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MicroRNAs (miRNAs) constitute a class of small cellular RNAs (typically 21-23nt) that function as post-transcriptional regulators of gene expression. Current estimates indicate that more than one third of the cellular transcriptome is regulated by miRNAs, although they are relatively few in number (less than 2000 human miRNAs). The high relative stability of miRNA in common clinical tissues and biofluids (e.g. plasma, serum, urine, saliva, etc.) and the ability of miRNA expression profiles to accurately classify discrete tissue types and disease states have positioned miRNA quantification as a promising new tool for a wide range of diagnostic applications. Furthermore miRNAs have been shown to be rapidly released from tissues into the circulation with the development of pathology. To facilitate discovery and clinical development of miRNA-based biomarkers, we developed a genome-wide Locked Nucleic Acid (LNA™)-based miRNA qPCR platform with unparalleled sensitivity and robustness. The platform allows high-throughput profiling of miRNAs from important clinical sources without the need for pre-amplification. Using this system, we have profiled thousands of biofluid samples including blood derived plasma and serum. An extensive quality control (QC) system has been implemented in order to secure technical excellence and reveal any unwanted bias coming from pre-analytical or analytical variables. We present our approaches to sample and RNA QC as well as data QC and normalization. Specifically we have developed normal reference ranges for circulating miRNAs in serum and plasma as well as a hemolysis indicator based on microRNA expression.

Fine-tuning of X chromosomal gene expression in Drosophila melanogaster involves the selective interaction of the Dosage Compensation Complex (DCC) with the male X chromosome, in order to increase the transcription of many genes. However, the X chromosomal DNA sequences determining DCC binding remain elusive. By adapting a 'one-hybrid' assay, we identified minimal DNA elements that direct the interaction of the key DCC subunit, MSL2, in cells. Strikingly, several such novel MSL2 recruitment modules have very different DNA sequences. The assay revealed a novel, 40 bp DNA element that is necessary for recruitment of DCC to an autosomal binding site in flies in the context of a longer sequence and sufficient by itself to direct recruitment if trimerized. Accordingly, recruitment of MSL2 to the single 40 bp element in cells was weak, but as a trimer approached the power of the strongest DCC recruitment site known to date, the roX1 DH site. This element is the shortest MSL2 recruitment sequence known to date. The results support a model for MSL2 recruitment according to which several different, degenerate sequence motifs of variable affinity cluster and synergise to form a high affinity site.

The ubiquitous tandem kinase JIL-1 is essential for Drosophila development. Its role in defining decondensed domains of larval polytene chromosomes is well established, but its involvement in transcription regulation has remained controversial. For a first comprehensive molecular characterisation of JIL-1, we generated a high-resolution, chromosome-wide interaction profile of the kinase in Drosophila cells and determined its role in transcription. JIL-1 binds active genes along their entire length. The presence of the kinase is not proportional to average transcription levels or polymerase density. Comparison of JIL-1 association with elongating RNA polymerase and a variety of histone modifications suggests two distinct targeting principles. A basal level of JIL-1 binding can be defined that correlates best with the methylation of histone H3 at lysine 36, a mark that is placed co-transcriptionally. The additional acetylation of H4K16 defines a second state characterised by approximately twofold elevated JIL-1 levels, which is particularly prominent on the dosage-compensated male X chromosome. Phosphorylation of the histone H3 N-terminus by JIL-1 in vitro is compatible with other tail modifications. In vivo, phosphorylation of H3 at serine 10, together with acetylation at lysine 14, creates a composite histone mark that is enriched at JIL-1 binding regions. Its depletion by RNA interference leads to a modest, but significant, decrease of transcription from the male X chromosome. Collectively, the results suggest that JIL-1 participates in a complex histone modification network that characterises active, decondensed chromatin. We hypothesise that one specific role of JIL-1 may be to reinforce, rather than to establish, the status of active chromatin through the phosphorylation of histone H3 at serine 10.

We analysed the nuclear organization of the Polycomb/Trithorax group response element (PRE/TRE) Fab-7 and of other PRE/TREs in larval tissues of D. melanogaster. The results show that pairing/clustering of transgenic and endogenous Fab-7 elements and of other endogenous PRE/TREs occurs only to a limited degree in a highly locus-specific and tissue-specific manner. However, transgenic Fab-7 elements as well as the Fab-7-regulated Abd-B gene and other endogenous loci preferentially occupied defined nuclear regions. Preferred association with the nuclear periphery was observed in the inactive state. However, also in the active state, Fab-7 was often found associated with the nuclear periphery as well as with the boundary of heterochromatin in a fly line- and tissue-specific manner. The boundary between heterochromatin and euchromatin revealed a highly complex architecture in the three-dimensional nuclear space with a close juxtaposition of active and repressed domains. The results suggest that such complex architectures create nuclear microenvironments sustaining specific states of activity of defined PRE/TREs. However, the data also show that the positional behaviour of the transgenic Fab-7 element does not apply to PRE/TREs in general. Altogether, this finding and the highly locus-, tissue-, and fly line-specific behaviour with regard to nuclear positioning and pairing/clustering suggest that the relationships between nuclear organization and functional regulation of PRE/TREs are highly complex and that simple models making general predictions might not be appropriate.

