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Hypothesis testing can be made by use of a vast number of more or less
complicated statistical methods. Statistical software enables non-statisticians
to perform the tests without knowledge about the actual calculations.
However, this comes with the risk of performing wrong tests and making
faulty conclusions. This possible limitation has been restricted by seeking
advice from relevant experts in the field.
Since falsely significant differences are identified by chance according to the
chosen significance level, and this applies to each test being made, it is often
advisable to correct the p-value for multiple testing when several tests are
made. The easiest way to do this is by use of the Bonferroni method, where
the p-values are simply multiplied by the number of tests that were
performed. However, these methods are often very conservative, as a
consequence real differences might be discarded.
Alleles in a randomly mating population without any evolutionary selection
pressures will occur in stably fixed proportions. This is called Hardy-
Weinberg equilibrium. In the gene association studies, Hardy-Weinberg
equilibrium was assessed by comparing theoretical genotype distributions,
calculated on the basis of observed allele frequencies, with observed
genotype frequencies using χ
2
-test. This ensures detection of genotyping
errors that could otherwise lead to false associations. However, Hardy-
Weinberg disequilibrium might also reflect natural selection as a
consequence of an actual association.
In gene association studies, risk estimations for carriers of the susceptibility
gene are relevant, not only to estimate the risk, but also as a measure of the
effect size of the studied gene. Since most diseases are uncommon, a
randomly selected cohort or a selection of carriers and non-carriers, are very
impractical because of the large sample size needed in such approaches. The
common way is instead to use a case-control design. However, this means
that a proper risk ratio cannot be calculated, since the study population was
not randomly selected from the whole population. Instead an odds ratio (OR)
has to be used. The OR represents the risk of an outcome and is interpreted in
relation to 1 which means that the subject under study has no effect on risk.
An OR of more than 1 means an increased risk, whereas an OR of a number
less than 1 should be interpreted as protective.
Synaptic elimination and the complement system in Alzheimer’s disease
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4
SUMMARY OF RESULTS
4.1
Association of RAGE with AD diagnosis
In an attempt to provide a link between recent animal studies showing that
the RAGE receptor mediates the LTP inhibiting effect of Aβ, and actual
human AD patients, we investigated the genetic variability in the gene
encoding RAGE (somewhat confusingly the gene is named advanced
glycation end-products receptor (AGER)), and it’s relation to AD.
While working on this project, a study (Li et al. 2010a) on the very same
subject was published. This group showed an association of the functional
82S SNP with AD in a Chinese cohort. In the field of genetics this is actually
a good thing, since independent replications in separate populations are
needed. In contrast to the study by Li et al. (2010) all AD cases in our
material were neurochemically confirmed and of European origin (further
demographics are given in Table 1, paper I).
Our study investigated some additional SNPs in AGER (Table 2, paper I),
although none of these were associated with AD, the 82S allele was
associated with an increased risk of AD (Pc=0.04, OR=2.0, 95% CI 1.2–3.4)
(Table 3, paper I). There was no genetic interaction between AGER 82S and
APOE ε4 in producing increased risk of AD (p=0.21) (Table 4, paper I), and
none of the AGER SNPs showed association with Aβ42, T-tau, Ptau181 or
MMSE scores.
RAGE may affect both production and accumulation of Aβ in the brain
(Chaney et al. 2005; Cho et al. 2009; Deane et al. 2003). Since the 82S
variant of RAGE has an increased ligand-binding affinity (Hofmann et al.
2002; Osawa et al. 2007) this could lead to increased signalling, which in
turn, would accelerate APP-processing through BACE1 (since BACE1 has
been shown to be positively regulated by RAGE (Cho et al. 2009)), and
thereby increase Aβ production. An increased transport of peripheral Aβ into
the brain would be expected as well, since RAGE has been shown to
transport Aβ across the blood-brain barrier into the brain (Deane et al. 2003).
However, we found no association between G82S genotype and Aβ levels.
Another possibility is that the 82S variant may be more effective in mediating
the LTP-inhibiting effect of Aβ (Arancio et al. 2004; Origlia et al. 2009;
Origlia et al. 2008), a hypothesis that remains to be tested.
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In conclusion, the present results, together with those from Li et al. (2010),
suggest that AGER is a susceptibility gene for AD.
4.2
Complement mediated synapse
elimination in the hippocampus
The complement system has been shown to be involved in elimination of
retino geniculate synapses (Stevens et al. 2007) and synapses in the
sensorimotor cortex (Chu et al. 2010) of mice.
Considering that the first brain region to be affected in AD is the
hippocampus, and that loss of synapses is perhaps the most relevant feature
of the disease; investigation of the mechanisms of synapse elimination in the
hippocampus is highly warranted.
Thus we decided to investigate if the number of synapses is altered in mice
lacking C3. The hypothesis was that these mice should have an increased
number of synapses as a result of impaired synaptic elimination.
An increased number of synapses should lead to an increased synaptic
efficacy, when measured as the average synaptic response to a given number
of stimulated axons. Surprisingly, the input/output measurements showed no
difference between C3 KO and WT animals (Fig. 1, paper II). However, there
was an increased PPR, indicating lower release probability in the C3 KO
mice (Fig. 2, paper II). According to the formula for synaptic efficacy, n*p*q,
a decreased release probability could potentially mask an increased synapse
number. In order to ascertain the decreased release probability in the C3 KO
mice, we recorded NMDAR EPSPs in the presence of MK-801. The resulting
MK-curves showed that the release probability was indeed lowered (Fig. 3,
paper II), thus supporting the original hypothesis that C3 KO mice have an
increased number of synapses.
This conclusion assumes that the quantal size remain unchanged. To test this,
we made whole cell patch clamp recordings of miniature EPSCs. Since no
difference in the size of the AMPAR EPSCs was observed (Fig. 4, paper II),
we conclude that quantal size is not changed in C3 KO mice. In conjunction
with a decreased release probability and unaltered input/output, the only
sensible conclusion is that the C3-deficient mice must have an increased
number of synapses.
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