Partial Cloning of Type 2 Iodothyronine Deiodinase in Suncus murinus

The house musk shrew, Suncus murinus (Insectivora, Soricidae), has been reported as having low type 1 iodothyronine deiodinase (D1) activity, but its serum levels in T 4 and T 3 are compatible with those in other mammalians. Previously, we demonstrated that D2 activity was very high in the suncus brain and brown adipose tissue (BAT). Therefore, it has been hypothesized that D2 plays a key role in maintaining serum T 3 levels in the suncus. To study D2 expression under various conditions, we needed a cDNA probe to detect D2 mRNA. For this purpose we planned to clone the suncus D2 cDNA. We amplified a 346-bp cDNA fragment by RT-PCR. This fragment had high sequence homology with the corresponding regions of other mammalian D2s. In the sequence we also found a TGA codon, which is expected to be translated as selenocysteine in the presence of the Selenocysteine Insertion Sequence (SECIS). Northern blot analysis using this fragment as a probe showed that the expression of D2 mRNA was depicted as a band at about 6-kb in BAT and brain and at a lesser amount in pituitary and testis. The size and tissue distribution as well as sequence homology with other mammalian species indicated that the cDNA fragment cloned in the present study encodes a part of suncus D2.