Genes and the Achilles Tendon 27

269

27

of 292 Hungarian patients with primary Achilles tendon ruptures or re-ruptures and 540 patients with various other tendon ruptures with a control group consisting of 1.2 million subjects represen- tative of the Hungarian population. The frequency of blood group O was signifi cantly higher (53.1%) in the patients with Achilles tendon ruptures than for the general Hungarian population (31.1%).

The O blood group was also higher in all the other investigated tendon ruptures, which included rupture of the long head of biceps (48.2%), exten- sor pollicis longus (53.9%), and the quadriceps (49.0%). Interestingly, 68.7% of the patients with multiple tendon ruptures (48) or re-ruptures (35) were of blood group O. No association was found in this study between the rhesus group and any of the tendon rupture groups.

Blood group O was still signifi cantly higher in the combined tendon rupture group (54%) when 443 additional subjects were recruited and ana- lyzed.

Kujala et al.

studied a group consisting of 917 Finnish patients diagnosed with a variety of musculoskeletal soft-tissue injuries. This group included 86 patients diagnosed with Achilles tendon ruptures and 63 with chronic Achilles ten- dinopathy. The frequency of the blood group O among the patients diagnosed with Achilles tendon ruptures (31.2%) was not signifi cantly higher than in the control group (31.4%). The blood group O frequency was, however, higher in the patients diagnosed with Achilles tendinopathy (42.9%). The A/O ratios for the groups with rupture (1.0) or tendinopathy (0.7) of the Achilles tendon were lower than for the control population (1.42). The ABO blood group was not associated Gene therapy delivers genetic material to cells to

alter synthesis and function, and can be achieved via viral vectors or liposomes.

Several animal studies have investigated the feasibility of gene transfer to tendons. Liposome constructs have been used to deliver β-galactosidase to rat patellar tendons.

In vivo and ex vivo adenoviral transduc- tion of the lac Z gene into rabbit patellar tenocytes has been reported (Fig. 27.1). Gene expression lasted for 6 weeks, possibly long enough for clini- cal applications.

Apart from direct injection of vectors, gene transfer has been achieved via intra- arterial injection of liposomes.

Animal studies have demonstrated that gene therapy can be used to alter the healing environ- ment of tendons. Adenoviral transfection of Focal Adhesion Kinase (FAK) into partially lacerated chicken fl exor tendons resulted in an expected increase in adhesion formation.

Although this study reports an adverse outcome, it proves the feasibility of gene therapy as a management modality.

Genetic Susceptibility to Achilles Tendon Injury

Genetic factors may be associated with an individ- ual’s susceptibility to Achilles tendon injury.

This was originally proposed in studies reporting an association between the ABO blood group and Achilles tendon ruptures or chronic Achilles tendinopathy.

In a retrospective study, Jozsa et al.

compared the frequencies of the ABO and Rh blood groups

Genes and the Achilles Tendon

Adam Ajis and Nicola Maffulli

with any of the other soft-tissue injuries. These soft-tissue injuries included rotator cuff impinge- ment (n = 142), patellar dislocation (n = 92), ante- rior cruciate ligament rupture (n = 205), spondylolisthesis (n = 177), and intervertebral disc herniation (n = 152).

Årøen et al.

have suggested that there may be a genetic predisposition toward tendon ruptures as they observed that 9 of the 10 subjects who suf- fered a contralateral Achilles tendon rupture were of blood group B. The investigators, however, did not put forward suggestions why blood group B instead of blood group O was associated with Achilles tendon rupture in their study.

Contrary to these fi ndings, other studies inves- tigating Finish, German, and Scottish populations have not found an association between the ABO blood groups and Achilles tendon ruptures.

Since the ABO gene encodes for transferases, some investigators have suggested that the differ-

ent enzymes produced by the ABO gene deter- mined not only the structure of the glycoprotein antigens on the red blood cells but also the struc- ture of some of the glycoproteins found in the ground substance of tendons.

Others have, how- ever, suggested that the association of the ABO gene with tendon injuries is not directly linked to the ABO blood group antigens. These investiga- tors have proposed that other genes, closely linked to the ABO gene on the tip of the long arm of chro- mosome 9q32-q34, which encode for components of the extracellular matrix, are more likely to be associated with Achilles tendon pathology.

