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features are due to an impairment of the transcription function of XPD or XPB,
whereas the photosensitivity is a consequence of affecting the repair function of
XPD or XPB. The repair/transcription syndrome hypothesis provided a guidingprinciple
to study the molecular defect underlying the non-NER symptoms of TID
and CS. Although the concept is consistent with all observations, unequivocal proof
for it has yet to be obtained. Early embryonic death of XPD knockout mice was
consistent with the essential role of XPD in transcription (Chapter 3). In vitro
growth experiments with preimplantation stage embryos obtained from heterozygous
XPD knockout intercrosses showed a significant higher fraction of embryos that died at
the 2-cell stage, compared to wild-type embryos. These results establish the essential
function of the XPD protein in mammals and in cellular viability and are consistent
with the notion that mostly single point mutations are found in the XPD gene of XP,
XP/CS and TID patients.
By establishing a mouse model for the photosensitive form of TID (Chapter 4),
we have unequivocally demonstrated that the full clinical spectmm of the disease is
caused by a single point mutation in the XPD gene. TTD is a very pleiotropic and
probably partially systemic disease. Osteoporosis and female infertility may be
secondary effects of starvation. In TID mice, the skin is the only tissue where it is
almost certain that the observed symptoms (hyperkeratosis and reduction of
cysteine-rich matrix proteins in the hairs) are caused by a cell autonomous defect
(probably at the level of transcription). Therefore, the skin is of first choice to study
the role of transcription insufficiency in the onset of TID symptoms. In Chapter 4,
we describe that the skin abnormalities are associated with reduced expression of
the late-terminal differentiation gene SPRR2. An unsolved but important
biochemical issue is the consequence of TID and XP/CS mutations in XPD on the
properties of the TFIIH complex: is it an effect on stability of the complex or on the
enzymatic properties alone? In a recent paper, Coin et al. (Nat. Gen. 20:184-188,
1998) demonstrated that certain patient-specific XPD mutations alter the proteinprotein
interaction between XPD and the TFIIH subunit p44. This may affect
stability of the complex and consequently results in reduced transcription capacity
of the cell, thereby causing TTD. TID mice provide a valuable experimental model
to test these predictions.
A breakthrough in the molecular TID puzzle is expected from the identification of
genes involved in NER-proficient TID. A candidate approach was performed in our
lab by sequencing XPD and other genes encoding TFIIH subunits of nonphotosensitive
TID patients but unfortunately no mutations were identified. The
rarity and heterogeneity of the disease precludes identification of candidate genes
via positional cloning. Perhaps Drosophila could be used as a more versatile genetic
tool to search for genes causing non-UV sensitive TID.
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