Dental abnormalities in rare genetic bone diseases: Literature review

Currently, over 500 rare genetic bone disorders are identified. These diseases are often accompanied by dental abnormalities, which are sometimes the first clue for an early diagnosis. However, not many dentists are sufficiently familiar with phenotypic abnormalities and treatment approaches when they encounter patients with rare diseases. Such patients often need dental treatment but have difficulties in finding a dentist who can treat them appropriately. Herein we focus on major dental phenotypes and summarize their potential causes and mechanisms, if known. We discuss representative diseases, dental treatments, and their effect on the oral health of patients and on oral health-related quality of life. This review can serve as a starting point for dentists to contribute to early diagnosis and further investigate the best treatment options for patients with rare disorders, with the goal of optimizing treatment outcomes.

Rescue of a cherubism bone marrow stromal culture phenotype by reducing TGFβ signaling

We utilized a bone marrow stromal culture system to investigate changes in TGFβ signaling in a mouse model for cherubism (Sh3bp2^KI/KI). Interestingly, bone marrow cultures derived from cherubism mice not only displayed impaired osteoblast differentiation, but also had spontaneous osteoclast formation. PAI1, a target gene of TGFβ signaling, was elevated 2-fold in cherubism CD11b^-,CD45^- cells compared to wild type cells, while the expression of BAMBI, an inhibitor of TGFβ signaling, was down-regulated. We also discovered that treatment of cherubism cultures with antagonists of the TGFβ signaling pathway could largely rescue osteoblast differentiation and markedly reduce spontaneous osteoclast formation. Treatment with the type I TGFβ receptor small molecule inhibitor SB505124 increased osteoblast reporter gene Col1a1-2.3 expression 24-fold and increased the expression of osteoblast gene markers Osterix (Sp7) 25-fold, Bone Sialoprotein (BSP) 7-fold, Osteocalcin (Bglap1) 100-fold, and Dentin Matrix Protein 1 (DMP1) 35-fold. In contrast, SB505124 treatment resulted in a significant reductions in osteoclast number and size. Gene expression analyses for RANKL, a positive regulator of osteoclast formation was 2.5-fold higher in osteoblast cultures derived from Sh3bp2^KI/KI mice compared to wild type cultures, whereas OPG, an inhibitor of RANKL was 5-fold lower. However, SB505124 treatment reduced RANKL almost back down to wild type levels, while increasing OPG expression. Our studies also implicate a role for TGFβ ligands in the etiology of cherubism. Blocking of TGFβ ligands with the monoclonal antibody 1D11 increased Col1a1-2.3 reporter expression 4-fold and 13-fold in cultures derived from Sh3bp2^KI/+ and Sh3bp2^KI/KI mice, respectively. Serum levels of latent TGFβ1 were also 2-fold higher in SH3BP2^KI/KI mice compared to wild type littermates. Taken together, these studies provide evidence that elevated levels of TGFβ signaling may contribute to the disease phenotype of cherubism and a reduction in pathway activity may be an effective therapeutic approach to treat this rare disease.

Genetic study of an Indian family with cherubism

Cherubism (OMIM : 118400) is an autosomal dominant disorder affecting mainly facial bones leading to disfigurement of face needing medical and surgical attention besides impairing the self esteem of person. At present, there is no medical cure and there is limited indication for surgery in such cases. So, correct diagnosis is of paramount importance to both treating physician and family. Here, the authors report a family with two affected members (mother and daughter) who were tested positive for a known pathogenic mutation and thus offered timely treatment and adequate genetic counseling.

3BP2-deficient mice are osteoporotic with impaired osteoblast and osteoclast functions

A fine balance between bone resorption by osteoclasts and bone formation by osteoblasts maintains bone homeostasis. In patients with cherubism, gain-of-function mutations in 3BP2, which is encoded by SH3-domain binding protein 2 (SH3BP2), cause cystic lesions with activated osteoclasts that lead to craniofacial abnormalities. However, little is known about the function of wild-type 3BP2 in regulating bone homeostasis. Here we have shown that 3BP2 is required for the normal function of both osteoblasts and osteoclasts. Initial analysis showed that Sh3bp2-/-mice developed osteoporosis as a result of reduced bone formation despite the fact that bone resorption was impaired. We demonstrated using reciprocal bone marrow chimeras, a cell-intrinsic defect of the osteoblast and osteoclast compartments in vivo. Further, Sh3bp2-/- osteoblasts failed to mature and form mineralized nodules in vitro, while Sh3bp2-/- osteoclasts spread poorly and were unable to effectively degrade dentine matrix in vitro. Finally, we showed that 3BP2 was required for Abl activation in osteoblasts and Src activation in osteoclasts, and demonstrated that the in vitro defect of each cell type was restored by the respective expression of activated forms of these kinases. These findings reveal an unanticipated role for the 3BP2 adapter protein in osteoblast function and in coordinating bone homeostatic signals in both osteoclast and osteoblast lineages.