There are examples of other pathologies, such as the nail-patella syndrome, where the ABO blood group was initially shown to be associated with the condition. It was subsequently discov- ered that the LMX1B gene, closely linked to the ABO gene, encoded for a protein responsible for the pathology.

Vector binds to cells membrane

Modified DNA injected in vectoor

Vector is packaged in vescicle

Vescicle breaks down releasing vector

Cell makes protein using new gene Adenovirus gene

therpay Viral

DNA

Viral DNA

Vector (Adenovirus)

Vector injects new gene into nucleus New

Gene

New gene

FIGURE 27.1. Schematics of adenovirus gene therapy.

Since the ABO blood group has been shown to be associated with Achilles tendon pathology in some studies, possible candidate genes were iden- tifi ed, located on the tip of the long arm of chro- mosome 9, (9q32-q34), closely linked to the ABO gene that might be associated with Achilles tendon pathology. Two of these genes, namely tenascin-C gene and COL5A1, encode for structural compo- nents of tendons.

The COL5A1 gene encodes for the pro alpha 1(V) collagen chain, found in most of the isoforms of type V collagen. The major isoform of type V collagen is a heterotrimer consisting of two pro alpha 1(V) chains and one pro alpha 2(V) chain.

Trace amounts of type V collagen are found in tendons, where it forms heterotypic fi bers with type I collagen.

Most investigators have specu- lated, based on the function of type V collagen in the cornea, that it plays an important role in regu- lating fi brillogenesis and modulating fi bril growth in tendons.

Dressler et al.

reported an age-dependent increase in the content of type V collagen, with a decrease in fi bril diameter and biomechanical properties in the rabbit patellar tendon. In addi- tion, Goncalves-Neto et al.

demonstrated an increase in collagen types III and V, together with a reduction in the content of type I collagen, in biopsy samples of tendons from patients with pos- terior tibial tendon dysfunction syndrome. A BstUI restriction fragment length polymorphism within its 3 ′-untranslated region of the COL5A1 gene has been shown to be associated with Achilles tendon pathology and more specifi cally chronic Achilles tendinopathy (p = 0.0009).

In addition, individuals with the A2 allele of this gene were less likely to present with symptoms of tendinopathy (odds ratio of 2.6; 95% CI 1.5–4.5, p = 0.0005).

Although the COL5A1 gene is an ideal candi- date gene for Achilles tendon pathology and more specifi cally chronic Achilles tendinopathy, the fi ndings discussed above do not prove that type V collagen is involved in the etiology. It is possible that another gene closely linked to the COL5A1 and ABO genes on the tip of the long arm of chro- mosome 9 encodes for a protein directly involved in the pathogenesis of Achilles tendon injuries.

One such gene, the tenascin-C or hexabrachion gene, is expressed in tendons.

Since tenascin-C is able to bind to various components of the extra-

cellular matrix and to cell receptors, it may play an important role in regulating cell–matrix inter- actions.

In normal adult tendons, tenascin-C is localized predominantly in regions responsible for transmitting high levels of mechanical force such as myotendinous and osteotendinous junc- tions.

Tenascin-C is also localized around the cells and the collagen fi bers.

In addition, Järvinen et al.

have shown that expression of the tenas- cin-C gene is regulated in a dose-dependent manner by mechanical loading in tendons. Iso- forms of tenascin-C with distinct functions are produced by alternative splicing of the primary transcript.

Riley et al.

have shown that healthy tendons express a small 200 KDa tenascin-C isoform, while degenerate tendons express a func- tionally distinct, larger 300 KDa isoform. In support of this fi nding, Ireland et al.

but not Alfredson et al.

have reported an increase in tenascin-C expression in biopsy samples of chronic Achilles tendinopathies using cDNA arrays.

The GT dinucleotide repeat polymorphism within intron 17 of the tenascin-C gene is also associated with Achilles tendon injury.

Alleles containing 12 and 14 GT repeats were overrepre- sented in individuals with the injury, while the alleles containing 13 and 17 repeats were under- represented. Individuals who were homozygous or heterozygous for the underrepresented alleles (13 and 17 repeats), but did not contain an over- represented allele (12 and 14 repeats), had a lower risk of developing Achilles tendon injury (odds ratio of 6.2, 95% CI 3.5–11.0).

