Molecular characterization of a patient presumed to have prader-willi syndrome

Transcriptomics of the Prader-Willi syndrome hypothalamus

Prader-Willi syndrome (PWS) is a complex neurodevelopmental disorder, arising from a loss of paternity expressed genetic material on the imprinted chromosome locus 15q11-q13. Despite increasing clarity on the underlying genetic defects, the molecular basis of the condition remains poorly understood. Hypothalamic dysfunction is widely recognized as the basis of the core symptoms of PWS, which include a deficiency in growth hormone and reproductive hormones, circadian rhythm abnormalities, and a lack of satiety, leading to an extreme obesity, among others. Genome-wide gene expression analysis (transcriptomics) offers an unbiased interrogation of complex disease pathogenesis and a potential window into the dysregulated pathways involved in disease. In this chapter, we review the findings from recent work investigating the PWS hypothalamic transcriptome, discuss the significance of the findings in relation to the clinical presentation and molecular underpinnings of PWS, and highlight future research directions.

The posterior pituitary expresses the serotonin receptor 2C

The serotonin receptor 2C (5HT2C) is an important drug target to treat obesity and depression. Its pre-mRNA undergoes alternative splicing, encoding a short RNA1 isoform that is localized intracellularly and a full-length isoform (RNA2) that can reach the cell membrane. These splicing isoforms are deregulated in Prader-Willi syndrome (PWS), due to the loss of a trans-acting regulatory RNA, SNORD115. Here we show that the 5HT2C mRNA is expressed in the posterior pituitary, suggesting that 5HT2C mRNA is generated in the hypothalamus and subsequently conveyed by axonal transport. In the pituitary, the ratio of 5HT2C isoforms is regulated by feeding, and can be manipulated using a splice-site changing oligonucleotide injected into the blood. The pituitary expression of the 5HT2C mRNA may constitute a previously unknown mechanism whereby serotonin in the circulation or drugs targeting the 5HT2C might induce side-effects. Finally, the deregulation of 5HT2C splicing isoforms in PWS could contribute to the known hormonal imbalances.

Non-coding RNAs and disease: the classical ncRNAs make a comeback

Many human diseases have been attributed to mutation in the protein coding regions of the human genome. The protein coding portion of the human genome, however, is very small compared with the non-coding portion of the genome. As such, there are a disproportionate number of diseases attributed to the coding compared with the non-coding portion of the genome. It is now clear that the non-coding portion of the genome produces many functional non-coding RNAs and these RNAs are slowly being linked to human diseases. Here we discuss examples where mutation in classical non-coding RNAs have been attributed to human disease and identify the future potential for the non-coding portion of the genome in disease biology.

SNORD116 and SNORD115 change expression of multiple genes and modify each other's activity

The loss of two gene clusters encoding small nucleolar RNAs, SNORD115 and SNORD116 contribute to Prader-Willi syndrome (PWS), the most common syndromic form of obesity in humans. SNORD115 and SNORD116 are considered to be orphan C/D box snoRNAs (SNORDs) as they do not target rRNAs or snRNAs. SNORD115 exhibits sequence complementarity towards the serotonin receptor 2C, but SNORD116 shows no extended complementarities to known RNAs. To identify molecular targets, we performed genome-wide array analysis after overexpressing SNORD115 and SNORD116 in HEK 293T cells, either alone or together. We found that SNORD116 changes the expression of over 200 genes. SNORD116 mainly changed mRNA expression levels. Surprisingly, we found that SNORD115 changes SNORD116's influence on gene expression. In similar experiments, we compared gene expression in post-mortem hypothalamus between individuals with PWS and aged-matched controls. The synopsis of these experiments resulted in 23 genes whose expression levels were influenced by SNORD116. Together our results indicate that SNORD115 and SNORD116 influence expression levels of multiple genes and modify each other activity.

Valproic acid causes proteasomal degradation of DICER and influences miRNA expression

Valproic acid (VPA) is a commonly used drug to treat epilepsy and bipolar disorders. Known properties of VPA are inhibitions of histone deacetylases and activation of extracellular signal regulated kinases (ERK), which cannot fully explain VPA's clinical features. We found that VPA induces the proteasomal degradation of DICER, a key protein in the generation of micro RNAs. Unexpectedly, the concentration of several micro RNAs increases after VPA treatment, which is caused by the upregulation of their hosting genes prior to DICER degradation. The data suggest that a loss of DICER protein and changes in micro RNA concentration contributes to the clinical properties of VPA. VPA can be used experimentally to down regulate DICER protein levels, which likely reflects a natural regulation of DICER.

The 5' untranslated region of the serotonin receptor 2C pre-mRNA generates miRNAs and is expressed in non-neuronal cells

The serotonin receptor 2C (HTR2C) gene encodes a G protein-coupled receptor that is exclusively expressed in neurons. Here, we report that the 5′ untranslated region of the receptor pre-mRNA as well as its hosted miRNAs is widely expressed in non-neuronal cell lines. Alternative splicing of HTR2C is regulated by MBII-52. MBII-52 and the neighboring MBII-85 cluster are absent in people with Prader–Willi syndrome, which likely causes the disease. We show that MBII-52 and MBII-85 increase expression of the HTR2C 5′ UTR and influence expression of the hosted miRNAs. The data indicate that the transcriptional unit expressing HTR2C is more complex than previously recognized and likely deregulated in Prader–Willi syndrome.