Biomedical Engineering Reference
In-Depth Information
So, why do some translocations appear much more frequently than others do? Is
this because of the higher-order chromatin structure or DSB movement?
7
Moreover,
in latter case, is there a higher chance that some loci will move to a particular
nuclear domain and will mutually interact more frequently because of higher-order
structure? Or, is a combination of both hypotheses required?
Since DSB movement was observed when cells were irradiated with densely
[
195
], but usually not sparsely [
197
,
199
], ionizing radiation, one explanation of the
contradictory results regarding DSB mobility could be that high-LET IR fragments
chromatin [
247
], due to high energy deposition along the particle track. It is indeed
hardly conceivable that these different mechanisms (one processing DSBs at the
sites of origin and one that requires their directed movement into repair factories)
would operate depending on the quality of damaging IR or kind of DSBs produced.
In support to this, our experiments in which DSBs were induced by low-LET
”
H2AX, NBS1 or 53BP1 foci revealed generally
little movement of DSBs, and, at the same time, showed that some foci had an
exceptionally high mobility [
199
,reviewedin
135
]. In agreement with the results
of P. Jeggo's group [
66
-
71
] that showed a specific repair mechanism for hete-
rochromatic DSBs (Sec.
20.4.4
), mobile foci were almost exclusively located inside
the condensed heterochromatin or at its border with decondensed euchromatin
domains (called “chromatin holes” because of their low staining with DNA dyes in
interphase nuclei) [
199
]. In addition, the seemingly chaotic movement of “mobile
DSBs” was, in fact, directed from the condensed chromatin into chromatin holes,
where it occasionally ended with the clustering of two or rarely more
-rays and immuno-detected as
”
H2AX
foci [
199
;reviewedin
135
]. Since heterochromatic DSBs largely colocalized
with epigenetic markers suggesting chromatin decondensation (Sec.
20.4.4
), their
movement probably reflects the opening of dense heterochromatin domains that
is initiated by DSB repair. The “directed” nature of this movement, although
unexpected if it should result from “random” decondensation, can be also simply
described since protrusion of damaged chromatin into chromatin holes is frequently
easier than decondensation of the whole affected chromatin domain, especially
when it comes to DSBs located close to the border with the chromatin hole [
199
;
reviewed in
135
](Fig.
20.1
d, e).
Importantly, the limited space within chromatin holes sometimes causes temporal
or stable clustering of repair foci that protrude into the same hole [
199
]. So, what
in fact represent these
”
H2AX clusters: complex multiple DSBs, repair factories
or by-products of DSB repair with an increased risk of formation of chromosomal
translocations? Since
”
H2AX clusters persist longer than single foci in cell nuclei,
the last possibility seems to be most probable. A model that illustrates the potential
relationships between higher-order chromatin structure, DSB repair, and formation
of chromosomal translocations, is described in a detail in the review of Falk et al.,
2010 [
135
](Fig.
20.1
d,e). Briefly, euchromatic DSBs are repaired at, or close to,
their individual positions. Therefore, in accordance with the “Position First Theory,”
”
7
provoked either by DSB induction or repair
