There could be two potential reasons.

1. If your transgenic line is heterozygous and the insertion locus follows mono-alellic expression pattern (similar to X inactivation), then you will see mosaic expression of your transgene.

2. Plasmid backbone could cause some random silencing. See Tasic B, Luo L, PNAS 108: 7902–7907. PMC3093482

Hope one of these answers your question.

There could be two potential reasons.

1. If your transgenic line is heterozygous and the insertion locus follows mono-alellic expression pattern (similar to X inactivation), then you will see mosaic expression of your transgene.

2. Plasmid backbone could cause some random silencing. See Tasic B, Luo L, PNAS 108: 7902–7907. PMC3093482

Hope one of these answers your question.

Thanks a lot Hui Zong, that was very helpful!

Yes Susan they are, but they behave the same when hemizygous also. The location is unknown, we haven't done whole genome sequencing and exon sequencing might not be able to help..

Many apologies for not spotting and responding to this question sooner ... I hope that the following is of some help:

The most parsimonious model assumes that transgene inactivation, causing mosaic expression, is a single stochastic silencing (or activation) event that occurs during development and is then stably inherited by daughter cells. We have made similar observations in a steroid 21 hydroxylase/Lac Z transgenic mice which we have subsequently exploited for cell lineage marking.

For further information, can I refer you to the introduction in the following publication: Relative transgene expression frequencies in homozygous versus hemizygous transgenic mice.

Chang SP, Opsahl ML, Whitelaw CB, Morley SD, West JD.

Transgenic Res. 2013 Dec;22(6):1143-54. doi: 10.1007/s11248-013-9732-5

Introduction

In independent transgenic lines, produced by microinjection of the same construct, the transgene may be expressed in all cells of the appropriate tissues or it may show a variegated pattern of expression (mosaic transgene expression). In addition, the degree of mosaicism may vary both among and within the different mosaic lines (Dobie et al. 1996; Morley et al. 1996; Opsahl et al. 2003). Even large yeast artificial chromosome transgenes, with their own regulatory sequences, may be subject to stochastic silencing (Alami et al. 2000). It is widely assumed that a stochastic transgene silencing event occurs during development, perhaps before the transgene is actually expressed, after which the potential for transgene expression is stably inherited by daughter cells. Transgene expression may be controlled in a binary way, so that the transgene is either fully active or silent l (Walters et al. 1995), or regulatory elements may cause variable levels of transgene expression among positive cells (Guy et al. 1997).

In a few cases, mosaic expression is caused by integration of the transgene into the X chromosome and subsequentX chromosome inactivation in females (Tan et al. 1993; Hadjantonakis et al. 2001). Stochastic silencing of transgenes integrated into autosomes may be attributable to position effect variegation related to effects of heterochromatin, which makes the transgene inaccessible to the transcriptional machinery and, therefore, non-permissive for expression. This may occur, for example, if the transgene integrates near an endogenous heterochomatic site, such as the centromere (Dobie et al. 1996). It may also occur if the transgene integrates as a series of repeated copies (Garrick et al. 1998), which can trigger the formation of local heterochromatin. In addition, transgene silencing may involve methylation (Preis et al. 2003), which can be subject to imprinting differences, depending on whether the transgene is transmitted via males or males or females (Preis et al. 2003; Williams et al. 2008).

Continued …

Thank you for your reply Steven! It is very interesting to hear about your model, and the information in your reply. We have been focusing on PEV or epigenetic mechanisms of gene silencing, but I will read up on SSS/SSA that you suggest and get back to you with any questions, if I may. Is it possible to read about your model/find out more? Thank you once again.

Key references (and references therein) as follows ... will be interested to hear what you think. We wanted to drill down into the mechanism of transgene inactivation and have some preliminary data on methylation, which keys into the dietary epigenetic modification of mouse coat colour story, but could not get the funding.

BW

Steve Morley

Chang S-P, Morrison HD, Nilsson F, Kenyon CJ, West JD, Morley SD (2013). Cell proliferation, movement and differentiation during maintenance of the adult mouse adrenal cortex. PLoS ONE 8(12): e81865. doi:10.1371/journal.pone.0081865 http://www.ncbi.nlm.nih.gov/pubmed/24324726

Chang S-P, Opsahl ML, Whitelaw CB, Morley SD, West JD (2013). Relative transgene expression frequencies in homozygous versus hemizygous transgenic mice. Transgenic Res. 22: 1143-1154. doi: 10.1007/s11248-013-9732-5 http://www.ncbi.nlm.nih.gov/pubmed/23873612

Chang S-P, Mullins JJ, Morley SD, West JD (2011). Transition from organogenesis to stem cell maintenance in the mouse adrenal cortex. Organogenesis 7: 267-280. doi.org/10.4161/org.7.4.18060

http://dx.doi.org/10.4161/org.7.4.18060

Morley SD, Viard I, Parker KL, Mullins JJ (1996). Adrenocortical-specific transgene expression directed by steroid hydroxylase gene promoters. Endocrine Res. 22: 631-639.

http://dx.doi.org/10.1080/07435809609043757