Download There are a variety of diseases commonly ascribed to antigenic

Noah Fram
Biology 303 (Genetics) term paper
November 3, 2007
Association between environmental and genetic causes of cerebral hemorrhaging
There are a variety of diseases commonly ascribed to environmental causes,
including such commonly known ones as obesity and hypertension, which are in fact
largely the responsibility of genetic mutations. Cerebral hemorrhaging, or the rupture of
capillaries supplying the brain with oxygen, is among the most deadly of these (3rd
highest cause of death in the United States). The most commonly known form of it,
stroke, often causes mental retardation and occasionally can result in severe mental
impairment (inability to feed oneself, inability to use language, undeveloped or
nonexistent motor control, etc.) Until the genomic era, the only method for stroke
prevention was relative abstinence from alcohol, or at least refraining from overindulgence.
While alcohol is still accepted as a major risk factor for stroke (8), twin studies
and family aggregation analysis pointing to a genetic component soon led many
biologists to start seeking a genetic mutation which would cause lesions in cranial blood
vessels. They tended to focus on a specific type of cerebral hemorrhage, cerebral
cavernous malformation (CCM), a
known autosomal dominant
Mendelian disorder, and soon
pinpointed three genes keying for
CCM: KRIT1 on 7q (7),
MGC4607 on 7p (3), and PDCD10
on 3q (4). The KRIT1 region was
detected first because most of the
studies focused on Hispanics, who
demonstrated a high expression of
the CCM trait, and the vast
majority of Hispanics with CCM
have the KRIT1 mutation.
One of the chief problems
Fig. 1 Radiological and histological features of familial cavernous angiomas.
with studying cerebral
a, Cerebral magnetic resonance of a patient: multiple lesions are observed.
b, Histological section of a cerebral cavernous angioma showing juxtaposition
hemorrhaging is its all-tooof thin vascular cavities lined by endothelium and collagen without intervening
frequent lethality. Because of
brain parenchyma (Modified Gomori Trichrome Stain, ﾗ 40 magnification,
courtesy of F. Chapon)(7).
this lethality, there have been a
number of studies investigating
certain mutations in model organisms, frequently zebrafish. Studying fish embryos is
much simpler than mammalian ones because even in vitro, fish are free-swimming. Of
course, fish and mammals have adapted to different environments, so their systems do
not show as much conservation of human processes as, for example, mice. What makes
zebrafish so common in studies of vascular development and integrity is that their genetic
makeup in this area is remarkably similar to that of mammals (1).
David Buchner and his team conducted an experiment to determine which gene
was causing one of a variety of types of cerebral hemorrhaging in zebrafish (1). While
gene pathways such as Notch, TGFβ, and VEGF had already been identified as causing
cerebral hemorrhaging (mutants mush for brains, bubblehead, migraine, gridlock, and
leaky heart), no pathway had been labeled as causing the redhead strain.
The rhd (redhead) gene was mapped to a specific section of zebrafish
chromosome 2 using bulked segregant analysis. After identifying the rhd
nonrecombinant interval as
lying between the D2Umi14
and D2Umi17 markers, they
decided to focus on four of
the seven genes present in
this interval. Among these
genes was pdcd10-like,
which they chose for special
investigation because its
human ortholog, PDCD10,
plays a major role in CCM.
Direct sequence comparison
between mutant and wild type
(A) Genetic map of the rhd nonrecombinant interval. Seven genes were identified as
potential canditates for the rhd phenotype, with all but golph4-like and pdcd10-like
embryos demonstrated that
within the refined nonrecominant interval between D2Umi17 and D2Umi18 (B). (1)
while pdcd10-like did not play
any obvious role in the rhd phenotype, that two other candidate genes, golph4-like and
pak2a both could be important. They finally pinpointed pak2a as the mutant gene
through RT-PCR (real-time polymerase chain reaction), which when combined with
pak2a’s unique position as within the refined nonrecombinant region between markers
D2Umi17 and D2Umi18 clearly identifies pak2a as
the gene responsible for rhd.
Pak2a’s human ortholog, PAK2, is also
responsible for in vitro cerebral hemorrhaging.
However, there is very little experimental research
about the human PAK2 gene because in vitro
hemorrhaging is lethal in humans. This is not
necessarily true for zebrafish embryos, however. For
this reason, most population-oriented experiments with
cerebral hemorrhaging are done with model organisms.
In fact, CCM is such a popular topic with stroke
(D,E) RT-PCR results for golph4-like (D) and pak2a
researchers just because it tends to strike in vivo, and
(E). M indicates 1-kb Plus DNA ladder
is not universally lethal. Understanding mutations in
(Invitrogen). (1)
the KRIT1 gene or its relations is not nearly as
daunting a task as understanding a mutant which causes humans to die in embryo.
