Polymorphic X-chromosome inactivation of the human TIMP1 gene

X inactivation silences most but not all of the genes on one of the two X c hromosomes in mammalian females. The human X chromosome preserves its activ ation status when isolated in rodent/human somatic-cell hybrids, and hybrid s retaining either the active or inactive X chromosome have been used to as sess the inactivation status of many X-linked genes. Surprisingly, the X-li nked gene for human tissue inhibitor of metalloproteinases (TIMP1) is expre ssed in some but not all inactive X-containing somatic-cell hybrids, sugges ting that this gene is either prone to reactivation or variable in its inac tivation. Since many genes that escape X inactivation are clustered, we exa mined the expression of four genes (ARAF1, ELK1, ZNF41, and ZNF157) within similar to 100 kb of TIMP1. All four genes were expressed only from the act ive X chromosome, demonstrating that the factors allowing TIMP1 expression from the inactive X chromosome are specific to the TIMP1 gene. To determine if this variable inactivation of TIMP1 is a function of the hybrid-cell en vironment or also is observed in human cells, we developed an allele-specif ic assay to assess TIMP1 expression in human females. Expression of two all eles was detected in some female cells with previously demonstrated extreme skewing of X inactivation, indicating TIMP1 expression from the inactive c hromosome. However, in other cells, no expression of TIMP1 was observed fro m the inactive X chromosome, suggesting that TIMP1 inactivation is polymorp hic in human females.