Montreal Children's Hospital Research Institute, Department of Pediatrics; McGill University
The term parental imprinting refers to differential behavior of each copy of genetic material based on the sex of the parent from whom it was inherited. Functionally, the most important consequence of imprinting is differential levels of expression from the two copies of certain genes, typically complete silencing of either the maternal or the paternal copy. However, parental imprinting is not an all-or-none phenomenon. Imprinted expression of any given gene may be tissue-specific, developmentally regulated, and polymorphic among individuals of the same species. We have studied imprinting as a polymorphic human trait using the following models.
1) Polymorphic imprinting of IGF2 in human leukocytes. Unlike most human tissues which express only the paternal copy, leukocytes show relaxation of IGF2 imprinting. RNA transcripts derived from the maternal copy can be readily detected in most individuals, albeit at levels generally lower than that of the paternal. The paternal/maternal ratio is reproducible in the same individual and in identical twins, and shows familial clustering compatible with oligogenic genetic control. Expression of the closely linked and reciprocally imprinted H19 gene on the maternal chromosome is believed to be responsible for silencing IGF2 expression from the same chromosome in cis. This is compatible with our observation that H19 transcripts are absent from the leukocytes of individuals with relaxed IGF2 imprinting, but can be readily detected in individuals who severely repress the maternal copy in these cells. We have extensively tested a number of polymorphisms in the INS-IGF2-H19 region, and failed to find association of any alleles with the extent of maternal IGF2 reactivated in leukocytes. Thus we conclude that this polymorphic relaxation of IGF2 imprinting in leukocytes is not determined in cis. Genome-wide linkage studies are planned, to identify loci that influence parent-of-origin specific expression of IGF2 in trans.
2) Polymorphic imprinting of IGF2R in the human. The murine Igf2r gene is expressed only from the maternal copy, but most human fetuses express both copies. We have observed that in multiple tissues of approximately one third of human second-trimester fetuses, IGF2R is transcribed from the paternal chromosome at levels less than half of the maternal. This partial polymorphic imprinting of IGF2R is developmentally regulated, as it is never observed in term placenta and is very rare in post-natal tissues. We have statistical evidence that aberrant post-natal persistence of IGF2R imprinting predisposes to Wilms tumor, an embryonal malignancy that has been etiologically linked to disrupted imprinting and overexpression of IGF2. In both normal human fetuses and the normal kidney of Wilms tumor patients, chromosomes that can be silenced upon passage from the paternal germline are marked by specific alleles of an intragenic CA repeat polymorphism. This linkage disequilibrium constitutes strong statistical evidence that IGF2R imprinting in humans is controlled in cis, by a sequence variant of the imprinted locus itself. We have preliminary evidence that, as in the mouse, an anti-sense RNA is transcribed from the long second intron of IGF2R on the paternal chromosome of imprinting individuals, and may play a role in silencing IGF2R in cis. Cloning of this RNA and its regulatory sequences will provide a focus for the search of sequence variants determining the polymorphic imprinting of human IGF2R.