Clin Genet 2000: 58: 270 279
Printed in Ireland. All rights reser ed
Developmental Biology: Frontiers for Clinical Genetics
Collagens: building blocks at the end of the
development line
Section Editor: Roderick R McInnes
e-mail: [e-mail]
PH Byers
Byers PH. Collagens: building blocks at the end of the development
line.
Departments of Pathology and Medicine,
Clin Genet 2000: 58: 270 279. © Munksgaard, 2000
University of Washington, Seattle, WA
98195, USA
Collagens are some of the major building blocks of the vertebrate
body. In addition to their structural role, they are important for cell
Key words: basement membranes  bone
guidance during development and for maintaining tissue integrity. In
 cartilage  collagen  development 
their absence, phenotypes range from lethal to mild. These studies genes  mutations  procollagen
demonstrate that collaens,in their rich array, play important roles in
Corresponding author: PH Byers, Depart-
development and are significant elements in reading the developmental
ment of Pathology, University of Washing-
code.
ton, Seattle, WA 98195, USA. Tel: +1 206
543 4206; fax: [phone]; e-mail:
[e-mail]
Received 6 July 2000, revised and ac-
cepted for publication 19 July 2000
light because of the great biological efficiency of
Our physical appearance is the end result of the
our surveillance system for developmental errors.
initiation of a complex developmental program,
the cellular memory of the instructions from that
Collagens: nature s building blocks
program, and the cell-, tissue-, and organ-specific
expression of a set of morphogenetic molecules. In
In the real world of dysmorphology, syndromol-
rare instances, the entire morphogenetic program
ogy, and genetics there is increasing recognition of
goes awry presumably because the early program-
disorders that result from mutations in molecules
ming events in development are miscued (EB
that function to organize the developmental role.
White s  Stuart Little being one of the best docu-
At the same time, it is clear that  end molecule
mented examples, although still poorly understood
failure leads to some of the more common disor-
at the molecular level almost half a century after
ders in which body form is distorted. The distinc-
the first report). Sometimes, these instances of
tive phenotypes resulting from mutations that
poor planning lead to rearrangements that can
affect the amount or structure of type I collagen
only be seen with detailed studies of internal
(osteogenesis imperfecta [OI] in all its forms) or
anatomy (such as situs in ersus) while others are
type II collagen (with a phenotypic range from
more easily appreciated by simple observation (ab-
achondrogenesis to Stickler syndrome) are striking
sence or duplication events, such as forms of ectro-
examples (1). Here, the basic plan is laid out, but
dactyly or polydactyly). These disorders flow from
the building materials are not delivered appropri-
fundamental errors in the specification of the body
ately or are unusable because of design flaws.
plan and can be attributed to early molecular
In this context, the most fundamental of all the
events that occur at specific times in embryonic
building blocks available are the collagens, until
development. Although these morphogenetic er- just the last couple of decades considered to be
rors are easy to spot, they are probably the mildest some of the least interesting biological molecules
of the consequences of mutations that affect the because of their long-term stability. The reputation
expression or structure of this set of early deter- of these dynamic, multi-functional molecules was
mining molecules, the most severe rarely coming to not enhanced when one of the pioneers of modern
270
Collagen building blocks
molecular biology hoped (perhaps this is apoc- abundance or modified in shape. However, it is
ryphal) that the structure of DNA would not be good to keep in mind that some collagens have
as dull as the triple helix of collagen.  Collagen other functional components and mutations in
has, however, survived and now presents a fasci- them may have quite different effects on, for ex-
nating tale of evolutionary (almost revolutionary) ample, vessel formation.
adaptation to fill a large number of both general
and highly specialized niches in morphogenesis
The biosynthetic pathway is complex
and tissue integrity (2).
