An in vitro Assay System for Molecular Understanding of Genomic Imprinting involved in Human Diseases

Makiko Meguro

Genomic imprinting is a biological phenomenon in which parental alleles are genetically marked, ensuring functional inequality of paternal and maternal genomes. This phenomenon plays an important role in early development, and may provide an explanation for certain genetic disorders that do not exhibit Mendelian inheritance. Beckwith-Wiedemann syndrome (BWS), which maps to 11p15.5, and Prader-Willi (PWS) and Angelman (AS) syndromes, which map to 15q11-q13, have taken center stage in human genetic studies of genomic imprinting.

With the goal of clarifying the mechanisms of genomic imprinting and its role in human disease, we have developed human monochromosomal hybrids retaining single individual chromosomes of defined parental origin via microcell-mediated chromosome transfer. This in vitro assay system has recently led to the identification of a novel imprinted locus (LIT1/KCNQ10T1) and a snoRNA, (HBII-85), associated with BWS and PWS, respectively. In addition, targeting experiments using the DT40 cell shuttle system demonstrated that the human LIT1/KCNQ10T1 locus could act as a negative regulator in cis for the coordination of the local control of imprinting on chromosome l1p15.5. Thus, our in vitro assay system can be used to define regulatory elements that confer the long-range control of gene activity within chromosomal domains. Using the same method, we are investigating the function of PWS and AS imprinting centers (ICs). The PWS and AS ICs are proposed to regulate the initiation of imprint switching for all genes in a 2-Mb imprinted domain during gametogenesis; however, the functions and mechanisms of ICs are still poorly understood. Moreover, modified human chromosome can be introduced to any other cells using microcell transfer. Since human genes on transferred chromosomes are regulated in a proper cell type-specific manner in recipient cells, the imprinting status can be analyzed in specific cell types.