Differential hybridization was used to detect repair defects in
xeroderma pigmentosum (XP) that are not amenable to current analyses. cDNA libraries were constructed from cytoplasmic
RNA of normal and XP fibroblast strains (complementation groups A and D) and analyzed for differential gene expression. More than 40,000 lambda gt10
cDNA clones were differentially screened with in vitro transcripts made from
cDNA in the pBluescript vector. Six differential clones were detected in the libraries of the XP group A and D strains which caused stronger or weaker signals when probed with transcripts from XP strains than with those from the normal strains. Two clones coded for mitochondrial genes: mitochondrial 16 S rRNA and
ATPase 6L. Overexpression of mitochondrial genes in XP may indicate that functions of the
ATP-generating system are impaired since such functions are intensified whenever they become insufficient, for example as a consequence of DNA damage. It is tempting to assume that abnormal mitochondria are one of the causes for the neurological malfunctions in XP. Furthermore, densitometric analysis of Northern blots revealed that
mRNA of
lactate dehydrogenase, chain M, was less abundant in four XP group A strains (extent of reduction: 70%) and in two
XP group D strains (extent of reduction: 58%).
Enzyme activity was also diminished. In addition,
mRNA of the gene for
glyceraldehyde-3-phosphate dehydrogenase was less expressed in the same XP group A and D fibroblast strains investigated (reduction in both complementation groups: 50%). Both glycolytic
enzymes have nuclear functions apart from their role in
sugar metabolism.
Lactate dehydrogenase, chain M, is identical to a helix-destabilizing
protein; it is closely associated with
chromatin and unfolded
DNA, suggesting a role in
DNA synthesis and transcription. The 37-kDa subunit of
glyceraldehyde-3-phosphate dehydrogenase is involved in transcription and was shown to be identical to
uracil-DNA glycosylase, a base-excision repair
enzyme. We presume that the nuclear functions of these glycolytic
enzymes may be thwarted in the XP strains investigated and may account for malfunctions in XP, particularly for neurological disturbances.