A single gene, or a group of genes, on the tip of the long arm of chromosome 9 are highly unlikely to be exclusively associated with the development of the symptoms of Achilles tendon injury. It is more probable that this condition is polygenic, and other genes that encode for important struc- tural components of tendons are also associated with Achilles tendon injury. Since the COL5A1 and tenascin-C genes have been shown to be asso- ciated with Achilles tendon injury, any gene that encodes for proteins involved in the same biologi- cal processes as type V collagen and tenascin-C within tendons would also be ideal genetic markers for tendon injury.

As mentioned above, type V collagen is involved

in the formation of type I collagen–containing

fi bers. Several other proteins are also involved in

fi brillogenesis, including collagen types XI, XII, and XIV, the proteoglycans decorin, lumican, and fi bromodulin, as well as the matricellular protein, thrombospondin 2.

In addition to tenascin-C, other proteins such as type XII collagen and type XIV collagen are also expressed in both tendons and ligaments and regulated by mechanical stretch.

It has also been postulated that colla- gen types XII and XIV play an important role in the regulation of fi bril assembly due to their ability to interact with proteoglycans such as decorin, lumican, and fi bromodulin.

In addition, at immunoelectron microscopy, both these collagen types were associated with the surface of collagen fi brils, suggesting that they might be able to form interfi brillar connections and mediate fi bril interaction with other extracel- lular and cell surface molecules.

The gene (COL12A1) that encodes for the alpha chain found in type XII collagen has been mapped to chromo- some 6q12-q14. In addition, the COL9A1, COL10A1, and COL19A1 genes have also been mapped to chromosome 6, while the COL14A1 gene has been mapped to chromosome 8q23. Both collagen types XII and XIV are homotrimers and belong, together with collagen types IX, XVI, XIX, and XX, to the subfamily of fi bril-associated colla- gens with interrupted triple helices (FACIT).

Although only two specifi c genetic elements, namely the COL5A1 and tenascin-C genes, have been shown to be associated with Achilles tendon injury to date, neither these nor any other genes have been shown to be associated with any other overuse tendon or acute ligament injuries. There- fore, any genes located on the tip of the long arm of chromosome 9 (COL5A1, tenascin-C gene, COL15A1, COL27A1, and LAMC3) and on chro- mosome 6 (COL9A1, COL10A1, COL12A1, and COL19A1) that encode for a protein found in tendons could be ideal candidate genetic markers for tendon and ligament pathologies. In addition, genes that encode for proteins involved in the structure of tendons and ligaments (type I and III collagens, elastin, and fi bronectin), fi brillogenesis (type V and XI collagens, decorin, lumican, fi bro- modulin, and thrombospondin 2), and are regu- lated by mechanical loading (tenascin-C and type XII and XIV collagens), could also be ideal candi- date genetic markers for tendon and ligament pathologies.

Experimental Applications of Gene Therapy

During healing, levels of collagen type V increase, and persistently elevated levels have been found up to 52 weeks after injury in the rabbit medial collateral ligament. Elevated levels of collagen type V may favor the formation of smaller type I collagen fi brils, which in turn results in reduced mechanical strength.

Human patellar teno- cytes transfected with specifi c antisense oligonu- cleotides synthesized reduced amounts of collagen type V.

Complementary deoxyribonucleic acid (cDNA) for platelet-derived growth factor B was trans- fected into rat patellar tendons using liposomes.

The medial half of the patellar tendon was tran- sected. Platelet-derived growth factor B resulted in an early increase in angiogenesis, and collagen deposition and matrix synthesis was greater at 4 weeks. However, there were no differences between the treated and control groups by 8 weeks.

Bone morphogenetic protein 12 (BMP-12) is the human analog of murine GDF-7.

BMP-12 increases gene expression of procollagen types I and III in human patellar tenocytes and is found at sites of tendon remodeling.

BMP-12 increased collagen type I synthesis by 30% in chicken fl exor tenocytes, and, when tenocytes transfected with the BMP-12 gene were applied to a chicken fl exor tendon laceration model, a twofold increase in tensile strength and load to failure was seen after 4 weeks.

Conclusion

There is a high incidence of tendon and ligament injuries during exercise activities, but the exact etiology of this condition is not fully understood.

Some studies have suggested that there is, at least

in part, a genetic component involved in suscep-

tibility to Achilles and other tendon injuries. Poly-

morphisms within the COL5A1 and tenascin-C

genes are associated with Achilles tendon injuries

in a physically active population, but further

research is needed to determine which other genes

are involved.

