Biology Reference
In-Depth Information
54
Figure 4.8.
A hypothetical
scheme for the evolution of the
exons in a region of the
a
2(I)
collagen (
COL1A2
; 7q22) gene
coding for the helical region of
the polypeptide (Li, 1997). The
number of base-pairs in each
exon is given. The dotted line
denotes the position of an
unequal crossover. An unequal
crossover between two 54 bp
exons could give rise to an
exon of 99 bp and an exon of
9 bp whereas an unequal
crossover between an exon of
99 bp and exon of 54 bp could
give rise to an exon of 108 bp
and an exon of 45 bp.
54
54
54
54
54
99
54
54
54
54
108
99
54
54
54
and fusions. The recurring sizes of many of the exons that encode the extant
human collagens provides ample evidence for the exon amplification and con-
traction processes that must have fashioned them. Although the evolution of sev-
eral families of related collagen genes has been studied in some detail, the
evolution of the gene family in its entirety has not yet been pieced together. Since
both the fibrillar and nonfibrillar collagens have a very ancient origin, this task is
one of some very considerable complexity.
Genes for the fibroblast growth factors and their receptors.
The fibroblast
growth factors (FGF) constitute a family of polypeptide growth factors that have
multiple functions in mitogenesis, angiogenesis and wound healing. They con-
tain an extracellular portion with either 2 or 3 immunoglobulin-like domains, a
transmembrane domain and a cytoplasmic tyrosine kinase domain. The FGF
family of proteins interacts with the membrane-associated FGF receptors
(FGFR) and these interactions are important for a variety of developmental
processes such as the formation of the mesoderm during gastrulation, the integra-
tion of growth, budding and patterning during early post-implantation, and the
development of various tissues including the skeletal system.
A total of 14 FGF genes have been identified in the human genome:
FGF1
(5q31),
FGF2
(4q25-q27),
FGF3
(11q13),
FGF4
(11q13),
FGF5
(4q21),
FGF6
(12p13),
FGF7
(15q15-q21),
FGF8
(10q24),
FGF9
(13q12),
FGF10
(5p12-p13),
FGF11
(17q21),
FGF12
(3q28),
FGF13
(Xq26) and
FGF14
(13q34). Thus, with
the exception of the closely linked and tandemly repeated
FGF3
and
FGF4
genes, the FGF genes are dispersed in the human genome. FGF genes are pre-
sent in invertebrate genomes and probably expanded in number after the diver-
gence of protostomes and deuterostomes. The mammalian FGF gene family has
emerged by a process of phased duplication and divergence (
Figure 4.9
; Coulier
et
al
., 1997; Johnson and Williams, 1993). Some of the paralogous mammalian FGF
