Demonstration of all-or-none loss of imprinting in mRNA expression in single cells

Loss of imprinting (LOI) is the reactivation of the silenced allele of an imprinted gene, leading to perturbation of monoallelic expression. We tested the hypothesis that LOI of PLAGL1, a representative maternally imprinted gene, occurs through an all-or-none process leading to a mixture of fully imprinted and nonimprinted cells. Herein using a quantitative RT-PCR-based experimental approach, we measured LOI at the single cell level in human trophoblasts and demonstrated a broad distribution of LOI among cells exhibiting LOI, with the mean centered at ∼100% LOI. There was a significant (P < 0.01) increase in expression after 2 days of 5-aza-2′-deoxycytidine (AZA) treatment and a significant (P < 0.01) increase in LOI after both 1 and 2 days of AZA treatment, while the distribution remained broad and centered at ∼100% LOI. We propose a transcriptional pulsing model to show that the broadness of the distribution reflects the stochastic nature of expression between the two alleles in each cell. The mean of the distribution of LOI in the cells is consistent with our hypothesis that LOI occurs by an all-or-none process. All-or-none LOI could lead to a second distinct cell population that may have a selective advantage, leading to variation of LOI in normal tissues, such as the placenta, or in neoplastic cells

Allelic Imbalance in TOR1A mRNA Expression in Manifesting and Non-Manifesting Carriers of the GAG-Deletion

Early onset dystonia (EOD) is associated with a 3bp-(ΔGAG) in-frame deletion in the TOR1A gene, which encodes for torsinA. Carriers of the mutant (ΔGAG) allele can either develop or escape a dystonic phenotype (~30% penetrance). The expression ratio of the two alleles could be important for the manifestation or prevention of the disease since wild-type (WT) torsinA is thought to have protective function. Absence of an antibody discriminating WT from ΔE torsinA has precluded the determination ΔE and WT torsinA levels in manifesting and nonmanifesting carriers. We performed quantitative analysis of TOR1A allele expression in manifesting (MC) and nonmanifesting (NMC) carriers using quantitative allele-specific PCR (qASPCR) to determine the levels of mutant versus WT torsinA mRNA. The technique described showed high degree of specificity in detecting the two alleles. The present study represents the first comprehensive analysis of biallelic expression of the TOR1A gene in lymphoblast and brain samples from patients and NMC relatives. We demonstrate that mRNA is transcribed from both the WT and ΔGAG allele in peripheral and neural tissues with a trend for increased expression of the ΔGAG allele compared to the WT in carriers regardless of their phenotype and thus cannot account for the reduced penetrance