Most mutations in collagen genes act in a Collagen biosynthesis involves one of the richest
dominant manner, probably because the processing pathways of all proteins. Almost all
molecules they form are trimers and incorpora- collagen genes have dozens to more than 100 ex-
tion of a single abnormal chain interferes with ons. The COL7A1 gene has 118 exons, the largest
the assembly of three chains in the molecule, the number of all known vertebrate genes, while the
folding of the triple helix, or interactions with COL10A1 gene has only two, making it the most
other molecules in the matrix (3). There are rare intron poor of all the collagen genes. The fibrillar
recessively inherited disorders that result from collagen genes have between 51 and 66 exons.
mutations in fibrillar collagen genes. Recessive The complexity of these genes and the deleterious
variants of some disorders, dystrophic epidermol- consequences of exon-skipping and of the use of
ysis bullosa that results from mutations in type cryptic splice sites provide a rich substrate for
VII collagen, can result from homozygosity, or mutations. The mature mRNAs are translated on
compound heterozygosity, for functional null alle- membrane-bound polysomes and the precursor
les in collagen genes (4). These experiments of chains are inserted into the lumen of the rough
nature and the deliberate (or fortuitous) inactiva- endoplasmic reticulum. During and after transla-
tion of collagen genes in model organisms tion, most prolyl residues that precede glycine
provide the few instances in which the develop- residues in the triple-helical domain are hydroxy-
mental effects of the absence of these molecules lated, as are a variable number of lysyl residues,
can be assessed without the interference of the the number depends on the collagen type. Chains
abnormal molecule. associate through regions in the carboxyl-terminal
propeptide, which also specifies binding partners,
and triple helix propagates from that end toward
More than a molecule: collagen is a family of genes
the amino-terminal end of the molecule. Prolyl
with characteristic features
4-hydroxylase, a tetramer of two -chains and
 Collagen is now known to be encoded by a fam- two -chains that are identical to protein disulfide
ily of at least 30 genes (see Table 1). The protein isomerase (PDI), HSP47 (a collagen-specific chap-
products are diverse in size, structure, distribu- erone), and several other proteins chaperone the
tion, and abundance. Each collagen gene encodes association and folding. Intact precursor
a precursor chain, known as pro chains, of spe- molecules are secreted, or in the case of type XIII
cified number and type, which contain sequences and type XVII remain membrane-bound through
that direct them to assemble with specific part- transmembrane components, are processed prote-
ners into molecules that contain three chains  olytically in the extracellular space, and form
some are homotrimers and some are hetero- fibrillar or meshwork complexes as their final
trimers. In aggregate, collagens represent the structure.
most abundant proteins of the body and, for the
most part, function as extracellular building
Mutations: the common language of dysfunction
blocks. Some, notably type XIII and type XVII,
are transmembrane molecules, and others, type For the large part, the phenotypes that result
XV and type XVIII, house other functional from mutations in collagen genes reflect the tissue
molecules  endostatins  that appear to regulate and organ distribution of the expression of these
vasculogenesis and are released from the parent genes (Tables 1 3). The developmental role of
molecule by proteolysis (5, 6). Thus, it should collagens can be difficult to define when muta-
come as no surprise that mutations in many of tions induce additional phenotypes. It is probably
these genes lead to phenotypes characterized by only in the presence of homozygosity for  null
altered shape, although not altered form. That is, mutations that the role of a particular collagen,
the basic instructions for laying down the body modifying enzyme, or chaperone can be assessed
plan are uncompromised; it is only the final con- because of the ability of the organism to adapt to
struction units that are either present in low structural alterations.
271
Byers
Table 1. Collagen genes and their disorders
Collagen type Gene Chromosomal lo- Protein Disorders
cation
I COL1A1 17q21.31-q22.05 pro 1(I) Osteogenesis imperfecta
Ehlers Danlos syndrome type VIIA
COL1A2 7q22.1 pro 2(I) Osteogenesis imperfecta
Ehlers Danlos syndrome type VIIB
Ehlers Danlos syndrome type II
II COL2A1 12q13.11-q13.2 pro 1(II) Stickler syndrome, type I
Wagner syndrome type II
Spondylepiphyseal dysplasia congenita
Kniest dysplasia
Hypochondrogenesis
Achondrogenesis type II
Spondylo-metaphyseal-epiphyseal dysplasia (SMED), Strudwick type
III COL3A1 2q31 pro 1(III) Ehlers Danlos syndrome type IV
Ehlers Danlos syndrome type III (?)