Gene therapy can be used to manipulate the healing environment for up to 8–10 weeks. This may be long enough to be clinically signifi cant.

Many genes may prove benefi cial to tendon healing, and further research is required to estab- lish the most advantageous genes to transfer.

Many of the studies reviewed above have been conducted in tendon transection models, but gene therapy may also improve healing in tendinopathy.

References

1. Nakamura N, Timmermann SA, Hart DA, Kaneda Y, Shrive NG, Shino K, et al. A comparison of in vivo gene delivery methods for antisense therapy in ligament healing. Gene Ther 1998; 5(11):1455–

1461.

2. Nakamura N, Shino K, Natsuume T, Horibe S, Mat- sumoto N, Kaneda Y, et al. Early biological effect of in vivo gene transfer of platelet-derived growth factor (PDGF)-B into healing patellar ligament.

Gene Ther 1998; 5(9):1165–1170.

3. Nakamura N, Horibe S, Matsumoto N, Tomita T, Natsuume T, Kaneda Y, et al. Transient introduc- tion of a foreign gene into healing rat patellar liga- ment. J Clin Invest 1996 Jan 1; 97(1):226–231.

4. Gerich TG, Kang R, Fu FH, Robbins PD, Evans CH.

Gene transfer to the rabbit patellar tendon:

Potential for genetic enhancement of tendon and ligament healing. Gene Ther 1996; 3(12):1089–

1093.

5. Gerich TG, Kang R, Fu FH, Robbins PD, Evans CH.

Gene transfer to the patellar tendon. Knee Surg Sports Traumatol Arthrosc 1997; 5(2):118–123.

6. Ozkan I, Shino K, Nakamura N, Natsuume T, Mat- sumoto N, Horibe S, et al. Direct in vivo gene trans- fer to healing rat patellar ligament by intra-arterial delivery of haemagglutinating virus of Japan lipo- somes. Eur J Clin Invest 1999; 29(1):63–67.

7. Lou J, Kubota H, Hotokezaka S, Ludwig FJ, Manske PR. In vivo gene transfer and overexpression of focal adhesion kinase (pp125 FAK) mediated by recombinant adenovirus-induced tendon adhesion formation and epitenon cell change. J Orthop Res 1997; 15(6):911–918.

8. Kannus P, Natri A. Etiology and pathophysiology of tendon ruptures in sports. Scand J Med Sci Sports 1997 Apr; 7(2):107–112.

9. Aroen A, Helgo D, Granlund OG, Bahr R. Contra- lateral tendon rupture risk is increased in individu- als with a previous Achilles tendon rupture. Scand J Med Sci Sports 2004 Feb; 14(1):30–33.

10. Mokone GG, Gajjar M, September AV, Schwellnus MP, Greenberg J, Noakes TD, Collins M. The guanine-thymine dinucleotide repeat polymor- phism within the tenascin-C gene is associated with Achilles tendon injuries. Am J Sports Med 2005 Jul;

33(7):1016–1021.

11. Jozsa L, Balint JB, Kannus P, Reffy A, Barzo M.

Distribution of blood groups in patients with tendon rupture: An analysis of 832 cases. J Bone Joint Surg Br 1989 Mar; 71(2):272–274.

12. Kujala UM, Jarvinen M, Natri A, Lehto M, Neli- markka O, Hurme M, Virta L, Finne J. ABO blood groups and musculoskeletal injuries. Injury 1992;

23(2):131–133.

13. Aroen A, Helgo D, Granlund OG, Bahr R. Contra- lateral tendon rupture risk is increased in individu- als with a previous Achilles tendon rupture. Scand J Med Sci Sports 2004 Feb; 14(1):30–33.

14. Leppilahti J, Puranen J, Orava S. ABO blood group and Achilles tendon rupture. Ann Chir Gynaecol 1996; 85(4):369–371.

15. Maffulli N, Reaper JA, Waterston SW, Ahya T. ABO blood groups and Achilles tendon rupture in the Grampian Region of Scotland. Clin J Sport Med 2000 Oct; 10(4):269–271.

16. Kujala UM, Jarvinen M, Natri A, Lehto M, Neli- markka O, Hurme M, Virta L, Finne J. ABO blood groups and musculoskeletal injuries. Injury 1992;

23(2):131–133.

17. Bongers EM, Gubler MC, Knoers NV. Nail-patella syndrome: Overview on clinical and molecular fi ndings. Pediatr Nephrol 2002 Sep; 17(9):703–712.

