Cytogenetics
and Cellular Phenotype
Cited
literature
Disease
Description
NBS
cellular phenotype is characterised by:
-
spontaneous chromosome instability
- radiosensitivity
- impaired cell cycle checkpoint control
Spontaneous
chromosome instability. Phytohemagglutinin (PHA)-stimulated lymphocytes of NBS patients
show a high frequency (>10%) of chromosome breakages and rearrangements:
chromosome (chrb) and chromatid (chtb) breaks, acentric fragments (ace), asymmetric multiradial
configurations and sporadic or clonal translocations and inversions.
Translocations and inversions preferentially involve chromosomes 7 and 14 with breakpoints
at bands
7p14,
7q35, 14q11 and 14q32, where immunoglobulin
heavy-chain (IgH) and T-cell receptor (TCR) genes are located.
Inv(7)(p14;q35) is the most frequently detected aberration,
followed by t(7;14)(p14;q11), t(7;14)(q35;q11), t(7;7)(p14;q35) and t(14;14)(q11;q32).
Click on the pictures below to see some examples of chromosome aberrations
detected in lymphocytes of a NBS patient
t(7;14)(q35;q11)
t(1;18)
chtb
multiple aberrations:
chtb;
inv(7)(p14;q35);
t(4;17);
ace
Radiosensitivity.
Chromosome instability can be further induced by exposure of cell lines to
ionising radiation or radiomimetic chemicals. The frequency of induced chromosome
aberrations is higher in NBS cells (both lymphocytes and fibroblasts) than in control
cell lines.
Radiosensitivity can also be assessed in a colony survival assay or as a
decreased colony forming
ability after irradiation.
Recently,
the missense mutation R215W (643C>T) has been found at the compound
heterozygous state with the classic 657del5 mutation in two monozygous twins
with a clinically severe form of NBS with neurological abnormalities (Seemanova
et al., 2006).
In the lymphocytes of the tested patient, no increase in the number of
chromosome breaks was observed, and no translocations or inversions involving
chromosomes 7 and 14 were detected.
Radiosensitivity (assessed by counting chromosome breaks in response to
0.5, 1.0, and 2.0 Gy radiation in lymphoblastoid cell lines of both twins) was
not increased when compared to a control cell line, and lower than what observed
in NBS lymphoblastoid cell lines.
The lymphoblastoid cell lines of the patients were
also tested for phosphorylation of nibrin by ATM, phosphorylation of ATM, and
nibrin-promoted phosphorylation/stabilisation
of p53 by ATM, after irradiation.
ATM phosphorylation was barely detectable at 2 Gy, suggesting that the
nibrin-Trp215 is even less able to sustain ionizing radiation
induced ATM activation than the 70 kDa carboxy terminal nibrin fragment
synthesized
from the 657del5 allele (as in 657del5 homozygous cells, ATM phosphorylation is
detected at 2 Gy, indicating a fourfold reduction in ATM activation in these
cells).
On the contrary, nibrin-Trp215 is itself efficiently phosphorylated by ATM.
Phosphorylation of a
downstream target of ATM, p53, is promoted by nibrin. As in cells from patients homozygous for the 657del5 mutation, the 657del5/643C>T(R215W) compound heterozygous cells also phosphorylate p53 poorly, and this is
accompanied by a failure to stabilise p53 after irradiation (see also Clinical
Phenotype and Molecular
Biology).
Radioresistant DNA synthesis and cell cycle checkpoint control.
NBS cells exhibit impaired cell cycle checkpoint control after DNA damage, i.e. irradiated NBS cells
go through cell cycle with
significantly reduced inhibition compared to control cells (which repair
radiation-induced DNA damage before proceeding to the next phase of the cell
cycle). A defect in the S-phase
checkpoint was early recognised in NBS, as radioresistant DNA synthesis (RDS). It has now become clear
that NBS cells
not only fail to suppress DNA synthesis after irradiation, but they also have impaired G1/S and G2/M
transition checkpoints, being unable to stop entry in both S-phase and mitosis. These
defects are partial and radiation-dose dependent, appearing
after exposure to low but not high radiation doses.
Click on the picture below to see a schematic representation of the cell cycle
and of NBS cell defects in checkpoint control.
(click here before opening
the picture if you want to have a legend)
Other features.
NBS cells seem to be defective in telomere (distal end of the chromosomes)
maintenance and degradation protection. This may lead to termino-terminal
fusions and chromosome instability. On the contrary, in a
pulsed field gel electrophoresis (PFGE) assay, NBS cells seem to rejoin DNA double strand breaks
(DSBs) proficiently, even if more sensitive techniques
might be needed to detect specific defects.
Page
last updated on: 13th April 2006