IV COL4A1 13q34 pro 1(IV)
COL4A2 13q34 pro 2(IV)
COL4A3 2q36-q37 pro 3(IV) Alport syndrome, recessive
COL4A4 2q36-q37 pro 4(IV) Alport syndrome, recessive
COL4A5 Xq22 pro 5(IV) Alport syndrome, X-linked
COL4A6 Xq22 pro 6(IV) Alport syndrome, X-linked Leiomyomatosis
V COL5A1 9q34.2-q34.3 pro 1(V) Ehlers Danlos syndrome type I
Ehlers Danlos syndrome type II
COL5A2 2q31 pro 2(V) Ehlers Danlos syndrome type I
COL5A3 Not mapped pro 3(V)
VI COL6A1 21q22.3 pro 1(VI) Bethlem myopathy
COL6A2 21q22.3 pro 2(VI) Bethlem myopathy
COL6A3 2q37 pro 3(VI) Bethlem myopathy
VII COL7A1 3p21.3 pro 1(VII) Epidermolysis bullosa, recessive dystrophic
Epidermolysis bullosa, dominant dystrophic
Epidermolysis bullosa, pretibial
VIII COL8A1 3q12-q13.1 pro 1(VIII)
COL8A2 1p34.4-p32.3 pro 2(VIII)
IX COL9A1 6q13 pro 1(IX) Multiple epiphyseal dysplasia
COL9A2 1p33-p32.2 pro 2(IX) Multiple epiphyseal dysplasia, type II
COL9A3 20q13.3 pro 3(IX) Multiple epiphyseal dysplasia, type III
X COL10A1 6q21-q22.3 pro 1(X) Metaphyseal chondrodysplasia, Schmid type
Spondylometaphysealdysplasia, Japanese type
XI COL11A1 1p21 pro 1(XI) Stickler syndrome, type III
Marshall syndrome
COL11A2 6p21.3 pro 2(XI) Stickler syndrome, type II
Otospondylomegaepiphyseal dysplasia (OSMED)
Weissenbacher-Zweymuller syndrome
Non-syndromic deafness (DFNA13)
XII COL12A1 6 pro 1(XII)
XIII COL13A1 10q22 pro 1(XIII)
XIV COL14A1 8q23 pro 1(XIV)
XV COL15A1 9q21-q22 pro 1(XV)
XVI COL16A1 1p34 pro 1(XVI)
XVII COL17A1 10q24.3 pro 1(XVII) Epidermolysis bullosa, generalized atrophic benign
XVIII COL18A1 21q22.3 pro 1(XVIII)
XIX COL19A1 6q12-q14 pro 1(XIX)
272
Collagen building blocks
Table 2. Collagen types, chain composition, and tissue distribution
Collagen type Chains Molecules Tissue distribution
Fibrillar collagens
I 1(I), 2(I) 1(I)2 2(I) Ubiquitous in hard and soft tissues, major protein of bone, skin.