18. Silver FH, Freeman JW, Seehra GP. Collagen self-assembly and the development of tendon mechanical properties. J Biomech 2003 Oct;

36(10):1529–1553.

19. Birk DE. Type V collagen: Heterotypic type I/V col- lagen interactions in the regulation of fi bril assem- bly. Micron 2001 Apr; 32(3):223–237.

20. Riley G. The pathogenesis of tendinopathy: A molecular perspective. Rheumatology (Oxford) 2004 Feb; 43(2):131–142.

21. Dressler MR, Butler DL, Wenstrup R, Awad HA, Smith F, Boivin GP. A potential mechanism for age- related declines in patellar tendon biomechanics.

J Orthop Res 2002 Nov; 20(6):1315–1322.

22. Goncalves-Neto J, Witzel SS, Teodoro WR, Carv- alho-Junior AE, Fernandes TD, Yoshinari HH.

Changes in collagen matrix composition in human posterior tibial tendon dysfunction. Joint Bone Spine 2002 Mar; 69(2):189–194.

23. Mokone GG, Schwellnus MP, Noakes TD, Collins M. The COL5A1 gene and Achilles tendon pathol- ogy. Scand J Med Sci Sports 2006 Feb; 16(1):19–26.

24. Chiquet M, Fambrough DM. Chick myotendinous antigen: I. A monoclonal antibody as a marker for tendon and muscle morphogenesis. J Cell Biol 1984 Jun; 8(6):1926–1936.

25. Chiquet M, Fambrough DM. Chick myotendinous antigen: II. A novel extracellular glycoprotein complex consisting of large disulfi de-linked sub- units. J Cell Biol 1984 Jun; 98(6):1937–1946.

26. Jarvinen TA, Jozsa L, Kannus P, Jarvinen TL, Kvist M, Hurme T, Isola J, Kalimo H, Jarvinen M.

Mechanical loading regulates tenascin-C expres- sion in the osteotendinous junction. J Cell Sci 1999 Sep; 112(Pt 18):3157–3166.

27. Jones FS, Jones PL. The tenascin family of ECM glycoproteins: Structure, function, and regulation during embryonic development and tissue remod- eling. Dev Dyn 2000 Jun; 218(2):235–259.

28. Jarvinen TA, Jozsa L, Kannus P, Jarvinen TL, Hurme T, Kvist M, Pelto-Huikko M, Kalimo H, Jarvinen M.

Mechanical loading regulates the expression of tenascin-C in the myotendinous junction and tendon but does not induce de novo synthesis in the skeletal muscle. J Cell Sci 2003 Mar 1; 116(Pt 5):857–866.

29. Jones PL, Jones FS. Tenascin-C in development and disease: Gene regulation and cell function. Matrix Biol 2000 Dec; 19(7):581–596.

30. Bosman FT, Stamenkovic I. Functional structure and composition of the extracellular matrix.

J Pathol 2003 Jul; 200(4):423–428.

31. Riley GP, Harrall RL, Cawston TE, Hazleman BL, Mackie EJ. Tenascin-C and human tendon degeneration. Am J Pathol 1996 Sep;

149(3):933–943.

32. Ireland D, Harrall R, Curry V, Holloway G, Hackney R, Hazleman B, Riley G. Multiple changes in gene expression in chronic human Achilles tendinopa- thy. Matrix Biol 2001 Jun; 20(3):159–169.

33. Alfredson H, Lorentzon M, Backman S, Backman A, Lerner UH. cDNA-arrays and real-time quanti- tative PCR techniques in the investigation of chronic Achilles tendinosis. J Orthop Res 2003 Nov;

21(6):970–975.

34. Fichard A, Kleman JP, Ruggiero F. Another look at collagen V and XI molecules. Matrix Biol 1995 Jul;

14(7):515–531.

35. Reed CC, Iozzo RV. The role of decorin in collagen fi brillogenesis and skin homeostasis. Glycoconj J 2002 May–Jun; 19(4–5):249–255.

36. Chakravarti S. Functions of lumican and fi bromod- ulin: Lessons from knockout mice. Glycoconj J 2002 May–Jun; 19(4–5):287–293.

