Molecular evolution of matrix metalloproteinase 20

Dental enamel is a hypermineralized tissue, containing only trace amounts of organic components. During enamel formation, matrix metalloproteinase 20 (MMP20) processes proteins comprising enamel matrix and facilitates hypermineralization. In the human genome, 24 distinct MMP genes have been identified. Among these genes, MMP20 is clustered with eight other genes, including MMP13, and all these clustered genes show phylogenetically close relationships. In this study, we investigated MMP20 and closely related MMP genes in various tetrapods and in a teleost fish, fugu. In the genome of the chicken, a toothless tetrapod, we identified degraded exons of MMP20, which supports the previous proposition that MMP20 is important specifically for enamel formation. Nevertheless, for unknown reasons, we failed to identify MMP20 in the platypus genome. In the opossum, lizard, and frog genomes, MMP20 was found clustered with MMP13. Furthermore, in the fugu genome, we identified an MMP20-like gene located adjacent to MMP13, suggesting that MMP20 arose before the divergence of ray-finned fish and lobe-finned fish. The teleost tooth surface is covered with enameloid, a hypermineralized tissue different from enamel. Thus, we hypothesize that MMP20 could have been used in an ancient hypermineralized tissue, which evolved into enameloid in teleosts and into enamel in tetrapods.