Impressions of imprints

Aberrant methylation of an imprinted gene U2af1-rs1(SP2) caused by its own transgene

Genomic imprinting refers to the parental allele-specific expression of genes. The precise mechanism underlying this phenomenon, which may involve DNA methylation, is not yet known. U2af1-rs1(SP2) is an imprinted gene expressed from the paternal allele and is methylated on the maternal allele. Here we report an artificial system in which expression and methylation of the endogenous imprinted gene U2af1-rs1 can be affected by interaction with its own transgene in the testis. We suggest that there is a mechanism in male gametogenesis by which the U2af1-rs1 gene is kept unmethylated to be expressed in the offspring in addition to a mechanism in female gametogenesis by which the U2af1-rs1 gene is methylated and is not expressed in the offspring.

An imprinted gene p57KIP2 is mutated in Beckwith-Wiedemann syndrome

p57KIP2 is a potent tight-binding inhibitor of several G1 cyclin/Cdk complexes, and is a negative regulator of cell proliferation. The gene encoding p57KIP2 is located at 11p15.5 (ref. 2), a region implicated in both sporadic cancers and Beckwith-Wiedemann syndrome, a cancer-predisposing syndrome, making it a tumour-suppressor candidate. Several types of childhood tumours including Wilms' tumour, adrenocortical carcinoma and rhabdomyosarcoma exhibit a specific loss of maternal 11p15 alleles, suggesting that genomic imprinting is involved. Genetic analysis of the Beckwith-Wiedemann syndrome indicated maternal carriers, as well as suggesting a role of genomic imprinting. Previously, we and others demonstrated that p57KIP2 is imprinted and that only the maternal allele is expressed in both mice and humans. Here we describe p57KIP2 mutations in patients with Beckwith-Wiedemann syndrome. Among nine patients we examined, two were heterozygous for different mutations in this gene-a missense mutation in the Cdk inhibitory domain resulting in loss of most of the protein, and a frameshift resulting in disruption of the QT domain. The missense mutation was transmitted from the patient's carrier mother, indicating that the expressed maternal allele was mutant and that the repressed paternal allele was normal. Consequently, little or no active p57KIP2 should exist and this probably causes the overgrowth in this BWS patient.

Genomic imprinting of p57KIP2, a cyclin-dependent kinase inhibitor, in mouse

p57KIP2 is a potent tight-binding inhibitor of several G1 cyclin/Cdk complexes, and is a negative regulator of cell proliferation. The gene encoding human p57KIP is located on chromosome 11p15.5 (ref. 2), a region implicated in both sporadic cancers and Beckwith-Wiedemann syndrome, a familial cancer syndrome, marking it a tumour suppressor candidate. Several types of childhood tumours including Wilm's tumour, adrenocortical carcinoma and rhabdomyosarcoma display a specific loss of maternal 11p15 alleles, suggesting that genomic imprinting plays an important part. Genetic analysis of the Beckwith-Wiedemann syndrome has indicated maternal carriers as well as suggested a role in genomic imprinting. Here, as a first step towards elucidating the genesis of human cancers in this region, we showed that a mouse homologue of p57KIP2 is genomically imprinted. The paternally inherited allele is transcriptionally repressed and methylated. This murine gene maps to the distal region of chromosome 7, within a cluster of imprinted genes, including insulin-2, insulin-like growth factor-2, H19 and Mash2 (refs 14-18).