Peter J. Roughley

What is Intervertebral Disc Degeneration, and What Causes It?

Study Design. Review and reinterpretation of existing literature. Objective. To suggest how intervertebral disc degeneration might be distinguished from the physiologic processes of growth, aging, healing, and adaptive remodeling. Summary of Background Data. The research literature concerning disc degeneration is particularly diverse, and there are no accepted definitions to guide biomedical research, or medicolegal practice. Definitions. The process of disc degeneration is an aberrant, cell-mediated response to progressive structural failure. A degenerate disc is one with structural failure combined with accelerated or advanced signs of aging. Early degenerative changes should refer to accelerated age-related changes in a structurally intact disc. Degenerative disc disease should be applied to a degenerate disc that is also painful. Justification. Structural defects such as endplate fracture, radial fissures, and herniation are easily detected, unambiguous markers of impaired disc function. They are not inevitable with age and are more closely related to pain than any other feature of aging discs. Structural failure is irreversible because adult discs have limited healing potential. It also progresses by physical and biologic mechanisms, and, therefore, is a suitable marker for a degenerative process. Biologic progression occurs because structural failure uncouples the local mechanical environment of disc cells from the overall loading of the disc, so that disc cell responses can be inappropriate or “aberrant.” Animal models confirm that cell-mediated changes always follow structural failure caused by trauma. This definition of disc degeneration simplifies the issue of causality: excessive mechanical loading disrupts a disc’s structure and precipitates a cascade of cell-mediated responses, leading to further disruption. Underlying causes of disc degeneration include genetic inheritance, age, inadequate metabolite transport, and loading history, all of which can weaken discs to such an extent that structural failure occurs during the activities of daily living. The other closely related definitions help to distinguish between degenerate and injured discs, and between discs that are and are not painful.

Biology of intervertebral disc aging and degeneration: involvement of the extracellular matrix.

Study Design. A review of current knowledge and opinions concerning the biologic changes that take place during development, maturation and degeneration of the intervertebral disc. Objective. To provide an overview of the changes that occur in structure and composition of the extracellular matrix of the intervertebral disc and to explain the origin of such changes and their functional consequences. Summary of Background Data. The structure of the intervertebral disc, and, in particular, the composition of its extracellular matrix, changes throughout life, ultimately resulting in tissue degeneration in the adult. Methods. A review of the published scientific literature. Results. In the young disc, the outer anulus fibrosus and inner nucleus pulposus have clear physical and molecular properties, although these differences become less distinct in the adult. The age changes are due to variations in both the abundance and structure of the macromolecules, particularly aggrecan, and the structural variations may be due to changes in both synthesis and degradation. It is not clear how many of the changes are by design to adapt to the altered environment of the growing spine. However, it is commonly thought that the degradative changes are detrimental to disc function, a property that is exacerbated by the inability of the mature avascular disc to remove and replace accumulated degradation products. The rate at which these detrimental changes occur may vary between individuals because of genetic, biomechanical, and nutritional differences. Such changes are thought to form the basis of tissue loss associated with disc degeneration. Conclusion. Changes in intervertebral disc structure throughout life ultimately result in tissue degeneration and the need for medical intervention. Current research is aimed at trying to restore the integrity of the degenerate disc matrix by biologic means, although at present it is not clear what the structure of the most appropriate repair tissue should be or how it can be achieved.

