SPINE Volume 32, Number 25, pp 2820 –2826 ©2007, Lippincott Williams & Wilkins, Inc. Expression of the Trp2 Allele of COL9A2 Is Associated With Alterations in the Mechanical Properties of Human Intervertebral Discs Darwesh M. K. Aladin, MPhil,* Kenneth M. C. Cheung, FRCS, FHKCOS, FHKAM(Orth),* Danny Chan, PhD,† Anita F. Y. Yee, BSc,* Jeffrey J. T. Jim, PhD,*† Keith D. K. Luk, FRCSE, FRCSG, FRACS, MCh(Orth), FHKAM(Orth),* and William W. Lu, PhD* Study Design. Biomechanical study into the associa- tion between genetic polymorphism in COL9A2 and me- chanical properties of human nucleus pulposus. Objective. To examine whether there is an association between genetic polymorphism in a structural gene, and alterations in the mechanical properties of the interverte- bral discs that may predispose to disc degeneration. Summary of Background Data. Genetic studies have demonstrated that a polymorphism (Trp2 allele) in COL9A2 coding for 2 chain of collagen IX predisposes the individual to disc degeneration. The mechanism of this predisposition is not known. Methods. Blood and whole disc samples were re- trieved from adolescents and young adults during scoli- osis surgery, degenerated discs were retrieved from pa- tients with back pain during anterior spinal fusion. Anulus fibrosus and nucleus pulposus from a set of the scoliosis discs were used to perform immunohistochemistry to demonstrate the presence of collagen IX in the scoliosis discs. For the remaining samples, DNA was extracted from blood to determine the Trp2 status by sequencing. Nondegenerated (Trp2-), nondegenerated (Trp2+), and degenerated (Trp2-) nucleus pulposus samples were tested in confined compression. Swelling pressure and compressive modulus were measured and compared be- tween groups. Results. Positive staining of collagen IX was detected in both anulus fibrosus and nucleus pulposus sections confirming its presence in the scoliosis discs. The mean swelling pressure and compressive modulus values of 6 nondegenerated (Trp2+) samples (swelling pressure = 0.0019 MPa, compressive modulus = 0.97 MPa) were significantly lower (P  0.05) than those of the 6 nonde- generated (Trp2-) samples (swelling pressure = 0.015 MPa; compressive modulus = 1.89 MPa). Conclusion. This is the first study to demonstrate an association between the Trp2 allele and disc mechanics, thus relating genetic variations and debilitating mechan- ical alterations that may ultimately result in intervertebral disc degeneration. Key words: intervertebral disc, degeneration, nucleus pulposus, Trp2, COL9A2, collagen, polymorphism, bio- mechanics. Spine 2007;32:2820 –2826 Intervertebral disc degeneration is a common problem in young individuals causing sciatica and low back pain. Although aging and loading patterns are known to influence the function of intervertebral discs, 1 re- cently genetic factors have been implicated as an im- portant predisposing factor. 2–5 A heterozygous substi- tution of tryptophan for either Gln 326 or Arg 326 (Trp2 allele) in COL9A2 coding for the 2 chain of collagen IX, is a predisposing genetic factor. Its relevance has been verified in both Finnish and Chinese populations. The prevalence of Trp2 has been reported in 4% in Finns, 6 and 20% in the Southern Chinese. 7 In the lat- ter study, individuals carrying the Trp2 allele were found to be 4 times more likely to develop tears in the anulus fibrosus between 30 to 39 years of age, with disc degeneration and endplate herniations occurring between 40 to 49 years of age. 7 The precise function of collagen IX, and how the alterations in coding sequence of the COL9A2 gene results in intervertebral disc degeneration is not known. Collagen IX molecules are cross-linked to one another, with covalent bonds forming between either 1(IX) or 3((IX) of the COL2 domains and the C- terminal NC1 domain of 3(IX). 8 The Trp change (Q326W) in 2(IX) is located only 3 amino acid resi- dues away from the NH 2 -terminal of the covalent cross-link between 3(IX) and collagen II. 6 Thus, there are a number of postulations. 9 First, with colla- gen IX’s physical location on collagen II fibrils and their multiple cross-links, collagen IX may play an important role in the physical integrity of the extracel- lular matrix; by forming an interconnected network, they resist the distension by aggrecan and water, to- gether helping to absorb and distribute loads applied to the spine, yet preventing excessive deformation of the matrix. Second, with its COL3 and NC4 domains forming an arm that projects into the perifibrillar ma- trix, it may participate in extracellular matrix ho- meostasis by acting as a molecular mediator. From the Departments of *Orthopaedics and Traumatology and †Bio- chemistry, University of Hong Kong, Hong Kong, China. Professor Cheung and Mr. Aladin contributed equally to the study and prepara- tion of the manuscript. Acknowledgment date: October 18, 2006. Revision date: March 6, 2007. Acceptance date: March 13, 2007. Supported by grants from the Hong Kong Research Grants Council (HKU 7378/04 M) and University Grants Committee (AoE/M-04/04). The manuscript submitted does not contain information about medical device(s)/drug(s). Other funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript. Address correspondence and reprint requests to William W. Lu, PhD, Department of Orthopaedics and Traumatology, LKS Faculty of Med- icine, University of Hong Kong, Pokfulam, Hong Kong; E-mail: wwlu@hkusua.hku.hk 2820