37. Bornstein P, Kyriakides TR, Yang Z, Armstrong LC, Birk DE. Thrombospondin 2 modulates collagen

fi brillogenesis and angiogenesis. J Investig Derma- tol Symp Proc 2000 Dec; 5(1):61–66.

38. Chiquet M. Regulation of extracellular matrix gene expression by mechanical stress. Matrix Biol 1999 Oct; 18(5):417–426.

39. Nishiyama T, McDonough AM, Bruns RR, Burge- son RE. Type XII and XIV collagens mediate inter- actions between banded collagen fi bers in vitro and may modulate extracellular matrix deformability.

J Biol Chem 1994 Nov 11; 269(45):28193–28199.

40. Ezura Y, Chakravarti S, Oldberg A, Chervoneva I, Birk DE. Differential expression of lumican and fi bromodulin regulate collagen fi brillogenesis in developing mouse tendons. J Cell Biol 2000 Nov 13;

151(4):779–788.

41. Svensson L, Narlid I, Oldberg A. Fibromodulin and lumican bind to the same region on collagen type I fi brils. FEBS Lett 2000 Mar 24; 470(2):178–182.

42. Danielson KG, Baribault H, Holmes DF, Graham H, Kadler KE, Iozzo RV. Targeted disruption of decorin leads to abnormal collagen fi bril morphol- ogy and skin fragility. J Cell Biol 1997 Feb 10;

136(3):729–743.

43. Schuppan D, Cantaluppi MC, Becker J, Veit A, Bunte T, Troyer D, Schuppan F, Schmid M, Acker- mann R, Hahn EG. Undulin, an extracellular matrix glycoprotein associated with collagen fi brils. J Biol Chem 1990 May 25; 265(15):8823–8832.

44. Keene DR, Lunstrum GP, Morris NP, Stoddard DW, Burgeson RE. Two type XII-like collagens localize to the surface of banded collagen fi brils. J Cell Biol 1991 May; 113(4):971–978.

45. Zhang X, Schuppan D, Becker J, Reichart P, Gelder- blom HR. Distribution of undulin, tenascin, and fi bronectin in the human periodontal ligament and cementum: Comparative immunoelectron micros- copy with ultra-thin cryosections. J Histochem Cytochem 1993 Feb; 41(2):245–251.

46. Walchli C, Koch M, Chiquet M, Odermatt BF, Trueb B. Tissue-specifi c expression of the fi bril-associated collagens XII and XIV. J Cell Sci. 1994 Feb; 107(Pt 2):669–681.

47. Shaw LM, Olsen BR. FACIT collagens: Diverse molecular bridges in extracellular matrices. Trends Biochem Sci 1991 May; 16(5):191–194.

48. Mayne R, Brewton RG. New members of the colla- gen superfamily. Curr Opin Cell Biol 1993 Oct;

5(5):883–890.

49. Olsen BR. New insights into the function of colla- gens from genetic analysis. Curr Opin Cell Biol 1995 Oct; 7(5):720–727.

50. Adachi E, Hayashi T. In vitro formation of hybrid fi brils of type V collagen and type I collagen:

Limited growth of type I collagen into thick fi brils

by type V collagen. Connect Tissue Res 1986;

14(4):257–266.

51. Niyibizi C, Kavalkovich K, Yamaji T, Woo SL. Type V collagen is increased during rabbit medial col- lateral ligament healing. Knee Surg Sports Trauma- tol Arthrosc 2000; 8:281–285.

52. Shimomura T, Jia F, Niyibizi C, Woo SL. Antisense oligonucleotides reduce synthesis of procollagen alpha1 (V) chain in human patellar tendon fi bro- blasts: Potential application in healing ligaments and tendons. Connect Tissue Res 2003; 44(3–4):167–

172.

53. Wolfman, NM, Celeste AJ, Cox K, Hattersley G, Nelson R, Yamaji N, et al. Preliminary characteriza- tion of the biological activities of rhBMP-12. J Bone Miner Res 1995; 10:s148.

54. Fu SC, Wong YP, Chan BP, Pau HM, Cheuk YC, Lee KM, et al. The roles of bone morphogenetic protein (BMP) 12 in stimulating the proliferation and matrix production of human patellar tendon fi bro- blasts. Life Sci 2003; 72(26):2965–2974.

55. Lou J, Tu Y, Burns M, Silva MJ, Manske P. BMP-12 gene transfer augmentation of lacerated tendon repair. J Orthop Res 2001; 19:1199–1202.