>G and c.212G>A. These mutations have a dominant negative effect that could explain the mild expression of the disease at the heterozygous state. Indeed, by site-directed mutagenesis we found that cells transfected with the mutations R71H and N417S, and co-transfected with the wild type allele, exhibit 30.5% and 26.5% of wild type alkaline phosphatase activity, respectively, [Fauvert
et al. manuscript in preparation].
We found in our series very good correlation between the severity of the symptoms and the AP value at time of diagnosis. Variation in clinical expression is currently known to correlate well with variable residual enzyme activity. Generally, the more severe the disease, the lower the serum AP activity level appropriate for age [
2]. Nevertheless, AP activity varies greatly with age and sex [
5]. Patient 4, at 1 year of age had a normal serum AP value which at first excluded the diagnosis of HP, but when tested again 2 years later he had a decreased value. Thus, molecular biology is crucial to establish the definitive diagnosis when biochemical and clinical data are not clear enough [
5].
Screening for the mutations in the ALPL gene is also of greater importance for prediction of recurrence risk in future pregnancies, carrier detection and prenatal diagnosis. The molecular diagnosis of each patient increases the knowledge of the level of severity of symptoms associated with each ALPL mutation and improves our ability to provide a better prognosis.
According to Hartsfield, premature exfoliation of primary teeth in children younger than 5 years of age should suggest a genetic or a systemic disease in the absence of trauma, especially, in children younger than 3 years of age [
26]. The differential diagnosis includes inherited immune diseases (Papillon-Lefèvre syndrome, Chediak-Higashi syndrome, neutropenias), inherited non-immune diseases (Ehlers-Danlos syndrome, Coffin-Lowry syndrome, hypophosphatemic rickets and dentinal dysplasia type I), sensory neuropathies neoplasias and early-onset periodontitis [
26]. In moderate HP, the premature and spontaneous loss of primary teeth is in most the cases the only symptom. There is no pain and no association with others oral features as inflammation of the gums, ulcerations or abscesses [
13,
21]. Thus, this symptom is a trigger sign in the milder forms of HP pointing to or predicting the diagnosis [
16,
26]. Premature and spontaneous loss of teeth alone without any history of trauma, should lead to further investigation. For any patient with this symptom, the diagnosis of HP should be considered.
Competing interests
The authors declare that they have no competing interests.