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PLEIOTROPY AND
GENETIC HETEROGENEITY
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Genetica per Scienze Naturali
a.a. 08-09 prof S. Presciuttini
The complexity of the phenotype
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At one level, geneticists tend to think of genes in isolation. In reality,
genes don't act in isolation. The proteins and RNAs they encode
contribute to specific cellular pathways that also receive input from
the products of many other genes. Furthermore, expression of a single
gene is dependent on many factors, including the specific genetic
backgrounds of the organism and a range of environmental
conditions, temperature, nutritional conditions, population density,
and so on.
Gene action is a term that covers a very complex set of events, and
there is probably no case where we understand all the events that
transpire from the level of expression of a single gene to the level of
an organism's phenotype.
Two important generalizations about the complexity of gene action:
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1. There is a one-to-many relationship of genes to phenotypes.
2. There is a one-to-many relationship of phenotypes to genes.
Genetica per Scienze Naturali
a.a. 08-09 prof S. Presciuttini
One-to-many relationship of genes to phenotypes
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This relationship is called pleiotropy. Pleiotropy is inferred
from the observation that mutations selected for their effect
on one specific character are often found to affect other
characters of the organism. This might mean that there are
related physiological pathways contributing to a similar
phenotype in several tissues.
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For example, the white eye-color mutation in Drosophila results in
lack of pigmentation not only in compound eyes but also in ocelli
(simple eyes), sheaths of tissue surrounding the male gonad, and
the Malpighian tubules (the fly's kidneys). In all these tissues,
pigment formation requires the uptake of pigment precursors by
the cells. The white allele causes a defect in this uptake, thereby
blocking pigment formation in all these tissues.
Genetica per Scienze Naturali
a.a. 08-09 prof S. Presciuttini
The AY allele as an example of pleiotropy
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An important question is how can a gene controlling coat color cause
death in an organism? Possibly in a single dose the allele causes a
yellowing of the coat, but when expressed in two doses, the gene
product kills the animal. Thus, this gene actually has an effect on two
phenotypes.
Pleiotropic gene - a gene that affects more than one phenotype
In this example the gene that causes yellowing of the coat also affects
viability and is termed a pleiotropic gene.
Genetica per Scienze Naturali
a.a. 08-09 prof S. Presciuttini
Pleiotropy is the rule rather than the exception
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Pleiotropy: A given gene affects more than one trait.
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Examples of pleiotropy abound; in fact, probably every gene is
pleiotropic to some extent.
Some instances of pleiotropy are not surprising.
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Mice that are bred for larger body size also produce litters with more pups. It is
not hard to understand how this can occur, since larger bodies have more room
for young inside and can supply more nutrients to the young.
A plant that lacks a functional gene involved in synthesis of abscisic acid has
leaves that wilt easily and seeds that do not become dormant in the normal way.
Genetica per Scienze Naturali
a.a. 08-09 prof S. Presciuttini
5. Phenylketonuria (PKU)
Another example is human
phenylketonuria, in which loss of an
enzyme involved in the breakdown
of excess phenylalanine causes
pleiotropic effects that include
elevated phenylalanine levels in the
blood plasma, urinary excretion of
intermediate products of
phenylalanine breakdown, severely
reduced IQ, changes in hair color,
and changes in head size.
Figure. Frequency distributions of
phenylketonurics (right) compared with
controls (left). A: d/s = 13, where d is the
difference in the means and s is the average
standard deviation of the two distributions.
B: d/s = 5.5; C: d/s = 2.0; D: d/s = 0.7
Genetica per Scienze Naturali
a.a. 08-09 prof S. Presciuttini
Gene mutations may affect apparently
unrelated traits
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Often, pleiotropy involves multiple events that are not
obviously physiologically related.
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For example, the dominant Drosophila mutation Dichaete causes
the wings to be held out laterally but also removes certain hairs on
the back of the fly; furthermore, the mutation is unviable when
homozygous.
