Pathophysiological Consequences of Protein N-Homocysteinylation - Homocysteine in Protein Structure/Function and Human Disease - page 115

6.5 N-Hcy-Collagen and Connective Tissue Abnormalities

CBS deficiency is characterized clinically by widespread deformities and malfunc-
tion of connective tissue including joint, laxity, kyphoscoliosis, pigeon breast, genu
valgum, severe osteoporosis, ectopia lentis, and vascular disease in which dilatation
of medium-sized arteries occur frequently [20]. Patients with homocystinuria due to
CBS deficiency often exhibit skeletal abnormalities resembling the phenotype of
patients with Marfan syndrome [20, 46], a connective tissue disease caused by
mutations in the fibrillin-1 gene, which affect the delivery of fibrillin-1 and/or its
assembly into microfibrils.
Collagen, the most abundant protein in mammals (comprising 25-35 % of the
whole-body protein content), is the major component of connective tissues. The
unusual mechanical stability of collagen fibrils and collagenous tissues is largely
dependent on the formation of covalent intramolecular pyridinoline cross-links
between collagen chains [402]. Because lysine residues are involved in the
pyridinoline cross-link formation that is essential for proper assembly of collagen
fibers and their mechanical properties, N-homocysteinylation of these lysine
residues by Hcy-thiolactone can prevent normal cross-linking and thus contribute
to connective tissues abnormalities observed in severe hyperhomocysteinemia.
Relatively low levels of N-linked Hcy in collagen can result in a structural defect
in the supramolecular organization of the connective tissue.
That skin collagen is defective in hyperhomocysteinemia has been shown by its
increased extractability from the skin of CBS-deficient patients: the extraction with
1-M NaCl and 0.5-M acetic acid solubilizes 7.8 % and 10.1 % of dermal collagen
from two CBS-deficient patients, whereas only 2.4 % and 2.9 % is extracted from
two age-matched control individuals [403]. Furthermore, collagen type I cross-links
(assayed by quantification of carboxyterminal telopeptide of collagen type I in
plasma) are reduced threefold in nine CBS-deficient patients relative to the 20
control individuals (1.14

g/L) [404]. Because the
extent of collagen biosynthesis (assessed by quantification of C-terminal propeptide
of type I procollagen and of N-terminal propeptide of procollagen type III) is the
same in patients with homocystinuria and in control subjects, these findings indicate
that connective tissue abnormalities in hyperhomocysteinemia are linked to dimin-
ished collagen cross-linking, but not biosynthesis [404].
Cbs-deficient mice have skin and bone abnormalities similar to those observed in
humans [305, 309, 405]. Analysis of N-linked Hcy content in collagen isolated from
Tg-I278T Cbs / mice shows that these mice have 18-fold higher skin N-Hcy-collagen
content than Tg-I278T Cbs +/+ littermates (89.9

0.24 μ g/L vs. 3.29 0.32 μ

2.4 pmol/mg skin)
[306]. Similar elevation in N-linked Hcy in the Cbs-deficient mice is observed in bone
collagen. These findings show that collagen is N-homocysteinylated in vivo in mice.
However, it remains to be determined whether N-homocysteinylation prevents colla-
gen cross-linking.

25.1 vs. 5.0