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sions have no counterpart in the human orthologues. Similarly, the observation
that triplet repeats associated with human disease are sometimes found to be both
polymorphic and expanded beyond the mammalian ancestral state in non-human
primates may also be viewed in terms of a bias of ascertainment rather than any
intrinsic propensity of triplet repeats to expand in primate genomes. Finally, not-
ing that most of the expanded triplet repeats are found in genes expressed in the
nervous system, Hancock (1996) proposed that triplet repeat expansion may have
occurred in parallel with the increase in brain complexity so characteristic of the
human lineage and may even have been instrumental in bringing about this
increase in complexity.
8.9.5 The unique case of involucrin
Involucrin is a unique protein in that it has evolved very rapidly in the primate
lineage through changes in the length and copy number of tandem repeats within
the coding region with the result that a full two thirds of its coding region has
been created within the anthropoid lineage (Green and Djian, 1992). The evolu-
tion of this gene/protein is therefore worth examining in some detail.
Involucrin is the most abundant protein component of the keratinocyte enve-
lope and is cross-linked to membrane proteins by a transglutaminase. The coding
1. Multiple duplications of the triplet CAG
CAG CAG CAG CAG CAG
(30 bp sequence 73 homologous
to future consensus sequence of
control segment)
2. Duplications of (CAG)
10
and mutations
Divergent repeats resembling
present flanking sequences
3. Duplications of one or
more divergent repeats
similar to future consensus
sequence of the central
segment
Central segment
4. Mutations in one repeat,
followed by duplications
First invariant block
5. Mutations in one repeat
of first invariant block,
followed by duplications
Second invariant block
Figure 8.14.
Proposed scheme for the evolution of the human involucrin (
IVL
) gene.