This example shows a real limitation in the way dominant and
recessive mutations are named. The reality is that a single
mutation can be both dominant and recessive, depending on which
aspect of its pleiotropic phenotype is under consideration. In
general, genetic terminology is not up to the task of representing
this level of pleiotropy and complexity in one symbol, and there is
a certain arbitrary or historical aspect as to how we name alleles.
Genetica per Scienze Naturali
a.a. 08-09 prof S. Presciuttini
Waardenburg syndrome in human
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In humans, heterozygosity for a gene called MITF causes a condition
called Waardenburg syndrome type 2, which involves iris defects,
pigmentation abnormalities, deafness, and inability to produce the
normal number of mast cells (a type of blood cell).
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1st quadrant = hearing loss
2nd quadrant = different colored eyes
3rd quadrant = white forelock
4th quadrant = premature graying of hair.
Genetica per Scienze Naturali
a.a. 08-09 prof S. Presciuttini
Pleiotropy and variable expressivity in WS
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The Waardenburg syndrome was named for Petrus Johannes
Waardenburg, a Dutch ophthalmologist (1886-1979) who was the first
to notice that people with two different colored eyes frequently had
hearing problems.
In this disease, the skin pigment, the iris of the eye, the inner ear
tissue, and the mast cells of the blood are not related to one another in
such a way that the absence of one would produce the absence of the
others.
Rather, all four parts of the body independently use the MITF protein
as a transcription factor.
In addition to different phenotypes, WS is also characterized by
variable expression of the various features.
Genetica per Scienze Naturali
a.a. 08-09 prof S. Presciuttini
A major challenge in genetics
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Pleiotropy occurs when a mutation in a single gene produces effects on more than
one characteristic, that is, causes multiple mutant phenotypes. In humans, this
phenomenon is most obvious when mutations in single genes cause diseases with
seemingly unrelated symptoms
A major challenge in the analysis of pleiotropic genes is determining whether all of
the phenotypes associated with a mutation result from the loss of a single function
or of multiple functions encoded by the same gene.
In addition to providing important information about gene function, distinguishing
between these two models is important for devising effective treatments and
analyzing drug side effects.
Classical genetic analysis attempts to resolve such issues by isolating and
characterizing multiple alleles of the same gene, with the goal of determining
whether these phenotypically defined functions are genetically separable.
Unfortunately, this type of approach is time consuming and often not feasible in a
clinical setting, which relies on the identification of naturally occurring alleles.
Genetica per Scienze Naturali
a.a. 08-09 prof S. Presciuttini
One-to-many relationship of phenotypes to genes
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This concept is based on the observation that many different genes
can affect a single phenotype. This is easy to understand in terms of a
character such as eye color, in which there are complex metabolic
pathways with numerous enzymatic steps, each encoded by one or
more gene products.
Genetic heterogeneity is the term used to refer to a given condition
that may be caused by different genes.
One goal of genetic analysis is to identify all the genes that affect a
specific phenotype and to understand their genetic, cellular,
developmental, and molecular roles. To do this, we need ways of
sorting mutations and genes.
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We first will consider how we can use genetic analysis to determine if two
mutants are caused by mutational hits in the same gene (that is, they are alleles)
or in different genes.
Later, we will consider how genetic analysis can be used to make inferences
about gene interactions in developmental and biochemical pathways.
Genetica per Scienze Naturali
a.a. 08-09 prof S. Presciuttini
The complementation test
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The allelism test that finds widest application is the complementation
test, which is illustrated in the following example.
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Consider a species of flower in which the wild-type color is blue. We have
induced three white-petaled mutants and have obtained pure-breeding strains
(all homozygous). We can call the mutant strains $, £, and ¥, using currency
symbols to avoid prejudicing our thinking concerning dominance. In each case
the results show that the mutant condition is determined by the recessive allele
of a single gene. However, are they three alleles of one gene, or of two or three
genes? The question can be answered by asking if the mutants complement
each other.
Complementation is the production of a wild-type phenotype
when two recessive mutant alleles are united in the same cell.
