Proc. Nat. Acad. Sci. USA Vol. 69, No. 9, pp. 2594-2598, September 1972 Reducible Crosslinks in Hydroxylysine-Deficient Collagens of a Heritable Disorder of Connective Tissue (skin/bone/cartilage/aminoacid analysis) DAVID R. EYRE AND MELVIN J. GLIMCHER* Department of Orthopedic Surgery, Harvard Medical School, Children's Hospital Medical Center, Boston, Massachusetts 02115 Communicated by Francis 0. Schmitt, July 3, 1972 ABSTRACT Reducible compounds that participate don, bone, and cartilage collagens (3-11). Each tissue reveals in crosslinking were analyzed in hydroxylysine-deficient a unique distribution of these reducible crosslinks that changes collagens of patients with a heritable disorder of connec- tive tissue. After treatment with [3H1sodium borohydride, as the tissue matures and ages (8, 10). new compounds, as well as a totally different pattern of Since connective tissues of patients with this disorder are tritiated compounds, were found in hydroxylysine-de- deficient in hydroxylysine, a crosslink precursor, it seemed ficient collagen from skin as compared with age-matched likely that the reducible crosslinks would either be absent or controls. The amount of desmosines detected indicated collagen cross- that more elastin was present in abnormal skin than in abnormal. Such a deficiency or abnormality in control skin. linking might be responsible for changes in the solubility Bone collagen, which was not as deficient in hydroxy- characteristics of the collagen (1), and for changes in the lysine as skin collagen, had the same compounds as normal structural properties of the tissues and the consequent skeletal bone collagen, but their relative proportions were altered, and connective tissue abnormalities. consistent with a deficiency of hydroxylysine, a precursor of the crosslinks. Although the content of hydroxylysine Biopsies of skin, bone, and cartilage removed during sur- in collagen of cartilage is essentially normal in these pa- gery were analyzed for the reducible components of collagen, tients, analysis after reduction revealed a different pat- and the results were compared to those obtained from the tern of reduced compounds from that of normal cartilage. It is speculated that Type II collagen, the major collagen component in cartilage, contains a normal amount of hydroxylysine, while Type I collagen, which is the major source of the crosslinks, is hydroxylysine-deficient. This Hydroxyysine - Defitient Skin distribution would explain the findings of an abnormal profile of reducible compounds despite an almost normal total hydroxylysine content. The finding that the deficiency of hydroxylysine in the 5 _ collagen of these patients is accompanied by changes in number, chemical nature, and, probably, distribution of ? crosslinkages, and the previously reported alterations in the solubility characteristics, suggest that at least some skeletal and connective tissue abnormalities are directly related to underlying molecular pathology. I.. 4 r Control SkinPost-Histt. Aminoacid analyses of biopsies from two siblings with a heri- -4 V~~~~~~~~~~~LNL table disorder of connective tissue revealed the presence of '4.. hydroxylysine-deficient collagen (1). Hydroxylysine was '4.. present in skin collagen at about 5% of the amount found in collagen of normal skin, and at higher, but still markedly reduced, amounts in fascia and bone, while the content in 4 Control Skin 'Post-Iistidinen amount. The basic cartilage was about 90% of the normal 14,4~HLNL~~~~~~~~ biochemical defect is an abnormally low activity of the en- zyme lysyl-protocollagen hydroxylase in cells that synthesize I0Z,0 collagen in the affected connective tissues (2). Hydroxylysine is a precursor in the biosynthesis of certain collagen crosslinks (3, 4). Borohydride-reducible crosslinks, which are apparently intermediates in the biosynthesis of more stable bonds, have been identified in normal skin, ten- 00 20 40 60 80 100 120 140 160 180 200 FRACTION NUMBER Abbreviations: HLNL, hydroxylysinonorleucine; HLHNL, hy- FIG. 1. Comparison of the elution profiles of tritiated com- droxylysinohydroxylnorleucine; DHNL, dihydroxynorleucine. pounds in hydrolysates of normal and hydroxylysine-deficient * Reprint requests to: Melvin J. Glimcher, M.D., Department of human skin collagens treated with [3H]borohydride. 10 mg dry Orthopedic Surgery, Children's Hospital Medical Center, 300 weight of each material were hydrolyzed in acid and applied Longwood Ave., Boston, Mass. 02115. to the aminoacid analyzer. HLNL, hydroxylysinonorleucine. 