Individual variability in the zinc inducibility of metallothionein-IIA mRNA in human lymphocytes

The metallothionein-IIA gene ( MT-IIA) is a major member of the human MT ge ne family. Metallothioneins (MTs) are low-molecular-weight, cysteine-rich p roteins that bind and detoxify heavy metals. At least two different MT-IIA polymorphisms have been identified in humans, one or both of which may affe ct susceptibility to metal toxicity. The purpose of this study was to inves tigate whether these different genotypes affect the inducibility of MT-IIA mRNA in human lymphocytes treated with zinc ( Zn), the major known inducer of MT-IIA in vitro. Fresh lymphocytes obtained from 16 healthy volunteers, aged 23-38 yr, were genotyped for the MT-IIA gene and tested for expression . A 435-bp HindIII-TaqI fragment of the MT-IIA promoter was used to probe f or the two known polymorphisms ( a 7.8-kb vs. a 5.3-kb fragment, and a 1.7- kb vs. a 1.6-kb fragment). The allele frequencies of the 16 subjects were 1 4% for 5.3-kb allele and 19% for 1.6-kb allele. In Northern blotting experi ments, MT-IIA mRNA levels were induced over a wide range of Zn concentratio ns during 2-h exposures; specifically, levels increased by 9- to 115-fold w ith exposure to 100 muM ZnCl2 and by 16- to 311-fold with exposure to 200 m uM ZnCl2. However, no significant differences in MT-IIA inducibility were f ound between the 7.8/5.3-kb allele pair ( n = 4) and the 7.8/7.8-kb allele pair (n = 12) or between the 1.7/1.6-kb allele pair (n = 5) and the 1.7/1.7 -kb allele pair (n = 11). Thus, MT-IIA is strongly inducible by Zn in human lymphocytes, but individual variations exceed those that can be attributed to the known promoter-region polymorphisms.