Biology Reference
In-Depth Information
regulated by TH. Surprisingly, there was little overlap in genes regulated by
T3 and T4, even though they have similar morphological outcomes. This
finding cannot be accounted for by differences in timing of T4 to T3 con-
version by deiodinases and awaits an explanation. For the inhibitors, similar
numbers of genes in similar categories were identified, and association anal-
ysis of genes regulated by MMI, PTU, and PER suggested they have similar
gene regulation changes. Thus, the inhibitors may have similar modes of ac-
tion on brain gene regulation. However, the rapid responses, that is, within
1-2 days, suggests that the mode of action may be a direct effect on brain
tissue, rather than blockade of TH synthesis with attendant delay in running
out of stores of hormone in the thyroid gland.
A complimentary analysis to the previous MAGEX brain study using the
same animals was carried out using hind limb and tail (
Helbing et al., 2007
).
As with the brain, the strategy was to find candidate genes using microarray that
could then be used in quantitative comparisons using qPCR. Fewer genes over-
all were identified in these tissues compared to the brain. As in the brain, T4 and
T3 had nonoverlapping sets of transcripts. Also, the sets of transcripts signifi-
cantlyregulatedbyMMI,PTU,andPERwereverydifferentfromeachother.
Some genes were identified as potential biomarkers, such as RNA helicase
II/Gu or DAD1 (defender against death 1). However, their use as biomarkers
is dependent on the tissue and time point after exposure. These studies revealed
the important issue that tissue and exposure duration need to be carefully
determined to identify biomarkers with high and robust predictive value.
In a recent microarray analysis, a meta-analysis approach was used to
identify robust TH-responsive genes as determined by detection across sev-
eral array platforms (
Searcy et al., 2012
). They used an Affymetrix array with
tail tissue treated for 6 and 48 h, with 20 nM TH. Many of the regulated
genes included those previously identified, such as transcription factors
and proteases. However, when comparing across platforms, they found
30% or less overlap of TH-regulated genes, which may be explained by
genes not represented on one of the arrays and the different TH doses used.
On the other hand, there was a high proportion of overlap with TH-
induced tail genes identified by subtractive hybridization. They reasoned
that overlap among platforms would represent reliable and robust genes
for use as biomarkers for TH disruption. As a proof of principle, they exam-
ined the effect of wastewater effluent (WWE) on four TH-response genes,
TR
b
, D2, CRHBP, FAP
a
. WWE augmented the induction of these tran-
scripts in the presence of TH, and transcripts of CRHBP and FAP
a
were
increased with WWE even without TH.