Increased myeloid cell responses to M-CSF and RANKL cause bone loss and inflammation in SH3BP2 "cherubism" mice

While studies of the adaptor SH3BP2 have implicated a role in receptor-mediated signaling in mast cells and lymphocytes, they have failed to identify its function or explain why SH3BP2 missense mutations cause bone loss and inflammation in patients with cherubism. We demonstrate that Sh3bp2 "cherubism" mice exhibit trabecular bone loss, TNF-alpha-dependent systemic inflammation, and cortical bone erosion. The mutant phenotype is lymphocyte independent and can be transferred to mice carrying wild-type Sh3bp2 alleles through mutant fetal liver cells. Mutant myeloid cells show increased responses to M-CSF and RANKL stimulation, and, through mechanisms of increased ERK 1/2 and SYK phosphorylation/activation, they form macrophages that express high levels of TNF-alpha and osteoclasts that are unusually large. M-CSF and RANKL stimulation of myeloid cells that overexpress wild-type SH3BP2 results in similar large osteoclasts. This indicates that the mutant phenotype reflects gain of SH3BP2 function and suggests that SH3BP2 is a critical regulator of myeloid cell responses to M-CSF and RANKL stimulation.

MMP-20 is predominately a tooth-specific enzyme with a deep catalytic pocket that hydrolyzes type V collagen

Matrix metalloproteinase-20 (MMP-20, enamelysin) has a highly restricted pattern of expression. In healthy tissues, MMP-20 is observed in the enamel organ and pulp organ of developing teeth and is present only as an activated enzyme. To identify other tissues that may express MMP-20, we performed a systematic mouse tissue expression screen. Among the non-tooth tissues assayed, MMP-20 transcripts were identified only in minute quantities within the large intestine. The murine Mmp20 promoter was cloned, sequenced, and assessed for potential tooth-specific regulatory elements. In silico analysis identified four promoter modules that were common to Mmp20 and at least two of three coregulated predominantly tooth-specific genes that encode ameloblastin, amelogenin, and enamelin. We asked if the highly restricted MMP-20 expression pattern was associated with a broad substrate specificity that might preclude its expression in other tissues. An iterative mixture-based random doedecamer peptide library screen with Edman sequencing of MMP-20 cleavage products revealed that, among MMPs previously screened, MMP-20 had unique substrate preferences. These preferences indicate that MMP-20 has a deep and wide catalytic pocket that can accommodate substrates with large aromatic residues in the P1' position. On the basis of matrices derived from the peptide library data, we identified and then confirmed that type V collagen is an MMP-20 substrate. Since type V collagen is not present in dental enamel but is an otherwise widely distributed collagen, and since only active MMP-20 has been observed in teeth, our data suggest that control of MMP-20 activity is primarily regulated by transcriptional means.

Clinical features of tricho-dento-osseous syndrome and presentation of three new cases: an addition to clinical heterogeneity

OBJECTIVE

Tricho-dento-osseous dysplasia is a rare autosomal dominant disorder which involves increased bone density, enamel hypoplasia, enlarged pulp chambers, and molar taurodontism. This study discusses the phenotypic variability of this condition and describes 3 new cases from a large family.

STUDY DESIGN

Three affected females and 1 unaffected female from the same family were clinically and radiographically evaluated. Mutation analysis was performed in the candidate gene DLX3. Phenotypes of affected individuals from 3 generations were compared to an unaffected control.

RESULTS

All affected subjects show increased bone density in long bones and increased thickness and bone density in the skull, especially the skull base. Mandibles are within the upper normal size limits and display increased trabeculation and bone density. No bone loss or regression of the alveolar ridge occurs in older subjects after teeth have been lost.

