Stochastic Loss of Silencing of the Imprinted Ndn/NDN Allele, in a Mouse Model and Humans with Prader-Willi Syndrome, Has Functional Consequences

Abstract

Genomic imprinting is a process that causes genes to be expressed from one allele only according to parental origin, the other allele being silent. Diseases can arise when the normally active alleles are not expressed. In this context, low level of expression of the normally silent alleles has been considered as genetic noise although such expression has never been further studied. Prader-Willi Syndrome (PWS) is a neurodevelopmental disease involving imprinted genes, including NDN, which are only expressed from the paternally inherited allele, with the maternally inherited allele silent. We present the first in-depth study of the low expression of a normally silent imprinted allele, in pathological context. Using a variety of qualitative and quantitative approaches and comparing wild-type, heterozygous and homozygous mice deleted for Ndn, we show that, in absence of the paternal Ndn allele, the maternal Ndn allele is expressed at an extremely low level with a high degree of non-genetic heterogeneity. The level of this expression is sex-dependent and shows transgenerational epigenetic inheritance. In about 50% of mutant mice, this expression reduces birth lethality and severity of the breathing deficiency, correlated with a reduction in the loss of serotonergic neurons. In wild-type brains, the maternal Ndn allele is never expressed. However, using several mouse models, we reveal a competition between non-imprinted Ndn promoters which results in monoallelic (paternal or maternal) Ndn expression, suggesting that Ndn allelic exclusion occurs in the absence of imprinting regulation. Importantly, specific expression of the maternal NDN allele is also detected in post-mortem brain samples of PWS individuals. Our data reveal an unexpected epigenetic flexibility of PWS imprinted genes that could be exploited to reactivate the functional but dormant maternal alleles in PWS. Overall our results reveal high non-genetic heterogeneity between genetically identical individuals that might underlie the variability of the phenotype.

Screening for rare epigenetic variations in autism and schizophrenia

“…One sample with a diagnosis of autism showed hypomethylation of NDN , whereas a second individual with schizophrenia showed hypomethylation at both NDN and the neighboring imprinted gene MAGEL2 . Aberrant imprinting of NDN has previously been linked with disruption of serotonergic neurons in mice (Rieusset et al, ), and thus it is possible that these imprinting anomalies contribute to the diagnosis of autism and schizophrenia. It should be noted, however, that due to the allele‐specific nature of methylation at imprinted loci, it is possible that the presence of an underlying heterozygous deletion at these regions could be incorrectly interpreted as an epivariation.…”

Cellular and disease functions of the Prader–Willi Syndrome geneMAGEL2

“…It is intriguing that a deletion of the entire gene leads to a milder phenotype than a truncating mutation. It is possible that the deletion of the complete paternal copy of the gene and its promoter as seen in the latter cases could lead to the leaky expression of the maternal copy of the MAGEL2 gene, as shown in the recent finding of stochastic loss of silencing of the imprinted Ndn allele in mice (104, 105). Alternatively, MAGEL2 is coded by a single exon and therefore mutations in MAGEL2 likely do not cause nonsense-mediated decay of the mRNA and may result in aberrant truncated protein products with potential dominant negative or other neomorphic effects.…”

Genomic imprinting in development, growth, behavior and stem cells

“…It is important to note, however, that mouse model systems do not recapitulate the full gamut of behavioral phenotypes exhibited by AS and PWS patients. For example, some, but not all, mice models for PWS exhibit increased appetite, which is also observed in human PWS patients (Rieusset et al, 2013), but this may not be from the lack of satiation that underlies these phenotypes in humans.…”