1(I)3 Uncommon, found in some tumors, amniotic fluid cells
II 1(II) 1(II)3 Cartilage, vitreous, intervertebral disk
See also type XI
III 1(III) 1(III)3 Soft tissues and hollow organs
V 1(V), 2(V), 3(V) 1(V)2 2(V) Soft tissues, placental, vessels, chorion
1(V) 2(V) 3(V) 2(V) can substitute for the 2(XI) chain in vitreous
See also type XI
XI 1(XI), 2(XI) 1(XI) 2(XI) 1(II) Cartilage
1(XI) 2(V) 1(II) Vitreous
Basement membrane collagens
IV 1(IV), 2(IV) 1(IV)2 2(IV) Basement membranes
3(IV), 4(IV) Others uncertain
5(IV), 6(IV)
Fibril-associated collagens with interrupted triple helices (FACIT)
IX 1(IX), 2(IX), 3(IX) 1(IX), 2(IX) 3(IX) Cartilage, vitreous
XII 1(XII) 1(XII)3 Soft tissues
XIV 1(XIV) 1(XIV)3 Soft tissues
Meshwork-forming collagens
VIII 1(VIII), 2(VIII) 1(VIII)2 2(VIII) Cornea, endothelium
X 1(X) 1(X)3 Hypertrophic zone of the growth plate
Anchoring-fibril collagen
VII 1(VII) 1(VII)3 Anchoring fibrils, dermal epidermal junction
Microfibril-forming collagens
VI 1(VI), 2(VI), 3(VI) 1(VI) 2(VI) 3(VI) Microfibrils in soft tissues and cartilage
Transmembrane collagens
XIII 1(XIII) 1(XIII)3 Cell surfaces, epithelial cells
XVII 1(XVII) 1(XVII)3 Epidermal cell surfaces
Endostatin forming collagens
XV 1(XV) 1(XV)3 Endothelial cells
XVIII 1(XVIII) 1(XVIII)3 Endothelial cells
Others
XVI 1(XVI) 1(XVI)3 Ubiquitous
XIX 1(XIX) 1(XIX)3 Ubiquitous
Type I collagen genes gene (9). In contrast, homozygosity for COL1A1
null mutations is lethal in the mouse, which serves
Type I collagen is the major protein of bone, skin,
to demonstrate the essential nature of the gene
tendons and ligaments, blood vessel walls, and
product (15, 16). These mice die at embryonic day
other connective tissues except cartilage. The ma-
11 because of loss of integrity of the vascular
jor phenotypes that are known to result from mu-
system under hydrostatic pressure. With the excep-
tations in the COL1A1 and COL1A2 genes are
tion of some alteration in branching during lung
forms of OI (1), Ehlers Danlos syndrome (EDS)
morphogenesis, the early embryos have no major
type VII (7, 8), an uncommon form of EDS type II
form alterations. Their cultured cells do, however,
(9), and rare disorders of blood vessel integrity
have multiple alterations of proteins in the extra-
(10). With rare exception, these disorders are in-
cellular matrix, which may contribute to the lack
herited in an autosomal dominant fashion and
of tissue integrity (17).
result either from haploinsufficiency mutations
The COL1A1 gene encodes the pro 1(I) chains
(11 14) or mutations that act in a dominant nega-
of type I procollagen, at least two of which are
tive fashion (1). Survival of individuals ho-
needed to make a functional molecule. Molecules
mozygous for  null mutations in the COL1A2 gene
cannot accommodate more than a single pro 2(I)
(moderately severe OI and a rare form of EDS
chain (encoded by the COL1A2 gene). Ho-
type II) demonstrate that this is not an essential
mozygosity for COL1A2 non-functional alleles
273
Table 3. Animal models, human disorders and type of mutations in collagen genes
Gene Animal models Human disorders
Type of mutation Type of mutation
Homozygous null (-/-) Heterogyzous Transgenic animals (over Homozygous null (-/-) Heterozygous null (+/-) Missense mutations; structural alterations
null (+/-) expression or missense
mutations)
COL1A1 Embryonic lethal at day 11 OI type I  like Range from lethal to moderate Not seen OI type I Wide range of phenotype from lethal to mild
in the mouse because of phenotype osteogenesis imperfecta
vascular rupture
COL1A2 OI type III Very mild os- NA OI type III, EDS type I/II ?mild osteopenia Wide range of phenotype from lethal to mild