CONCLUSION

The mutation in DLX3 has positive effects on bone density throughout life. Although the mutation in this family is identical with mutations found in other families it results in clinical and phenotypic variability.

Activation of NFkappaB signal pathways in keloid fibroblasts

Keloids are characterized as an "over-exuberant" healing response resulting in a disproportionate extracellular matrix (ECM) accumulation and tissue fibrosis. In view of the integral role of inflammation and cytokines in the healing response, it is logical to assume that they may play a part in orchestrating the pathology of this "abnormal" healing process. Tumor necrosis factor-alpha (TNF-alpha) is a potent proinflammatory cytokine involved in activation of signaling events and transcriptional programs, such as NFkappaB. This study attempts to determine the difference in NFkappaB and its related genes expression and DNA binding activity between keloid and normal skin fibroblasts. Three keloid and normal skin tissues (NSk) and their derived fibroblasts were used to determine NFkappaB signaling pathway expression using specific cDNA microarrays, Western blot analysis and immunohistochemistry. Electrophoretic mobility gel shift assay (EMSA) was used to assess NFkappaB-binding activity, all assays were performed in the presence and absence of TNF-alpha. TNF-alpha up-regulated 15% of NFkappaB signal pathway related genes in keloid fibroblast compared to normal skin. At the protein level, keloid fibroblasts and tissues showed higher basal levels of TNF- receptor-associated factors-TRAF1, TRAF2-TNF-alpha, inhibitor of apoptosis (c-IAP-1), and NFkappaB, compared with NSk. Keloid fibroblasts showed a constitutive increase in NFkappaB-binding activity in comparison to NSk both with and without TNF-alpha treatment. NFkappaB and its targeted genes, especially the antiapoptotic genes, could play a role in keloid pathogenesis; targeting NFkappaB could help in developing therapeutic interventions for the treatment of keloid scarring.

Genome Scans Provide Evidence for Keloid Susceptibility Loci on Chromosomes 2q23 and 7p11

Keloids are proliferative fibrous growths that result from an excessive tissue response to skin trauma. They often occur sporadically, but in some families a genetic predisposition to keloids has been observed. Here we studied two families with an autosomal dominant inheritance pattern of keloids. One African-American family showed a high degree of variability in the extent of keloid formation between family members, whereas the second family from Japan showed a pattern of full penetrance and the formation of only small keloids. We performed a genome-wide linkage search for genes predisposing to keloid formation in these two families. We identified linkage to chromosome 2q23 (maximal two-point LOD score of 3.01) for the Japanese family. The African-American family showed evidence for a keloid susceptibility locus on chromosome 7p11 (maximal two-point LOD score of 3.16). The observed locus heterogeneity in autosomal dominant keloid disease is consistent with the clinical heterogeneity of this scarring disorder. Dense microsatellite analysis in these two loci was performed and candidate genes were identified. This study provides the first genetic evidence for keloid susceptibility loci and serves as a basis for the identification of responsible genes.

Clinical genetics of familial keloids

BACKGROUND

Keloids are proliferative fibrous growths that result from an excessive tissue response to skin trauma. Most keloids occur sporadically, but some cases are familial. However, the genetics of keloid formation have only rarely been documented, and the mode of inheritance is not known.

OBJECTIVE

To elucidate the clinical genetic characteristics of keloid wound-healing disorder.

OBSERVATIONS

We studied the clinical and genetic characteristics of 14 pedigrees with familial keloids. The ethnicity of these families is mostly African American (n = 10), but also white (n = 1), Japanese (n = 2), and African Caribbean (n = 1). The pedigrees account for 341 family members, of whom 96 displayed keloids. Of the affected family members, 36 are male and 60 are female. The age of onset varies from early childhood to late adulthood. There is variable expression of keloids within the same families: some affected members have only minor earlobe keloids, whereas others have very severe keloids affecting large areas of the body. In the described pedigrees, 7 individuals are obligate unaffected carriers, revealing nonpenetrance in about 6.8% of keloid gene carriers. Syndromes associated with keloids, namely Rubinstein-Taybi and Goeminne syndrome, were not found in these families. Additionally, linkage to the gene loci of these syndromes and X-chromosomal linkage were excluded.

CONCLUSIONS

The pattern of inheritance observed in these families is consistent with an autosomal dominant mode with incomplete clinical penetrance and variable expression. This is the most comprehensive collection of keloid families described to date, and it allows for the first time the elucidation of the clinical genetic characteristics of the familial form of this wound-healing disorder.