Dosage compensation involves fine-tuning of gene expression at the level of entire chromosomes. The principles that assure selective targeting of the male X chromosome in Drosophila and the mechanism by which transcription levels are adjusted in a twofold range are still mysterious. We discuss the prevalent models in the context of recent experimental observations.

The Drosophila trithorax group gene kismet (kis)was identified in a screen for extragenic suppressors of Polycomb(Pc) and subsequently shown to play important roles in both segmentation and the determination of body segment identities. One of the two major proteins encoded by kis (KIS-L) is related to members of the SWI2/SNF2 and CHD families of ATP-dependent chromatin-remodeling factors. To clarify the role of KIS-L in gene expression, we examined its distribution on larval salivary gland polytene chromosomes. KIS-L is associated with virtually all sites of transcriptionally active chromatin in a pattern that largely overlaps that of RNA Polymerase II (Pol II). The levels of elongating Pol II and the elongation factors SPT6 and CHD1 are dramatically reduced on polytene chromosomes from kis mutant larvae. By contrast, the loss of KIS-L function does not affect the binding of PC to chromatin or the recruitment of Pol II to promoters. These data suggest that KIS-L facilitates an early step in transcriptional elongation by Pol II.

The dosage compensation complex (DCC) in Drosophila melanogaster is responsible for up-regulating transcription from the single male X chromosome to equal the transcription from the two X chromosomes in females. Visualization of the DCC, a large ribonucleoprotein complex, on male larval polytene chromosomes reveals that the complex binds selectively to many interbands on the X chromosome. The targeting of the DCC is thought to be in part determined by DNA sequences that are enriched on the X. So far, lack of knowledge about DCC binding sites has prevented the identification of sequence determinants. Only three binding sites have been identified to date, but analysis of their DNA sequence did not allow the prediction of further binding sites. We have used chromatin immunoprecipitation to identify a number of new DCC binding fragments and characterized them in vivo by visualizing DCC binding to autosomal insertions of these fragments, and we have demonstrated that they possess a wide range of potential to recruit the DCC. By varying the in vivo concentration of the DCC, we provide evidence that this range of recruitment potential is due to differences in affinity of the complex to these sites. We were also able to establish that DCC binding to ectopic high-affinity sites can allow nearby low-affinity sites to recruit the complex. Using the sequences of the newly identified and previously characterized binding fragments, we have uncovered a number of short sequence motifs, which in combination may contribute to DCC recruitment. Our findings suggest that the DCC is recruited to the X via a number of binding sites of decreasing affinities, and that the presence of high- and moderate-affinity sites on the X may ensure that lower-affinity sites are occupied in a context-dependent manner. Our bioinformatics analysis suggests that DCC binding sites may be composed of variable combinations of degenerate motifs., Synopsis In fruit flies, just like in humans, the two sexes are distinguished by different sex chromosomes. Females have two X chromosomes and hence a double dose of all X-linked genes when compared to males, which only have a single X chromosome. This different gene dosage needs to be compensated for by adjusting transcription levels such that male and female cells synthesize equal amounts of gene products. In Drosophila melanogaster, dosage compensation occurs by doubling the transcription of many genes on the single male X. This chromosome-wide control is achieved by a male-specific dosage compensation complex (DCC), which contains enzymes, structural proteins, and non-coding RNA. How is the DCC able to distinguish the X chromosome from the autosomes for selective interaction? In the following article, the authors identify and characterize several novel DNA sequences on the X chromosome that can recruit the DCC. Their results suggest that the X chromosome contains a large number of binding sites for the DCC, which are made up of combinations of degenerate sequence elements. These elements constitute binding sites with varying affinities for the complex. Collectively, their abundance on the X chromosome restricts the action of DCC to the X chromosomal territory.

Twofold differences in gene expression levels can be vital for an organism. This is beautifully illustrated by the process of `dosage compensation' in Drosophila, which doubles transcription from the single male X chromosome to equal the mRNA levels originating from the two X chromosomes in female cells. Failure of the process leads to male-specific lethality. A number of recent publications have furthered our understanding of the ribonucleoprotein complex, which mediates dosage compensation and how it targets the male X chromosome. Deciphering the principles of X chromosome recognition and the nature of the chromatin configuration, that allows fine-tuning of transcription, remain the most interesting challenges.