teopenia osteogenesis imperfecta
COL2A1 Neonatal or late fetal lethal  Stickler-like Range from severe lethal to Not seen Stickler syndrome Wide range of phenotype from achondrogene-
mild chondrodysplasia sis type II to spondyloepiphy-seal dysplasias
COL3A1 Early death from arterial rupture Normal NA Not seen EDS type IV EDS type IV
COL4A1 Point mutations are embryonic Not known Not known Not known
lethal in C. elegans
COL4A2 Not known Not known Not known
COL4A3 Homozygosity produces autosomal recessive
Alport syndrome
COL4A4 Homozygosity produces autosomal recessive
Alport syndrome
COL4A5 Hemizygosity produces Alport Hemizygosity produces Alport syndrome
syndrome
COL4A6 Combined with loss of COL4A5
produces Alport syndrome and
enteric leiomyomatosis
COL5A1 EDS type I/II EDS type I/II
COL5A2 Deletion of exon 6 (contains EDS type I/II
N-propeptide cleavage site)
produces EDS type I/II like
picture in homozygous mice
COL6A1 Early onset severe myopathy Bethlem myopathy
COL6A2 Bethlem myopathy
COL6A3 Bethlem myopathy
COL7A1 Recessive dystrophic Normal Recessive dystrophic epi- Normal Dominant dystrophic epidermolysis bullosa
epidermolysis bullosa dermolysis bullosa
COL9A1 Late onset mild degenerative Normal Multiple epiphyseal dysplasia
joint disease
COL9A2 Multiple epiphyseal dysplasia
COL9A3 Multiple epiphyseal dysplasia
COL10A
COL11A1 Perinatal lethal in the cho/cho Normal Stickler and Marshall syndromes Stickler and Marshall syndromes
mouse
COL11A2 Stickler syndrome
COL17A1 Generalized benign epidermolysis bullosa
274
Byers
Collagen building blocks
produces much milder phenotypes of OI (18) or and the abnormal molecules, although poorly
EDS (9). Thus, although the COL1A2 gene secreted, interfere with fibrillogenesis. The pheno-
product provides some plasticity for the type I type of the heterozygous null mutation in humans,
procollagen molecules (particularly, it seems, in Stickler syndrome (1), resembles the heterozygous
permitting normal bone mineralization and normal null mouse.
vascular and skin integrity), it is not essential for Type IX and type XI collagens interact with type
survival. In contrast, the absence of COL1A1 gene II collagen in the cartilage matrix to form het-
products means that no type I procollagen erotypic fibrils (that is fibrils that contain all three
molecules can be made. Under these circumstances molecules). Although both are present in much
all mechanical integrity of tissues is lost. This phe- lower amounts than type II collagen, the effects of
notype would be expected in a quarter of all preg- homozygosity for null mutations (premature termi-
nancies initiated by two parents, both of whom nation codons) in the Col11a1 gene in the cho/cho
have OI type I (which results from heterozygosity mouse are lethal as a consequence of abnormalities
for COL1A1 null mutations). in the cartilage of limbs, ribs, the mandible and
trachea (22). Mutations in the COL11A1 gene in
people give rise to a variant of Stickler syndrome,
Type II collagen and other collagens expressed in cartilage
Marshall syndrome, or an intermediate phenotype
Cartilage is home to several collagens: types II, IX, (23). This variation may depend on the outcome of
X, and XI. Type II collagen is the most abundant splice site mutations. Homozygosity for a null
collagen in the matrix of cartilage, in the early Col9a1 gene in mice yields a minimal phenotype of
anlage of developing bone, and the vitreous of the late onset mild degenerative joint disease (24), even
eye. Type IX and type XI collagens have similar though the protein product is required for assem-
patterns of expression and distribution. They form bly of the entire type IX collagen protein (25).