Type V Osteogenesis Imperfecta: A New Form of Brittle Bone Disease

Osteogenesis imperfecta (OI) is commonly subdivided into four clinical types. Among these, OI type IV clearly represents a heterogeneous group of disorders. Here we describe 7 OI patients (3 girls), who would typically be classified as having OI type IV but who can be distinguished from other type IV patients. We propose to call this disease entity OI type V. These children had a history of moderate to severe increased fragility of long bones and vertebral bodies. Four patients had experienced at least one episode of hyperplastic callus formation. The family history was positive for OI in 3 patients, with an autosomal dominant pattern of inheritance. All type V patients had limitations in the range of pronation/supination in one or both forearms, associated with a radiologically apparent calcification of the interosseous membrane. Three patients had anterior dislocation of the radial head. A radiodense metaphyseal band immediately adjacent to the growth plate was a constant feature in growing patients. Lumbar spine bone mineral density was low and similar to age‐matched patients with OI type IV. None of the type V patients presented blue sclerae or dentinogenesis imperfecta, but ligamentous laxity was similar to that in patients with OI type IV. Levels of biochemical markers of bone metabolism generally were within the reference range, but serum alkaline phosphatase and urinary collagen type I N‐telopeptide excretion increased markedly during periods of active hyperplastic callus formation. Qualitative histology of iliac biopsy specimens showed that lamellae were arranged in an irregular fashion or had a meshlike appearance. Quantitative histomorphometry revealed decreased amounts of cortical and cancellous bone, like in OI type IV. However, in contrast to OI type IV, parameters that reflect remodeling activation on cancellous bone were mostly normal in OI type V, while parameters reflecting bone formation processes in individual remodeling sites were clearly decreased. Mutation screening of the coding regions and exon/intron boundaries of both collagen type I genes did not reveal any mutations affecting glycine codons or splice sites. In conclusion, OI type V is a new form of autosomal dominant OI, which does not appear to be associated with collagen type I mutations. The genetic defect underlying this disease remains to be elucidated.

Osteogenesis imperfecta type VI: a form of brittle bone disease with a mineralization defect.

Osteogenesis imperfecta (OI) is a heritable disease of bone in which the hallmark is bone fragility. Usually, the disorder is divided into four groups on clinical grounds. We previously described a group of patients initially classified with OI type IV who had a discrete phenotype including hyperplastic callus formation without evidence of mutations in type I collagen. We called that disease entity OI type V. In this study, we describe another group of 8 patients initially diagnosed with OI type IV who share unique, common characteristics. We propose to name this disorder “OI type VI.” Fractures were first documented between 4 and 18 months of age. Patients with OI type VI sustained more frequent fractures than patients with OI type IV. Sclerae were white or faintly blue and dentinogenesis imperfecta was uniformly absent. All patients had vertebral compression fractures. No patients showed radiological signs of rickets. Lumbar spine areal bone mineral density (aBMD) was low and similar to age‐matched patients with OI type IV. Serum alkaline phosphatase levels were elevated compared with age‐matched patients with type IV OI (409 ± 145 U/liter vs. 295 ± 95 U/liter; p < 0.03 by t‐test). Other biochemical parameters of bone and mineral metabolism were within the reference range. Mutation screening of the coding regions and exon/intron boundaries of both collagen type I genes did not reveal any mutations, and type I collagen protein analyses were normal. Qualitative histology of iliac crest bone biopsy specimens showed an absence of the birefringent pattern of normal lamellar bone under polarized light, often with a “fish‐scale” pattern. Quantitative histomorphometry revealed thin cortices, hyperosteoidosis, and a prolonged mineralization lag time in the presence of a decreased mineral apposition rate. We conclude that type VI OI is a moderate to severe form of brittle bone disease with accumulation of osteoid due to a mineralization defect, in the absence of a disturbance of mineral metabolism. The underlying genetic defect remains to be elucidated.

Cartilage in normal and osteoarthritis conditions.

The preservation of articular cartilage depends on keeping the cartilage architecture intact. Cartilage strength and function depend on both the properties of the tissue and on their structural parameters. The main structural macromolecules are collagen and proteoglycans (aggrecan). During life, cartilage matrix turnover is mediated by a multitude of complex autocrine and paracrine anabolic and catabolic factors. These act on the chondrocytes and can lead to repair, remodeling or catabolic processes like those that occur in osteoarthritis. Osteoarthritis is characterized by degradation and loss of articular cartilage, subchondral bone remodeling, and, at the clinical stage of the disease, inflammation of the synovial membrane. The alterations in osteoarthritic cartilage are numerous and involve morphologic and metabolic changes in chondrocytes, as well as biochemical and structural alterations in the extracellular matrix macromolecules.

