Embryonic Expression of the Human 40-kD Keratin: Evidence from a Processed Pseudogene Sequence

Analysis of the cytoskeletal components of early murine embryos has detected expression of two keratin proteins, K#8 and K#18, at the 4-8-cell stage. Comparable data for human embryos do not exist, although several processed pseudogenes corresponding to K#8 and K#18 have been discovered in the human genome. Because only genes that are expressed in pre-germ-line and germ-line cells can give rise to processed pseudogenes, the existence of human K#8 and K#18 processed pseudogenes is prima facie evidence for expression of keratins K#8 and K#18 in the early human embryo. We have cloned and determined the complete sequence of a processed pseudogene corresponding to another acidic human keratin. Comparison of its sequence with known sequences of other mammalian keratins indicates that the pseudogene arose from a reverse transcript of a correctly initiated and terminated functional human K#19 gene. This implies expression of K#19 keratin in addition to K#8 and K#18 in the early human embryo. We have proposed previously that K#19 evolved specifically to redress unbalanced production of various basic keratins, and our current evidence, that it is expressed at an early stage of development, implies that K#19 may fulfill this same role during human embryogenesis.