Genetica per Scienze Naturali
a.a. 08-09 prof S. Presciuttini
Performing the complementation test
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In a diploid organism the complementation test is performed by intercrossing
homozygous recessive mutants two at a time and observing whether or not the
progeny have wild-type phenotype. If recessive mutations represent alleles of the
same gene, then obviously they will not complement because they both represent
lost gene function. Such alleles can be thought of generally as a’ and a", using
primes to distinguish between two different mutant alleles of a gene whose wildtype allele is a+. These alleles could have different mutant sites but would be
functionally identical. The heterozygote a’/a" would be
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However, two recessive mutations in different genes would have wild-type function
provided by the respective wild-type alleles. Here we can name the genes a1 and a2,
after their mutant alleles. Heterozygotes would be a1/+ ; +/a2 (unlinked genes) or
a1+/+a2 (linked genes), and we can diagram them as follows:
Genetica per Scienze Naturali
a.a. 08-09 prof S. Presciuttini
Mutants that complement
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We now return to the flower example and intercross the mutant strains
to test for complementation. Assume the results of intercrossing
mutants $, £, and ¥ are as follows:
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From this set of results we would conclude that mutants $ and £ must
be caused by alleles of one gene (say w1) because they do not
complement; but ¥ must be caused by a mutant allele of another gene
(w2).
The molecular explanation of such results is often in terms of
biochemical pathways in the cell. How does complementation work at
the molecular level? Although it is conventional to say that it is
mutants that complement, in fact the active agents in complementation
are the proteins produced by the wild-type alleles.
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Genetica per Scienze Naturali
a.a. 08-09 prof S. Presciuttini
Complementation
Three phenotypically identical white
mutants, $, £, and ¥, are intercrossed
to form heterozygotes whose
phenotypes reveal whether or not the
mutations complement each other.
(Only two of the three possible
crosses are shown here.) If two
mutations are in different genes (such
as £ and ¥), then complementation
results in the completion of the
biochemical pathway (the end
product is a blue pigment in this
example). If mutations are in the
same gene (such as $ and £), no
complementation occurs because the
biochemical pathway is blocked at
the step controlled by that gene, and
the intermediates in the pathway are
colorless (white).
Genetica per Scienze Naturali
a.a. 08-09 prof S. Presciuttini
Complementation test for rare human genetic
diseases
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Genetic complementation for rare and invalidating genetic diseases
cannot be observed in human pedigree, because affected people do not
marry
However, some mammalian somatic cells can be
cultured in a well-defined medium. In addition,
cultured cells can be fused to produce somatic
hybrids; although cell fusion occurs spontaneously
at very low rate, it can be increased in the presence
of certain viruses that have a lipoprotein envelope
similar to the plasma membrane of animal cells. A
mutant viral glycoprotein in the envelope promotes
cell fusion. Cell fusion is also promoted by
polyethylene glycol, which causes the plasma
membranes of adjacent cells to adhere to each other
and to fuse.
Genetica per Scienze Naturali
a.a. 08-09 prof S. Presciuttini
Xeroderma pigmentosum
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Cell fusion is the basis for the complementation test in human; if a
genetic defect is assayable in cultured cells, complementation analysis
by cell fusion can be undertaken.
For example, the autosomal recessive disease xeroderma
pigmentosum (XP) involves defects in repair of UV-induced damage
in DNA. Patients are abnormally sensitive to sunlight, developing skin
cancer after relatively brief exposure.
Multiple basocellular carcinomas on
the face of an XP patient. Thick arrow
points to a recent lesion, and thin
arrow to a scar of an old lesion
Genetica per Scienze Naturali
a.a. 08-09 prof S. Presciuttini
First demonstration of complementation groups in XP
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By fusing fibroblasts from various patients with XP, seven main
complementation groups have been defined
Genetica per Scienze Naturali
a.a. 08-09 prof S. Presciuttini
Different sensitivity to UV radiation of cells from
different complementation groups
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Hypersensitivity to UV radiation of XP cells in culture. Here the cells
from a number of complementation groups are shown. There is a
variation between complementation groups, but all are more sensitive
to UV radiation than are normal cells. The difference in UV
photosensitivity between normal and diseased cells is evident from the
survival curves of cultured cells treated with UV light
Genetica per Scienze Naturali
a.a. 08-09 prof S. Presciuttini