Osteogenesis imperfecta type VII: an autosomal recessive form of brittle bone disease.

Osteogenesis imperfecta (OI) is a heritable disease of bone with low bone mass and bone fragility. The disease is generally classified into four types based on clinical features and disease severity, although recently fifth and sixth forms have also been reported. Most forms of OI are autosomal dominant. Rarely, autosomal recessive disease has been described. We report the clinical, radiological, and histological features of four children (age 3.9-8.6 years at last follow-up; all girls) and four adults (age 28-33 years; two women) with a novel form of autosomal recessive OI living in an isolated First Nations community in northern Quebec. In keeping with the established numeric classification for OI forms, we have called this form of the disease OI type VII. The phenotype is moderate to severe, characterized by fractures at birth, bluish sclerae, early deformity of the lower extremities, coxa vara, and osteopenia. Rhizomelia is a prominent clinical feature. Histomorphometric analyses of iliac crest bone samples revealed findings similar to OI type I, with decreased cortical width and trabecular number, increased bone turnover, and preservation of the birefringent pattern of lamellar bone. The disease has subsequently been localized to chromosome 3p22-24.1, which is outside the loci for type I collagen genes. The underlying genetic basis for the disease remains to be determined.

The potential and limitations of a cell-seeded collagen/hyaluronan scaffold to engineer an intervertebral disc-like matrix.

Study Design. The use of a cell-seeded biomatrix for tissue engineering of the intervertebral disc. Objective. To evaluate the ability of a biomatrix to support the viability of intervertebral disc cells and to accumulate the extracellular matrix that they produce. Summary of Background Data. Intervertebral disc degeneration is a common occurrence during adult life that has adverse economic consequences on the health care system. Current surgical treatments are aimed at removing or replacing the degenerate tissue, which can alter the biomechanics of the spine and result in degeneration at adjacent disc levels. The ideal treatment of the degenerate disc would involve biologic repair, and tissue-engineering techniques offer a means to achieve this goal. Methods. Scaffolds of type I collagen and hyaluronan were seeded with bovine nucleus pulposus or anulus fibrosus cells and maintained in culture for up to 60 days in the presence of fetal calf serum or a variety of growth factors to try to generate a tissue whose properties could mimic those of the nucleus pulposus with respect to proteoglycan content. Results. During the culture period, various proteoglycans (aggrecan, decorin, biglycan, fibromodulin, and lumican) and collagens (types I and II) accumulated in the scaffold. Proteoglycan accumulation in the scaffold was greatest under conditions in which transforming growth factor-&bgr;1 was present, but under all conditions, more proteoglycan was lost into the culture medium than retained in the scaffold. Both the nucleus and anulus cells behaved in a similar manner with respect to their ability to synthesize matrix macromolecules and have them retained in the scaffold. By day 60 of culture, the proteoglycan content of the scaffolds never exceeded 10% of that present in the mature nucleus pulposus, although this figure could have been considerably increased if most of the proteoglycan being synthesized could have been retained. Furthermore, proteoglycan retention was not uniform within the scaffold, but increased near its periphery. Conclusions. This work demonstrates that although it is possible to maintain functional disc cells in a biomatrix, it will be necessary to optimize proteoglycan synthesis and retention if any resulting tissue is to be of value in the biologic repair of the degenerate disc. The ability of the anulus cells to replicate the matrix production of the nucleus cells, at least in the collagen/hyaluronan scaffold, suggests that repair may not be limited to the availability of authentic nucleus cells.