heterotypic (multiple types of collagen) fibrils with Type X collagen is found almost exclusively in
type II collagen. Because of its appearance early in the hypertrophic zone of the growth plate. Mice
the formation of endochondral bone, it seemed homozygous for inactivation mutants of the
likely that both the form and the growth of the Col10a1 gene have variable phenotypes with about
bones would be dependent on the presence of type 10% dying in the perinatal period and more by
II collagen. In mice homozygous for non-expres- early adult life. These animals all have significant
sion COL2A1 alleles, membranous and periosteal growth plate compression as well as hematopoietic
bone form normally but long bones are rudimen- alterations (26). They resemble, to some extent, the
tary, lack marrow cavities and the only mineralized mice produced by a dominant interference trans-
bone is formed from the periosteum. The mice gene (27). In people, dominantly inherited muta-
have no palate; their ribs are small and not prop- tions result in Schmid metaphyseal chondro-
erly mineralized so that the thorax is small. Alveoli dysplasia with short stature and bowed bones. No
are not distended and the animals do not survive. homozygous nulls are known but most of the
Calvarial mineralization is normal, although the human mutations may act as heterozygous nulls in
shape is altered (19, 20). In these animals the that they likely interfere with chain association or
notochord is not resorbed and vertebral body with multimer formation (28 30). Almost all the
structure is altered (21). It is thought, surprisingly, mutations in the COL10A1 gene are found in the
that most of these alterations, even the failure of domain that encodes the chain association domain.
palate formation and rotation, arise because of The relationship between the skeletal and hemato-
changes in the mechanical properties of tissues, not poietic findings is not clear at this point, but sug-
specification of the plan of the tissue (21). gests that the development of the latter may
Type II collagen is a homotrimer. In addition, depend on certain signals from an intact marrow.
the pro 1(II) chain is included with the pro 1(XI)
and pro 2(XI) chain in type XI collagen. Muta-
Type III collagen
tions that alter sequences in the triple-helical do-
main of the pro 1(II) chain interfere with the The homozygosity for a Col3a1 null mutation in
normal helix formation of 7/8 molecules made by the mouse results in a perinatal lethal phenotype
the cell and many of those alter secretion (1). The with vascular rupture (31). These mice have very
perinatal lethal disorder achondrogenesis type II thin vascular walls and thin skin. As type III
results from heterozygosity for mutations in the collagen is expressed early in the development of
triple-helical domain and the phenotype resembles these tissues, one proposal has been that the
the homozygous COL2A1 null mouse, probably molecules provide a part of the scaffolding on
because there are virtually no normal molecules which the mature organ is built. The published
275
Byers
mutations in the COL3A1 gene all alter the se- involved in the control of smooth muscle growth
quence of the chains (1, 32). A small number of located in the large intron 2 or if the presence of
premature termination mutations in the COL3A1 both gene products is important for that function
gene are known and also result in an EDS type IV (39, 40).
phenotype (vascular, bowel, uterine rupture with
early death), similar to the effects of heterozygous
Type VI collagen
dominant mutations (U Schwarze and PH Byers,
The type VI collagen genes, COL6A1, COL6A2,
unpublished).
and COL6A3, encode a collagenous protein that
forms a portion of the epimysium and is involved in
Type IV collagen
connections with many of the proteins of the muscle
fiber. Mutations in any of the three of these genes,
Basement membrane molecules come early and stay
the protein products of which contribute to a single
late. They are among the first matrix molecules
heterotrimer, result in Bethlem myopathy, an un-
synthesized by embryonic cells and the same
common form of muscle disease characterized by
molecules are important for isolation of organ-
muscular dystrophy and joint contracture (41 43).
forming buds from endodermal cells and in the
Homozygosity for a targeted disruption in the
separation of tissue formed from different cell
murine Col6a1 gene results in no clinical phenotype
pools. These molecules persist in the major base-
but histological evidence of myopathy (44). Type VI
ment membranes of the kidney, lung, skin, and
collagen is a heterotrimers of all three chains and
other regions in which cells of one origin are sepa-
the 1(VI) chain is essential for molecular assembly.
