INDIVIDUAL VARIABILITY IN THE ZINC INDUCIBILITY OF METALLOTHIONEIN-II_A mRNA IN HUMAN LYMPHOCYTES

Abstract

The metallothionein-II_A gene (MT-II_A) is a major member of the human MT gene family. Metallothioneins (MTs) are low-molecular-weight, cysteine-rich proteins that bind and detoxify heavy metals. At least two different MT-II_A polymorphisms have been identified in humans, one or both of which may affect susceptibility to metal toxicity. The purpose of this study was to investigate whether these different genotypes affect the inducibility of MT-II_A mRNA in human lymphocytes treated with zinc (Zn), the major known inducer of MT-II_A in vitro. Fresh lymphocytes obtained from 16 healthy volunteers, aged 23-38 yr, were genotyped for the MT-II_A gene and tested for expression. A 435-bp Hin dIII- Taq I fragment of the MT-II_A promoter was used to probe for the two knownpolymorphisms (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 14% for 5.3-kb allele and 19% for 1.6-kb allele. In Northern blotting experiments, MT-II_A mRNA levels were induced over a wide range of Zn concentrations during 2-h exposures; specifically, levels increased by 9- to 115-fold with exposure to 100 µ M ZnCl₂ and by 16- to 311-fold with exposure to 200 µ M ZnCl₂. However, no significant differences in MT-II_A inducibility were found 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-II_A is strongly inducible by Zn in human lymphocytes, but individual variations exceed those that can be attributed to the known promoter-region polymorphisms.