Tumor Necrosis Factorα Modulates Matrix Production and Catabolism in Nucleus Pulposus Tissue

Study Design. This study examines changes in the production of extracellular matrix molecules as well as the induction of tissue degradation in in vitro formed nucleus pulposus (NP) tissues following incubation with tumor necrosis factor (TNF)&agr;. Objective. To characterize the response of NP cells to TNF-&agr;, a proinflammatory cytokine present in herniated NP tissues. Summary of Background Data. TNF-&agr; is a proinflammatory cytokine expressed by NP cells of degenerate intervertebral discs. It is implicated in the pain associated with disc herniation, although its role in intervertebral disc degeneration remains poorly understood. Methods. In vitro formed NP tissues were treated with TNF-&agr; (up to 50 ng/mL) over 48 hours. Tissues were assessed for histologic appearance, proteoglycan and collagen contents, as well as proteoglycan and collagen synthesis. Reverse transcriptase polymerase chain reaction was used to determine the effect of TNF-&agr; on NP cell gene expression. Proteoglycan degradation was assessed by immunoblot analysis. Results. At doses of 1–5 ng/mL, TNF-&agr; induced multiple cellular responses, including: decreased expression of both aggrecan and type II collagen genes; decreases in the accumulation and overall synthesis of aggrecan and collagen; increased expression of MMP-1, MMP-3, MMP-13, ADAM-TS4, and ADAM-TS5; and induction of ADAM-TS dependent proteoglycan degradation. Within 48 hours, these cellular responses resulted in NP tissue with only 25% of its original proteoglycan content. Conclusions. Because low levels of TNF-&agr;, comparable to those present physiologically, induced NP tissue degradation, this suggests that TNF-&agr; may contribute to the degenerative changes that occur in disc disease.

The human lumican gene: Organization, chromosomal location, and expression in articular cartilage

A human lumican cDNA sequence was derived by polymerase chain reaction techniques from RNA obtained from intestine, placenta, and articular cartilage. A contiguous sequence of 1729 bases was obtained corresponding to an observed message size of 1.8 kilobases (kb). The cDNA sequence consists of an 80-base pair (bp) 5′-untranslated region, a 1014-bp coding sequence, and a 618-bp 3′-untranslated region terminating in a 17-bp poly(A) tail. The deduced lumican protein sequence has 338 amino acids, including a putative 18-residue signal peptide. The human lumican gene was shown to be spread over about 7.5 kb of genomic DNA and to be located on chromosome 12q22. The gene consists of 3 exons separated by introns of 2.2 and 3.5 kb. The shorter 5′-intron resides 21 bases prior to the translation initiation codon, and the 3′-intron resides 152 bases prior to the translation termination codon. The lumican message is expressed at high levels in adult articular chondrocytes but at low levels in the young juvenile. This age-related trend in message level is not, however, common to all tissues in which the lumican gene is expressed. Lumican is present in the extracellular matrix of human articular cartilage at all ages, although its abundance is far greater in the adult. In the adult cartilage lumican exists predominantly in a glycoprotein form lacking keratan sulfate, whereas the juvenile form of the molecule is a proteoglycan.

Lumican and decorin are differentially expressed in human breast carcinoma.

Previous studies have shown that lumican is expressed and increased in the stroma of breast tumours. Lumican expression has now been examined relative to other members of the small leucine‐rich proteoglycan gene family in normal and neoplastic breast tissues, to begin to determine its role in breast tumour progression. Western blot study showed that lumican protein is highly abundant relative to decorin, while biglycan and fibromodulin are only detected occasionally in breast tissues (n=15 cases). Further analysis of lumican and decorin expression performed in matched normal and tumour tissues by in situ hybridization showed that both mRNAs were expressed by similar fibroblast‐like cells adjacent to epithelium. However, lumican mRNA expression was significantly increased in tumours (n=34, p<0.0001), while decorin mRNA was decreased (p=0.0002) in neoplastic relative to adjacent normal stroma. This was accompanied by a significant increase in lumican protein (n=12, p=0.0122), but not decorin. Further evidence of altered lumican expression in breast cancer was manifested by discordance between lumican mRNA and protein localization in some regions of tumours but not in adjacent morphologically normal tissues. It is concluded that lumican is the most abundant of these proteoglycans in breast tumours and that lumican and decorin are inversely regulated in association with breast tumourigenesis. Copyright