Genomic imprinting is a mammal-specific epigenetic mechanism of the gamete or zygote leading to parent-of-origin-specific differential expression of the two alleles of a gene in somatic cells of offspring. Individual imprinted genes tend to cluster in a region of chromosome.
To identify novel imprinted genes in the chromosomal vicinity of a previously identified imprinted gene, a systematic method exploiting the clustering feature was attempted in this study. Peg1/Mest, a maternally imprinted gene is located in the sub-proximal end of mouse chromosome 6, which is previously suspected to contain imprinted genes as demonstrated by uniparental disomy studies. Polymorphisms of the genes and ESTs mapped to this region in mouse and human was identified and used to physically discriminate one of parental alleles from the other in the F1 hybrids between two inbred mouse strains, Mus musculus molossinus and Mus musculus domesticus (C57BL/6J). Restriction fragment length polymorphism analysis of RT-PCR products was carried out to investigate the allele-specific expression of genes and ESTs in the F1 hybrids. Calu, Cda15a02, Ube2h and Cappa2 genes showed biallelic expression and allelic expression bias was not detected in these genes. However, one of the mouse EST, tentatively named URF1, was obviously shown to be expressed only from the paternal allele in the brain of F1 hybrids. Thus URF1 is maternally imprinted in adult mouse brain. Expression pattern in the reciprocal cross unambiguously confirmed the maternal imprinting of URF1 gene. Imprinting of URF1 seems to be delicately regulated in tissue-specific manner. In adult heart and lung, imprinting status of URF1 was relaxed. The extent of relaxation was analyzed quantitatively. It was estimated that the percentage of C57BL/6J allele was 93% for F1 hybrid of (M × C) and 29% for hybrid of (C × M) in heart and 86% for hybrid of (M × C) and 50% for hybrid of (C × M) in lung. URF1 provides an example of endogenous gene that shows relaxation of imprinting across different tissues. Relaxed imprinting of URF1 implies that the function of this gene is regulated in tissue-specific manner.
각인된 유전자는 특정한 염색체 좌위 주변에 집합하여 존재하는 경향이 있다. 이러한 경향을 이용하여 각인된 유전자 Peg1/Mest가 위치하는 생쥐 염색체 6번의 일부에 위치된 여러 유전자들과 EST들의 유전자 각인 여부를 조사하였다. 이들 유전자들과 EST들은 사람과 생쥐의 염색체에서 Peg1/Mest 유전자와의 물리적인 연관성을 고려하여 가까울수록 선택적으로 조사되었다. 대부분의 유전자와 EST들은 제 1세대 잡종에서 양쪽 부모로부터 유전되어 전해진 대립형질을 모두 발현하는 것으로 조사되었으나 URF1이라 명명된 EST의 경우 생쥐 성체의 뇌에서 각인되어 있음이 확인되었다. URF1은 뇌와 신장, 간, 심장, 허파, 갑상선, 고환 등의 조직에서 발현되고 있음을 RT-PCR로 확인하였고, 이 중 허파와 심장에서의 유전자 각인 여부를 조사하였다. 허파와 심장에서 URF1은 완전한 각인을 보이지는 않았지만 양쪽 부모로부터 물려받은 각각의 대립형질이 뚜렷한 양적인 차이를 나타내며 각인된 발현 양상을 보임을 정량화된 방법으로 입증하였다. URF1의 rat homolog는 genome 상에 두 벌이 존재한다고 보고된 바에 반해, 생쥐에서는 한 벌만이 존재함을 Southern blot analysis 결과 알 수 있었다.
