Gains, Losses and Changes of Function after Gene Duplication: Study of the Metallothionein Family

Abstract

Metallothioneins (MT) are small proteins involved in heavy metal detoxification and protection against oxidative stress and cancer. The mammalian MT family originated through a series of duplication events which generated four major genes (MT1 to MT4). MT1 and MT2 encode for ubiquitous proteins, while MT3 and MT4 evolved to accomplish specific roles in brain and epithelium, respectively. Herein, phylogenetic, transcriptional and polymorphic analyses are carried out to expose gains, losses and diversification of functions that characterize the evolutionary history of the MT family. The phylogenetic analyses show that all four major genes originated through a single duplication event prior to the radiation of mammals. Further expansion of the MT1 gene has occurred in the primate lineage reaching in humans a total of 13 paralogs, five of which are pseudogenes. In humans, the reading frame of all five MT1 pseudogenes is reconstructed by sequence homology with a functional duplicate revealing that loss of invariant cysteines is the most frequent event accounting for pseudogeneisation. Expression analyses based on EST counts and RT-PCR experiments show that, as for MT1 and MT2, human MT3 is also ubiquitously expressed while MT4 transcripts are present in brain, testes, esophagus and mainly in thymus. Polymorphic variation reveals two deleterious mutations (Cys30Tyr and Arg31Trp) in MT4 with frequencies reaching about 30% in African and Asian populations suggesting the gene is inactive in some individuals and physiological compensation for its loss must arise from a functional equivalent. Altogether our findings provide novel data on the evolution and diversification of MT gene duplicates, a valuable resource for understanding the vast set of biological processes in which these proteins are involved.

Barley Metallothioneins: MT3 and MT4 Are Localized in the Grain Aleurone Layer and Show Differential Zinc Binding

“…Duplication within MT genes is a well-known phenomenon that has been characterized in Drosophila melanogaster (Maroni et al, 1987) and different oyster species (Tanguy et al, 2002). In humans, MT1 has undergone extensive duplications rendering 13 MT1 paralogs of which five have developed into pseudogenes (Moleirinho et al, 2011), and also in yeast, the copy number of the endogenous MT gene CUP1 varies from one to multiple tandem repeats and correlates positively with yeast Cu and Cd tolerance, showing that the duplicated genes encode functional MT proteins (Karin et al, 1984).…”

Evolutionary progress in the functions of metallothioneins in the ever changing ecosystems

“…In humans, the reading frame of all five MT-1 pseudogenes is reconstructed by sequence homology with a functional duplicate revealing that loss of invariant cysteines is the most frequent event accounting for pseudogeneisation. Expression analyses based on EST counts and RT-PCR experiments show that, as for MT-1 and MT-2, human MT-3 is also ubiquitously expressed while MT-4 transcripts are present in brain, testes, esophagus and mainly in thymus (Moleirinho et al, 2011).…”

“…Since its discovery (Palmiter et al, 1992), MT-3 has been frequently associated with the protection against neuronal injury (Chung & West, 2004;Hozumi et al, 1995). The mammalian MT3 protein shows a characteristic insertion of six residues at the a-domain when compared to that of MT-1 and MT-2 and an extra residue in the b domain (Thr), which is responsible for neuron growth inhibitory activity in Alzheimer disease (Cai et al, 2006;Romero-Isart et al, 2002).…”

“…MT-4 retains a high degree of conservation between humans and mice, yet it shows the highest sequence divergence when compared with any other MT family member (Moleirinho et al, 2011). It has been shown that mouse Mt-4 retains the capacity to bind Zn (Quaife et al, 1994;Tio et al, 2004), Cd and Cu as the ubiquitously expressed MT-1/MT-2, although the affinity to Cu is higher (Tio et al, 2004, Meloni et al, 2006.…”

“…It has been shown that mouse Mt-4 retains the capacity to bind Zn (Quaife et al, 1994;Tio et al, 2004), Cd and Cu as the ubiquitously expressed MT-1/MT-2, although the affinity to Cu is higher (Tio et al, 2004, Meloni et al, 2006. Suggestions have been made that MT-4 is inactive in some individuals (Moleirinho et al, 2011). It is tempting to assume that the loss (pseudogeneization) of MT-4 can be compensated by functional equivalents.…”