Gene for the human transmembrane-type protein tyrosine phosphatase H (PTPRH): genomic structure, fine-mapping and its exclusion as a candidate for Peutz-Jeghers syndrome

Mutations in the serine/threonine kinase STK11 lead to Peutz-Jeghers syndrome (PJS) in a subset of affected individuals. Significant evidence for linkage to a second potential PJS disease locus on 19q13.4 has previously been described in one PJS family (PJS07). In the current study, we investigated this second locus for PJS gene candidates. We mapped the main candidate gene in this region, the gene for the transmembrane-type protein tyrosine phosphatase H (PTPRH), within 15 kb telomeric to the marker D19S880. We determined its genomic structure, and performed mutation analysis of all exons and the exon-intron junctions of the PTPRH gene in the PJS07 family. No disease causing mutation was identified in PTPRH in affected individuals, suggesting the existence of an as yet not identified gene on 19q13.4 as a second PJS gene.

Mutations in the gene encoding c-Abl-binding protein SH3BP2 cause cherubism

Cherubism (MIM 118400) is an autosomal dominant inherited syndrome characterized by excessive bone degradation of the upper and lower jaws followed by development of fibrous tissue masses, which causes a characteristic facial swelling. Here we describe seven mutations in the SH3-binding protein SH3BP2 (MIM 602104) on chromosome 4p16.3 that cause cherubism.

Autosomal dominant craniometaphyseal dysplasia is caused by mutations in the transmembrane protein ANK

Craniometaphyseal dysplasia (CMD) is a rare skeletal disorder characterized by progressive thickening and increased mineral density of craniofacial bones and abnormally developed metaphyses in long bones. Linkage studies mapped the locus for the autosomal dominant form of CMD to an approximately 5-cM interval on chromosome 5p, which is defined by recombinations between loci D5S810 and D5S1954. Mutational analysis of positional candidate genes was performed, and we describe herein three different mutations, in five different families and in isolated cases, in ANK, a multipass transmembrane protein involved in the transport of intracellular pyrophosphate into extracellular matrix. The mutations are two in-frame deletions and one in-frame insertion caused by a splicing defect. All mutations cluster within seven amino acids in one of the six possible cytosolic domains of ANK. These results suggest that the mutated protein has a dominant negative effect on the function of ANK, since reduced levels of pyrophosphate in bone matrix are known to increase mineralization.

A double-deletion mutation in the Pitx3 gene causes arrested lens development in aphakia mice

The recessive aphakia (ak) mouse mutant is characterized by bilateral microphthalmia due to a failure of lens morphogenesis. We fine-mapped the ak locus to the interval between D19Umi1 and D19Mit9, developed new polymorphic markers, and mapped candidate genes by construction of a BAC contig. The Pitx3 gene, known to be expressed in lens primordia, shows zero recombination with the ak mutation on our intersubspecific intercross panel representing 1170 meioses. A recent report described a deletion in the intergenic region between Gbf1 and Pitx3 as the possible ak mutation. Our results differ in that we find not only the distant intergenic deletion, but also a much larger deletion directly in the Pitx3 gene, eliminating exon 1 and extending into intron 1 and the promoter region. Pitx3 transcript levels are severely reduced in ak/ak mice from E11.5 to newborn (5 +/- 1% of the wildtype levels at E13.5), while an involvement of the flanking Gbf1 and Cig30 genes in the aberrant lens development is highly unlikely based on expression analysis. We conclude that the ak mutation consists of two deletions, the larger of which removes part of Pitx3, indicating a crucial role of this gene in early lens development.

Craniosynostosis in cherubism

Cherubism is a rare autosomal dominant fibro-osseous disorder that affects almost exclusively maxilla and mandible. Extracranial skeletal involvement is rare. We report on three affected males in three generations. The youngest affected relative was examined at age 4 months. He also had craniosynostosis. His affected father and grandfather had cherubism and clubbing of the fingers. Cherubism was mapped to region 4p16. Because of the associated cranio-synostosis, we excluded the FGFR3 gene as a candidate gene for cherubism.

