Potential use of the levels of the mRNA of a specific metallothionein isoform (MT-20) in mussel (Mytilus edulis) as a biomarker of cadmium contamination

Metallothionein (MT) is a metal-inducible protein and a potential biomarker of metal pollution. Mussel MTs are characterized by having both monomeric (MT-10) and dimeric (MT-20) isoforms. We have carried out a series of experiments in which we studied mRNA levels of both MTs in Mytilus edulis after a brief exposure (6–72 h) to cadmium, zinc and copper. The induction of MT-10 and MT-20 mRNA in the heavy metal-treated mussels was detected by Northern blot analysis. The exposure of mussels to copper (40 μg l−1), cadmium (200 μg l−1) or zinc (1000 μg l−1) resulted in different MT-gene induction patterns. In the cadmium-intoxicated mussels levels of both MT-10 and MT-20 mRNA were increased. In the zinc-treated mussels, a rapid response of the MT-10 gene was detected within 6 h. Induction of MT-20 transcription was specific for cadmium, and can be envisaged as a potential biomarker of cadmium contamination. Copper had no effect on mRNA levels. We suggest the possibility that there is a hitherto-unknown copper-specific isoform.