rated from those of other origins. In mammals,
there are six type IV collagen genes that occur in
Type VII and type XVII collagens
three pairs, each oriented in a head-to-head fashion
and sharing a bifunctional promotor, no doubt
Mutations in the COL7A1 gene, the encoded
having evolved from a single gene duplication/in-
product of which forms anchoring fibrils at the
version event followed by duplication and disper-
dermal epidermal junction, produce different
sion of the two-gene structure. The two genes
forms of dystrophic epidermolysis bullosa (45).
expressed ubiquitously in these tissues are the
Targeted inactivation of the murine Col7a1 gene
COL4A1 and COL4A2 genes, located on chromo-
results in the recessive dystrophic epidermolysis
some 13. No human mutations in these genes are
bullosa picture, essentially unchanged from that in
known and no knockout mice appear to have been
humans (46). Mutations in another collagen gene
generated. The only clue to the fate of mutations in
expressed by basal keratinocytes that functions as a
these genes is offered in Caenorhabditis elegans. In
transmembrane protein, COL17A1, result in a
C. elegans, mutations in both the COL4A1 and
much milder form of bullous skin disease (47).
COL4A2 equivalent genes often have a lethal out-
come during embryogenesis, although others may
Genotype to phenotype: a well-trodden but poorly
permit development to proceed (33 35). In hu-
mapped pathway
mans, it is likely that most mutations in these genes
result in phenotypes that do not survive early Dramatic effects on development are seen, to date,
embryogenesis. only with null mutations in a small number of
The COL4A3, COL4A4, COL4A5, and COL4A6 genes: Col1a1 in mice and type IV collagen genes in
genes all appear to be expressed in kidney, either in C. elegans. Mutations in some of the genes of the
heterotrimers that contain a single chain each of the post-translational pathway, notably the prolyl 4-hy-
COL4A3, COL4A4, and COL4A5 products or in droxylase gene, may also be an embryonic lethal, at
trimers that contain two products of the COL4A5 least when examined in C. elegans (48). It is clear
and a single chain encoded by the COL4A6 gene. that, while there are complex interactions among
Homozygosity for knockout/null mutations in the collagens and other proteins, many of these are
murine Col4A3 gene results in a progressive important only at the final stages of growth and
glomerular disease (36) and cochlear abnormalities have relatively little importance in the translation of
(37), similar to those seen in people with autosomal other developmental signals.
recessive Alport syndrome. The most striking effect The tissue-specific distribution of gene expression
of mutations in the type IV collagen genes is the is one key to understanding the phenotypic effects
combined Alport syndrome and diffuse esophageal of mutations but it remains difficult to see
leiomyomatosis seen with deletions that extend how mutations that either alter the amount of a
from intron 2 of the COL4A6 gene into the chain synthesized or alter the structure in a very
COL4A5 gene (38). It is not clear if there is a gene subtle fashion give rise to the diverse phenotypes
276
Collagen building blocks
represented by dominant interference (dominant to the specificity of interactions lent by only a few
negative) or recessive mutations in collagen genes. residues of a protein backbone. In some cells, the
These pathways are most extensively explored for a constituent chains of three or more types of colla-
few of the fibrillar collagens, type I collagen and gen are synthesized simultaneously with few if any
bone disorders, and type II collagen for disorders errors occurring in the correct association of the
of cartilage growth and structure. half a dozen or more different gene products.