Collagen XII mutation disrupts matrix structure of periodontal ligament and skin

Collagen XII has been postulated to organize the extracellular matrix (ECM) architecture of dense connective tissues such as the periodontal ligament (PDL) and skin. The objective of this study was to test this hypothesis in transgenic mice carrying a dominant interference mutation of collagen XII. The truncated alpha1(XII) collagen minigene construct MXIINC3(-), driven by the mouse alpha2(I) collagen promoter, was prepared and used to generate transgenic mouse lines. The PDL matrix fibers of molar teeth lost the ordered architecture characteristic of ligament tissue without noticeable inflammation. Cellular cement appeared to be disrupted at the PDL insertion. By confocal laser scanning microscopy, the PDL of transgenic mice demonstrated swollen and irregularly arranged collagen fibers associated with internal porosity. The skin of transgenic mice revealed the lack of matrix fiber structure in the papillary dermis. These results indicated that the dominant interference mutation of collagen XII disorganized the ECM architecture of PDL and skin.

Structural variation of type XII collagen at its carboxyl-terminal NC1 domain generated by tissue-specific alternative splicing

This paper reports the identification of two structural variations in the NC1 domain of rat and mouse type XII collagen. The long NC1 domain encoding 74 amino acids showed homology to chicken type XII and XIV collagens. The short NC1 domain was composed of 19 amino acids. Through genomic DNA analyses, two alternative exons were identified, each of which contained the variable NC1 sequence. With the amino-terminal NC3 splicing alternatives, we propose here a new descriptive nomenclature: types XIIA-1 and XIIB-1 which include a long NC1 sequence encoded by exon 1 (from the 3'-end), and types XIIA-2 and XIIB-2 which include a short NC1 sequence encoded by exon 2. Types XIIA-1 and XIIB-1, the predominant transcripts in 15-day old mouse embryos, showed decreased expression in 17-day old embryos when type XIIB-2 expression was sustained at constant levels. In adult mice, type XIIB-1 associates with ligament and tendon, whereas type XIIB-2 is expressed in various other tissues. The long NC1 domain contains an extended acidic region (pI = 3.4) followed by a terminal basic region (pI = 13.8). Because the short NC1 domain lacks these features, structural variations in the type XII collagen NC1 domain suggests different functional roles in a tissue-specific fashion.

The gene for cherubism maps to chromosome 4p16

Cherubism is an autosomal dominant disorder that may be related to tooth development and eruption. It is a disorder of age-related bone remodeling, mostly limited to the maxilla and the mandible, with loss of bone in the jaws and its replacement with large amounts of fibrous tissue. We have used a genomewide search with a three-generation family and have established linkage to chromosome 4p16. Three other families affected with cherubism were also genotyped and were mapped to the same locus. The combined LOD score is 4.21 at a recombination fraction of 0, and the locus spans an interval of approximately 22 cM.

Variability in the upstream promoter and intron sequences of the human, mouse and chick type X collagen genes

The type X collagen gene is specifically expressed in hypertrophic chondrocytes during endochondral ossification. Transcription of the type X collagen gene by these differentiated cells is turned on at the same time as transcription of several other cartilage specific genes is switched off and before mineralization of the matrix begins. Analysis of type X collagen promoters for regulatory regions in different cell culture systems and in transgenic mice has given contradictory results suggesting major differences among species. To approach this problem, we have determined the nucleotide sequences of the two introns and upstream promoter sequences of the human and mouse type X collagen genes and compared them with those of bovine and chick. Within the promoter regions, we found three boxes of homology which are nearly continuous in the human gene but have interruptions in the murine gene. One of these interruptions was identified as a complex 1.9 kb repetitive element with homology to LINE, B1, B2 and long terminal repeat sequences. Regulatory elements of the human type X collagen gene are located upstream of the region where the repetitive element is inserted in the mouse gene, making it likely that the repetitive element is inserted between the coding region and regulatory sequences of the murine gene without interfering with its expression pattern. We also compared the sequences of the introns of both genes and found strong conservation. Comparisons of the mammalian sequences with promoter and first intron sequences of the chicken type X collagen gene revealed that only the proximal 120 nucleotides of the promoter were conserved, whereas all other sequences displayed no obvious homology to the murine and human sequences.