2594 Downloaded by guest on September 30, 2021 Proc. Nat. Acad. Sci. USA 69 (1972) Crosslinks in Hydroxylysine-Deficient Collagens 2595 collagens of site-matched and age-matched samples of normal Hydroxylysine- Deficient Skin tissues. METHODS Skin biopsies were scraped free of fat and stirred in 2 M NaBr for several hours to facilitate removal of epidermis (1). The dermis was washed thoroughly by stirring in saline and then water at 4°. Samples of cortical bone from spinous processes of thoracic vertebrae were washed by stirring in saline and then demineralized in 0.2 M EDTA (pH 7.9), at 40 for several days, followed by further washing in distilled water. Rib cartilage was stirred in saline and then water. Control Skin Samples of the washed tissues were cut into small pieces, HIS stirred in 0.1 M sodium phosphate (pH 7.4), and reacted with tritiated sodium borohydride (10 Ci/mol) at room tem- perature (250) for 45 min. Acetic acid was added to bring the solution to pH 3.0, and after 5 min of further stirring, the tissue pieces were thoroughly washed in several changes of distilled water. After the tissue was dried over P205, it was hydrolyzed in 3 N HCl in an evacuated, sealed tube for 48 hr at 1050, conditions that prevent certain participants of crosslinking from being destroyed (12). Amino acids were I_e--- r-------?- . fractionated on an aminoacid analyzer equipped for stream 155 160 165 170 175 180 splitting. Tritium activity was measured in a portion of the FRACTION NUMBER column effluent, and radioactive peaks were identified as described (13). FIG. 2. Regions of the two chromatograms depicted in Fig. 1 Site-matched and age-matched samples of skin, bone, and comparing the elution profiles of tritium activity and of ninhy- cartilage were taken at biopsy or autopsy from normal con- drin-positive material. - , absorbance at 570 nm; nective tissues and analyzed by the same procedures. 'H activity. RESULTS from the abnormal skin collagen eluted from the column later Skin than the major radioactive peak found in normal skin collagen, The elution profile of tritiated compounds from a hydrolysate indicating they were not the same compounds. of reduced collagen of abnormal skin was markedly different The radioactive peak eluting at fraction number 130 was from that of normal skin (Fig. 1). Three independent age- also of interest. This peak was much larger in borohydride- matched samples of normal skin revealed essentially the treated samples of diseased skin than in normal skin. It eluted same patterns. Skin biopsies from each of the two siblings in an identical position to the reduced desmosine peak from with the disease gave virtually the same abnormal elution a hydrolysate of reduced elastin. Identification as reduced profiles of tritium activity. The major peak from normal desmosine on this basis alone was not reliable since several skin collagen was apparently the material described as post- minor radioactive peaks also eluted in this region on analysis histidine compound (14, 15) or Fraction C (6, 9, 10). The of bone collagen. However, conventional aminoacid analysis, next most abundant compound was hydroxylysinonorleucine by reaction with ninhydrin, of a hydrolysate of a relatively (HLNL), derived by reduction of the aldimine crosslink, large sample (10 mg dry weight) of dermis, which had not dehydrohydroxylysinonorleucine, formed by addition between been treated with borohydride, confirmed that more of the a residue of hydroxylysine and a residue of allysine, the alde- elastin crosslinks, desmosine and isodesmosine, were present hyde from an enzymic oxidative deamination of a lysyl res- in abnormal skin than in a sample of control skin. There idue. The abnormal skin, however, did not reveal any of appeared to be four times the concentration of desmosines the normal "post-histidine" compound, and HLNL was per unit weight of collagen in one of the samples of diseased much diminished and possibly absent since its elution posi- skin compared to normal skin. Collagen was estimated by tion was now obscured by several minor peaks. Instead, a hydroxyproline content. new major peak of tritium activity eluted later than the normal "post-histidine" peak, and appeared as a partially Bone resolved double peak suggesting that more than one com- The hydroxylysine content of samples of diseased bone varied pound was present. A ninhydrin-positive double peak coin- considerably. Spongy bone from the interior of a vertebra cided precisely with the radioactive peak (Fig. 2). In terms of contained collagen with about 50% of the normal amount of lysine equivalents, this double peak represented in total one hydroxylysine (1). However, histological examination after to two lysine residues per molecule, or about 0.5 lysine res- staining with Safranin 0 showed that pockets of cartilage idue per a chain. It was not detected in the collagen without were included in the bone sample. In the present study, corti- prior borohydride treatment, indicating that the