Herman Hamersma

The natural history of sclerosteosis

Sclerosteosis (SCL) is a severe, progressive, autosomal‐recessive craniotubular hyperostosis (MIM 269500). The determinant gene (SOST) has been isolated, and genotype–phenotype correlations, as well as the elucidation of pathogenetic mechanisms, are dependent upon the documentation of the natural history of the condition. For this reason, the course and complications in 63 affected individuals in South Africa, seen over a 38‐year period, have been analyzed. Thirty‐four of these persons died during the course of the survey, 24 from complications related to elevation of intracranial pressure as a result of calvarial overgrowth. The mean age of death in this group of individuals was 33 years, with an even gender distribution. Facial palsy and deafness, as a result of cranial nerve entrapment, developed in childhood in 52 (82%) affected persons. Mandibular overgrowth was present in 46 (73%) adults and syndactyly in 48 (76%). In South Africa in 2002, 29 affected persons were alive, 10 being ≤20 years of age. It is evident that sclerosteosis is a severe disorder which places a considerable burden upon affected individuals and their families.

The syndromic status of sclerosteosis and van Buchem disease

We have examined 50 persons with sclerosteosis in the Afrikaner community of South Africa and 15 individuals with van Buchem disease in Holland. The clinical and radiographic manifestations of these conditions are very similar, the only notable differences being greater severity and syndactyly in the majority of the patients with sclerosteosis.

Patients with sclerosteosis and disease carriers: Human models of the effect of sclerostin on bone turnover

Sclerosteosis is a rare bone sclerosing dysplasia, caused by loss‐of‐function mutations in the SOST gene, encoding sclerostin, a negative regulator of bone formation. The purpose of this study was to determine how the lack of sclerostin affects bone turnover in patients with sclerosteosis and to assess whether sclerostin synthesis is decreased in carriers of the SOST mutation and, if so, to what extent this would affect their phenotype and bone formation. We measured sclerostin, procollagen type 1 amino‐terminal propeptide (P1NP), and cross‐linked C‐telopeptide (CTX) in serum of 19 patients with sclerosteosis, 26 heterozygous carriers of the C69T SOST mutation, and 77 healthy controls. Chips of compact bone discarded during routine surgery were also examined from 6 patients and 4 controls. Sclerostin was undetectable in serum of patients but was measurable in all carriers (mean 15.5 pg/mL; 95% confidence interval [CI] 13.7 to 17.2 pg/mL), in whom it was significantly lower than in healthy controls (mean 40.0 pg/mL; 95% CI 36.9 to 42.7 pg/mL; p < 0.001). P1NP levels were highest in patients (mean 153.7 ng/mL; 95% CI 100.5 to 206.9 ng/mL; p = 0.01 versus carriers, p = 0.002 versus controls), but carriers also had significantly higher P1NP levels (mean 58.3 ng/mL; 95% CI 47.0 to 69.6 ng/mL) than controls (mean 37.8 ng/mL; 95% CI 34.9 to 42.0 ng/mL; p = 0.006). In patients and carriers, P1NP levels declined with age, reaching a plateau after the age of 20 years. Serum sclerostin and P1NP were negatively correlated in carriers and age‐ and gender‐matched controls (r = 0.40, p = 0.008). Mean CTX levels were well within the normal range and did not differ between patients and disease carriers after adjusting for age (p = 0.22). Our results provide in vivo evidence of increased bone formation caused by the absence or decreased synthesis of sclerostin in humans. They also suggest that inhibition of sclerostin can be titrated because the decreased sclerostin levels in disease carriers did not lead to any of the symptoms or complications of the disease but had a positive effect on bone mass. Further studies are needed to clarify the role of sclerostin on bone resorption.

Sclerostin in Mineralized Matrices and van Buchem Disease

Sclerostin is an inhibitor of bone formation expressed by osteocytes. We hypothesized that sclerostin is expressed by cells of the same origin and also embedded within mineralized matrices. In this study, we analyzed (a) sclerostin expression using immunohistochemistry, (b) whether the genomic defect in individuals with van Buchem disease (VBD) was associated with the absence of sclerostin expression, and (c) whether this was associated with hypercementosis. Sclerostin was expressed by cementocytes in mouse and human teeth and by mineralized hypertrophic chondrocytes in the human growth plate. In individuals with VBD, sclerostin expression was absent or strongly decreased in osteocytes and cementocytes. This was associated with increased bone formation, but no overt changes in cementum thickness. In conclusion, sclerostin is expressed by all 3 terminally differentiated cell types embedded within mineralized matrices: osteocytes, cementocytes, and hypertrophic chondrocytes.