Genomics and proteomics have played a major role in making cerebral
hemorrhaging easier to study without having to rely on organisms such as zebrafish for
experimental data (5). For example, an Icelandic project involving 476 stroke patients
used a whole-genome scan to identify linkage to genes on chromosomes 5q12 and 13q1213 which have been named STRK1 and STRK2. Also, mutations in the genes for factor
V Leiden and prothrombin apparently are potential risk factors for stroke, especially in
women taking certain types of oral contraceptives. Some genetic factors affect different
types of stroke in different ways: elevated serum cholesterol is positively correlated with
ischemic (embolic) stroke risk but negatively with hemorrhagic stroke. Finally, some
genes play double duty, as the same genes which can cause diabetes and hypertension
also increase risk of stroke.
One of the most interesting points about the Hispanic-American population of
diagnosed CCM patients is that it displays a rather pronounced founder effect. In other
words, there is a high statistical probability that all or most of those cases are descended
from a single, ancient mutation, in a fashion
similar to Huntington’s disease. However, this
does not hold true for other populations. In
1999, Sophie Laberge et al published an article
in the European Journal of Human Genetics
which described a genetic analysis of a group of
36 French families (6). Haplotype analysis of
the families with the conditional linkage
probability >.95 (shown at left) showed that
none of them shared the Hispanic-American
haplotype. Comparative analysis also
demonstrated no evidence of a founder effect
within this French population.
Examined from an historical
perspective, this observation could lead to a
variety of very important conclusions in
understanding CCM. For example, given the
extensive history of genetic mixing between the
Spanish conquistadores and the South American
natives (whose progeny now dominate the
Hispanic-American population), a founder
effect could imply that the CCM1 mutation was
not present in the native population but was
brought across the Atlantic by the Spaniards.
Taking this one step further, perhaps that
particular mutation was more common among
Spaniards than the South Americans; this would
not only explain, to some extent, the founder
effect among Hispanic-Americans but also the
Conditional probability of association with CCM1 (KRIT1) gene in
lack of one among the French.
each of 51 families involved in an association study. Nine families
(yellow) had a condition probability of p>.95, while four had p<.05.
Essentially, genomic/proteomic analysis
of potential risks for cerebral hemorrhaging and other disorders like it is a relatively new
development, but one with a great deal of promise. Stroke itself does not require a
genetic component but, like obesity, understanding the hereditary aspects of stroke could
very well help scientists develop more effective preventative medications. In at least one
case, research into risk factors associated with metabolizing stroke medication (CYP450
enzymes in particular can cause varied sensitivity to a stroke medication called warfarin)
(5) could help doctors determine which drug in what dosage would be most appropriate
for that specific patient.
BIBLIOGRAPHY
1. Buchner, D. A.; et al. (2007) Pak2a mutations cause cerebral hemorrhage in
redhead zebrafish. PNAS, 104(35), 13996-14001. National Academy of
Sciences of the USA.
2. Craig, HD; et al. (1998) Multilocus linkage identifies two new loci for a
mendelian form of stroke, cerebral cavernous malformation, at 7p15-13 and
3q25.2-27. Human Molecular Genetics, 7, 1851-8. Oxford University Press.
3. Denier, C; et al. (2004) Mutations within the MGC4607 Gene Cause Cerebral
Cavernous Malformations. American Journal of Human Genetics, 74, 326-37.
American Society of Human Genetics.
4. Guclu, Bulent; et al. (2005) Mutations in apoptosis-related gene, PDCD10, cause
cerebral cavernous mutation 3. Neurosurgery, 57(5), 1008-13. Congress of
Neurological Surgeons.
5. Gwinn-Hardy, Katrina; Dawson, Valina. (2004) Genomic-Proteomics and
Stroke: Introduction. Stroke, 35(suppl 1), 2731-4. American Heart Association,
Inc.
6. Laberge, S; et al. (1999) Genetic heterogeneity and absence of founder effect in
a series of 36 French cerebral cavernous angiomas families. European Journal
of Human Genetics, 7(4), 499-504.
7. Laberge, S; et al. (1999) Truncating mutations in CCM1, encoding KRIT1,
cause hereditary cavernous angiomas. Nature Genetics, 23, 189-93. Nature
America, Inc.
8. Liu, Jin; et al. (2007) A βPix–Pak2a signaling pathway regulates cerebral
vascular stability in zebrafish. PNAS, 104(35), 13990-5. National Academy of
Sciences of the USA.
9. Woo, Daniel; et al. (2002) Genetic and environmental risk factors for
intracerebral hemorrhage: preliminary results of a population-based study.
Stroke, 33(5), 1190-7. American Heart Association, Inc.