Olivier Bénichou

Six novel missense mutations in the LDL receptor-related protein 5 (LRP5) gene in different conditions with an increased bone density.

Bone is a dynamic tissue that is subject to the balanced processes of bone formation and bone resorption. Imbalance can give rise to skeletal pathologies with increased bone density. In recent years, several genes underlying such sclerosing bone disorders have been identified. The LDL receptor-related protein 5 (LRP5) gene has been shown to be involved in both osteoporosis-pseudoglioma syndrome and the high-bone-mass phenotype and turned out to be an important regulator of peak bone mass in vertebrates. We performed mutation analysis of the LRP5 gene in 10 families or isolated patients with different conditions with an increased bone density, including endosteal hyperostosis, Van Buchem disease, autosomal dominant osteosclerosis, and osteopetrosis type I. Direct sequencing of the LRP5 gene revealed 19 sequence variants. Thirteen of these were confirmed as polymorphisms, but six novel missense mutations (D111Y, G171R, A214T, A214V, A242T, and T253I) are most likely disease causing. Like the previously reported mutation (G171V) that causes the high-bone-mass phenotype, all mutations are located in the aminoterminal part of the gene, before the first epidermal growth factor-like domain. These results indicate that, despite the different diagnoses that can be made, conditions with an increased bone density affecting mainly the cortices of the long bones and the skull are often caused by mutations in the LRP5 gene. Functional analysis of the effects of the various mutations will be of interest, to evaluate whether all the mutations give rise to the same pathogenic mechanism.

Localization of a gene for autosomal dominant osteopetrosis (Albers-Schonberg disease) to chromosome 1p21.

Albers-Schönberg disease, the classical form of osteopetrosis, is an autosomal dominant condition with generalized increased skeletal density due to reduced bone resorption. Characteristic radiological findings are generalized osteosclerosis, with, most typically, end-plate sandwichlike thickening of the vertebrae (Rugger-Jersey spine) and the bone-within-bone (endobones) phenomenon. We studied an extended kindred with Albers-Schönberg disease and found linkage with several markers from chromosome 1p21. The Albers-Schönberg gene is located in a candidate region of approximately 8.5 cM flanked by markers D1S486 and D1S2792. A maximum LOD score (Z(max)) of 4.09 was obtained in multipoint analysis at loci D1S239/D1S248. Possible linkage of osteopetrosis to this chromosomal region was analyzed because the CSF-1 gene, which is mutated in the op/op mouse model for osteopetrosis, is located in 1p21. However, SSCP and mutation analysis in patients did not reveal any abnormality, which excludes the CSF-1 gene as the disease-causing gene. This was confirmed by refined physical mapping of the CSF-1 gene outside the candidate region for the Albers-Schönberg gene. The identification of the molecular defect underlying Albers-Schönberg disease will therefore be dependent on the isolation of other genes from an 8.5-cM candidate region on chromosome 1p21.

Further Evidence for Genetic Heterogeneity Within Type II Autosomal Dominant Osteopetrosis

Type II autosomal dominant osteopetrosis (ADO II) is characterized by an increased bone mass that contrasts with the high frequency of fractures. Linkage analysis performed in an extensive Danish family recently provided evidence for the mapping of an ADO II gene to an 8.5‐cM region in chromosome 1p21 between microsatellite markers D1S486 and D1S2792. We recruited, phenotyped, and haplotyped 4F catheter ADO II families including 18 affected subjects and 29 unaffected subjects in order to narrow the candidate region and to search for genetic heterogeneity. ADO II diagnosis was ascertained by the observation of vertebral end plate thickening in at least 2 patients from successive generations. Linkage studies involved five microsatellite markers (D1S486, D1S206, D1S495, D1S248, and D1S2792) spanning 1p21. Haplotype analyses of two of our families clearly excluded the tested locus. The two remaining families gave poorly informative results. These results, combined with those previously reported in two American families, suggest that chromosomal region 1p21 is most likely a minor locus for ADO II.

Exclusion of the chromosomal 1p21 region in a large pedigree with a phenotypic variant of type II autosomal dominant osteopetrosis

Abstract Purpose. To describe a large family including nine subjects with a mild phenotypic variant of type II autosomal dominant osteopetrosis (ADO II). Methods. Clinical and radiological description. Results are compared to those previously obtained in 42 patients (23 families) with classical ADO II. The present family was haplotyped with five markers from the chromosomal 1p21 region, which was previously shown to be linked to ADO II in one out of five tested families. Results. Radiological manifestations included isolated vertebral endplate thickening (22% vs 6% in ADO II), or isolated ‘bone in bone’ sign (22% vs 0% in ADO II), or both (33% vs 94% in ADO II), or a high bone density without segmentary osteosclerosis (lumbar or femoral Z-score above +4 SD) (22% vs 0% in ADO II). Radiographic penetrance was estimated to be 47% (90% in ADO II). Fractures were rare (0.67 vs 3.3/patient in ADO II) and occurred after significant trauma. Non-insulin-dependent diabetes occurred in 11 family members, without obvious co-segregation with osteopetrosis. Linkage analysis excluded the mutated gene in this family from being located in the chromosomal 1p21 region. Conclusion. In the absence of a clear alternative diagnosis, the nine subjects reported here most likely have a mild form of ADO II, with a variable radiological expression. No definite evidence for this hypothesis can be provided until ADO II mutation(s) have been identified.