A review of the osteopetroses.

The osteopetroses are a group of conditions which are characterized by varying combinations of bony sclerosis and modelling defects. Classical osteopetrosis may be inherited as an autosomal dominant or autosomal recessive: the former variety is benign, heterogeneous and comparatively common, while the latter is precocious, potentially lethal and rare. Many other craniotubular dysplasias and hyperostoses are loosely grouped with the osteopetroses. The commonest of these is the autosomal dominant form of craniometaphyseal dysplasia, while the others which are well known include Pyle disease, and van Buchem disease. Sclerosteosis is a progressive condition in which massive cranial thickening is associated with syndactyly and gigantism. Each of these disorders has specific clinical and radiographic features, which permit recognition. Diagnostic accuracy is crucial for treatment, prognostication and effective genetic management.

Sclerosteosis - an autosomal recessive disorder.

Sclerosteosis is a rare, potentially lethal skeletal disorder in which massive bony overgrowth leads to facial distortion, cranial nerve compression and progressive rise in intra‐cranial pressure. Gigantism and syndactyly of the 2nd and 3rd fingers are associated features. In a nationwide investigation in South Africa, 25 affected individuals in 15 Afrikaner kindreds have been studied. The minimum prevalence of the condition in this community is 1 in 75,000. Analysis of pedigree data confirms that sclerosteosis is an autosomal recessive condition. The gene frequency in the Afrikaner people is estimated at 0.0035, with 10,000 clinically normal heterozygotes in this population. Heterozygote detection may be possible on a basis of recognition of minor changes which are apparent on skull radiographs.

The radiology of sclerosteosis

Sclerosteosis is an unusual disorder in which increased skeletal density is associated with abnormalities of bony modelling. The condition is progressive and complications arise due to cranial nerve involvement. Death from raised intracranial pressure occurs in early adulthood. In order to define the range and extent of the bony abnormalities, the radiographic features of 18 affected individuals have been analysed. The recognition of characteristic changes permits diagnostic precision.

Craniometaphyseal dysplasia--variability of expression within a large family.

Fifteen individuals in five generations of a kindred with branches in South Africa and England had the autosomal dominant form of craniometaphyseal dysplasia. The majority of affected adults had mild to moderate mandibular distortion, while paranasal bossing was a transient manifestation in childhood. Facial palsy with onset in childhood was present either unilaterally or bilaterally in about 30% of the patients, while 50% had auditory dysfunction which varied from mild impairment of hearing to total deafness. Stature and intellect were normal, and bone fragility, osteomyelitis and dyshaemopoesis were not features of the condition. Five other potentially affected family members had deafness or facial palsy of uncertain aetiology in the absence of other stigmata of CMD. It is uncertain whether these complications represent minor degrees of phenotypic expression of the abnormal gene.

Frontometaphyseal dysplasia: autosomal dominant or X-linked?

The clinical and radiographic manifestations in a 45-year-old male with frontometaphyseal dysplasia (FMD) are documented and depicted. Deafness and degenerative osteoarthropathy in weight-bearing joints were the main clinical problems. Widespread patchy cranial sclerosis was reminiscent of Pagets disease, while digital deformity resembled rheumatoid arthritis. On the basis of a review and tabulation of published reports, evidence emerges to support the concept of X-linked inheritance. The relationship between FMD and osteodysplasty remains a matter for speculation.

Oculodento-osseous dysplasia: heterogeneity or variable expression?

Oculodento‐osseous dysplasia (ODOD) has been recognised in three South African patients from two kindreds of Dutch descent. Their ocular, nasal, dental and digital stigmata resembled those of previously reported cases, but their cranial hyperostosis and mandibular overgrowth were of much greater degree. In addition, the two survivors had serious neurological complications consequent upon spinal cord compression at the base of the skull and calcification of the basal ganglia.