Type X collagen expression in osteoarthritic and rheumatoid articular cartilage

Type X collagen is a short chain, non-fibril-forming collagen synthesized primarily by hypertrophic chondrocytes in the growth plate of fetal cartilage. Previously, we have also identified type X collagen in the extracellular matrix of fibrillated, osteoarthritic but not in normal articular cartilage using biochemical and immunohistochemical techniques (von der Mark et al. 1992a). Here we compare the expression of type X with types I and II collagen in normal and degenerate human articular cartilage by in situ hybridization. Signals for cytoplasmic alpha 1(X) collagen mRNA were not detectable in sections of healthy adult articular cartilage, but few specimens of osteoarthritic articular cartilage showed moderate expression of type X collagen in deep zones, but not in the upper fibrillated zone where type X collagen was detected by immunofluorescence. This apparent discrepancy may be explained by the relatively short phases of type X collagen gene activity in osteoarthritis and the short mRNA half-life compared with the longer half-life of the type X collagen protein. At sites of newly formed osteophytic and repair cartilage, alpha 1(X) mRNA was strongly expressed in hypertrophic cells, marking the areas of endochondral bone formation. As in hypertrophic chondrocytes in the proliferative zone of fetal cartilage, type X collagen expression was also associated with strong type II collagen expression.

Determination of 5' ends of specific mRNAs by DNA ligase-dependent amplification

We established a novel way to clone 5' ends of unknown length and sequence of individual cDNAs. T4 DNA ligase is employed to ligate an annealed duplex of complementary primers, one of them with a 4-nucleotide-long randomized overlap, to first-strand cDNA, generating a new 5' end. Subsequent PCR with a down-stream primer and a primer with specificity for this new 5' end leads to products that can easily be cloned and sequenced. Considerations for the choice of primers for ligation and amplification are given. We have used this method to determine the 5' sequences of three independent mRNAs: the human collagen type-X gene, the chicken anchorin CII gene, and the human cytidine deaminase gene. We will discuss this method in comparison with other methods published for the amplification of unknown 5' ends of mRNA species.

Genomic organization and full-length cDNA sequence of human collagen X

We have determined the full-length cDNA sequence of the human alpha 1(X) collagen gene by sequence analysis of a genomic clone ERG [(1991) Dev. Biol. 148, 562-572], and of cDNA fragments generated from a reverse transcribed as alpha 1(X) mRNA by PCR. We defined the promoter region, the transcription initiation site and the full-length 5'-untranslated region. We also report the exon/intron boundaries of the transcript and the complete 3'-untranslated region as well as a 3'-flanking sequence containing two additional polyadenylation signals. The promoter region is homologous to chicken and mouse type X promoters within several highly conserved regions. The genomic organization shows high homologies to chicken and mouse.

In situ hybridization studies on the expression of type X collagen in fetal human cartilage

Type X collagen is a short, non-fibril-forming collagen restricted to the hypertrophic, calcifying zone of growth plate cartilage. It is developmentally regulated and found exclusively in hypertrophic cartilage. Here we report on the structure and distribution of human type X collagen based on the cloning of a PCR fragment covering 292 bp of the carboxy-terminal, non-triple-helical domain. Seventy-five percent of the sequence are identical to that of chicken type X collagen at nucleic acid level and 84% at amino acid level. This probe was used for in situ hybridization analyses of type X collagen expression in a human growth plate. Human fetal cartilage, which is different from the avian cartilage-bone transition zone, showed strong type X collagen expression confined to the lower hypertrophic zone of the growth plate. The upper zone of hypertrophic chondrocytes did not contain alpha 1(X) transcripts, indicating that type X collagen expression follows cellular hypertrophy. The distribution of type X collagen mRNA has been previously unreported in chondrocytes from zones of secondary ossification and in chondrocytes associated with endochondral bone trabecules containing calcified cartilage. In situ hybridization analyses with probes for type I and II collagen on consecutive sections indicated a spatial gradient in chondrocyte differentiation in the human epiphysis. Chondrocytes of low type II collagen expression in the resting zone are followed by proliferating columnar chondrocytes with strong type II collagen expression and a zone of hypertrophic chondrocytes synthesizing type X and type II collagen. In contrast to findings in avian growth cartilage in some of our samples of human epiphyseal cartilage hypertrophic chondrocytes continued to strongly express type II collagen down to the chondro-osseous junction. Transcripts of the alpha 2(I) collagen gene, however, were detected only in perichondrium, vascular cavities, and bone, but not in hypertrophic or any other chondrocytes. The above observations demonstrate that the isolation of the human type X collagen DNA will contribute to studies of pathways of chondrocyte differentiation in the mammalian growth plate.