Substitutions for glycine, exon-skipping, or
small in-frame deletions or insertions within the
The complexity of pathogenesis for mutations in
triple-helical domains of these chains do not inter-
type I collagen starts in the nucleus
fere with association, but do bring helix propaga-
Most known mutations in the COL1A1 and tion to a halt, awaiting sufficient energy to
COL1A2 genes give rise to forms of brittle bone overcome the barrier or re-initiating helix forma-
disease, OI (49, 1). These phenotypes range from a tion. These molecules remain bound to the modify-
subtle increase in the risk of fractures (a few during ing enzymes and their rate of secretion is
an individual s lifetime) to intrauterine or perinatal substantially slowed. These molecules remain in
lethality (50). The mildest of these forms (OI type the ER for extended periods. Many are degraded
I) results from failure to synthesize the products of by mechanisms that are not yet clear, but a small
one COL1A1 allele (11 14, 51). The mutations number can still traverse with secretory pathway,
that result in this phenotype are largely premature disregarding the many checkpoints on the lookout
termination codons along the length of this 51 for misfolded molecules, only to exit the cell and
exon gene. Three mechanisms are involved: frame continue their misadventures by interfering with
shifts due to small insertions or deletions, point fibrillogenesis or with, in the case of bone, mineral-
mutations that create termination codons, and ization. The only protection from this rude intru-
splice site mutations that result in inclusion of sion on order is the intracellular retention and
additional nucleotides or deletion of nucleotides possibly degradation of the abnormal molecules.
due to alternate splicing such that new termination Unfortunately for the organism, these processes
codons result at or downstream from the muta- are not entirely successful, so that some abnormal
tional event. These products are either extremely molecules are secreted and their deleterious effects
unstable or are segregated into the nucleus away prevail.
from the protein translation machinery and secre- Mutations that alter sequences in the carboxyl-
tory pathway. terminal region of chains can ablate chain associa-
tion, have no effect, or alter the rate of association.
Those that do not permit chain association act as
Goes on to the secretory pathway
 null alleles so that the phenotype would be ex-
In the case of mutations that alter the sequence of pected to be mild. Those that alter the rate of
the protein, but allow full-length, or nearly full- association activate the cellular  stress protein re-
length, chains to be synthesized, the cell brings a sponse and stimulate the synthesis of several chap-
very different armamentarium to bear in its attack erone proteins which then appear either to
on these purveyors of molecular chaos. The consti- facilitate chain association or interfere with the
tutive pro chains of type I procollagen (and of process (52 54). Although generally inefficient,
other fibrillar collagens) are synthesized on ribo- some of these molecules are secreted. The pheno-
somes bound to the membrane of the rough endo- types tend to be severe, but it is not clear whether
plasmic reticulum and inserted through the this is true for all such mutations or is biased by
membrane during elongation. Because chain asso- ascertainment.
ciation is determined by sequences in the carboxyl-
terminal 200 residues of these 1400 residue long
And ends in the extracellular matrix
chains, the chains lie quietly in the lumen  but
not alone. The modifying prolyl and lysyl hydroxy- Still lacking is a clear step-by-step understanding
lases, hydroxylysyl glycosidases, and additional of how mutations create the specific phenotype. It
chaperone proteins  probably BiP, GRP78, prolyl is all well and good to say that too little normal
cis trans isomerase, and HSP47 accompany these molecules create a mild phenotype while the pres-
chains in their travels. Once synthesis is completed, ence of abnormal molecules creates a more severe
the carboxyl-terminal regions fold, expose interact- picture. After all, although generally true, this is
ing surfaces, and chains assemble into appropriate not always the case. As a rule, for type I collagen
combinations. By itself, the correct combination of mutations, the phenotype reflects the nature of the
chains with their mates is a remarkable testament mutation, the location of the mutation in the
277
Byers
epidermolysis bullosa provides evidence for distinct molec-
molecule, the effect of the mutation on the chain,
ular mechanisms underlying defective anchoring fibril for-
and, for substitutions for glycine residues, the nature
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of the substituting amino acid. It appears that all
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mutations that alter the sequence within the triple- T. Complete primary structure of two variant forms of
human type XVIII collagen and tissue-specific differences in
helical domain of the chain interfere with folding
the expression of the corresponding transcripts. Matrix Biol
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However, it may well be the secreted molecules that
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are less efficiently incorporated into fibrils and when globular domain of collagen XVIII containing the angio-
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