Aberrant patterns of X chromosome inactivation in bovine clones

Abstract

In mammals, epigenetic marks on the X chromosomes are involved in dosage compensation. Specifically, they are required for X chromosome inactivation (XCI), the random transcriptional silencing of one of the two X chromosomes in female cells during late blastocyst development. During natural reproduction, both X chromosomes are active in the female zygote. In somatic-cell cloning, however, the cloned embryos receive one active (Xa) and one inactive (Xi) X chromosome from the donor cells. Patterns of XCIhave been reported normal in cloned mice, but have yet to be investigated in other species. We examined allele-specific expression of the X-linked monoamine oxidase type A (MAOA) gene and the expression of nine additional X-linked genes in nine cloned XX calves. We found aberrant expression patterns in nine of ten X-linked genes and hypomethylation of Xist in organs of deceased clones. Analysis of MAOA expression in bovine placentae from natural reproduction revealed imprinted XCI with preferential inactivation of the paternal X chromosome. In contrast, we found random XCI in placentae of the deceased clones but completely skewed XCI in that of live clones. Thus, incomplete nuclear reprogramming may generate abnormal epigenetic marks on the X chromosomes of cloned cattle, affecting both random and imprinted XCI.

Expression of X‐linked genes in deceased neonates and surviving cloned female piglets

“…Abnormal expression of genes on the X-chromosome have been reported in cloned bovine embryos and full-term calves as well as in cloned mouse embryos (Wrenzycki et al 2002;Xue et al 2002). This is possibly due to the insufficient reprogramming of the donor cells and failure to reactivate/inactivate the X chromosome during embryonic development (Nolen et al 2005).…”

“…Faulty or incomplete epigenetic reprogramming has been observed in different stages of development (Cezar et al 2003;Kang et al 2001b;Mann et al 2003). Even in cloned animals that survive to term, different aspects of epigenetic abnormalities are evident, although the majority of these animals appear to be healthy and normal (Archer et al 2003;Li et al 2004;Xue et al 2002).…”

“…The inactive X chromosomes in the donor cell requires reactivation/reprogramming during NT and provides a very good marker for reprogramming studies. Interestingly, aberrant patterns of X chromosome inactivation were discovered in bovine clones (Xue et al 2002), while in cloned mice both relatively normal (Eggan et al 2000) and abnormal XCI patterns (Nolen et al 2005;Senda et al 2004) have been reported. In order to investigate the normality of XCI in cloned pigs, we selected five X-linked genes and one ubiquitous autosomal gene to compare their expression levels in major organs of cloned piglets and age-matched controls by quantitative real time RT-PCR.…”

Mammalian nuclear transfer

“…It should be emphasized, however, that reprogramming that occurs postzygotically such as X-chromosome re-activation and subsequent inactivation (Eggan et al, 2000) and telomere length adjustment (Lanza et al, 2000a;Tian et al, 2000;Wakayama et al, 2000a;Betts et al, 2001) are faithfully accomplished in apparently normal cloned embryos. Nevertheless, X inactivation errors and chromosomal aberrations have been seen in visibly abnormal cloned embryos (Xue et al, 2002;Nolen et al, 2005).…”

A Continuity of X-Chromosome Silence from Gamete to Zygote

“…In 1971, Sharman and colleagues documented one of the earliest examples of genomic imprinting in mammals when they described a form of XCI in which the paternal X chromosome (X P ) is preferentially silenced (Sharman 1971). Imprinted XCI takes place in the extraembryonic tissues of some eutherians such as the rodent (Takagi and Sasaki 1975) and cow (Xue et al 2002). (In humans, the status of XCI in the placenta remains controversial [Ropers et al 1978;Migeon and Do 1979;Zeng and Yankowitz 2003].)…”