No abstract

Smith-Magenis syndrome (SMS) is a mental retardation/multiple congenital anomalies disorder associated with a heterozygous approximately 4-Mb deletion in 17p11.2. Patients with SMS show variability in clinical phenotype despite a common deletion found in>75-80% of patients. Recently, point mutations in the retinoic acid induced 1 (RAI1) gene, which lies within the SMS critical interval, were identified in three patients with many SMS features in whom no deletion was detected. It is not clear if the entire SMS phenotype can be accounted for by RAI1 haploinsufficiency, nor has the precise function of RAI1 been delineated. We report two novel RAI1 mutations, one frameshift and one nonsense allele, in nondeletion SMS patients. Comparisons of the clinical features in these two patients, three of the previously reported RAI1 point mutation cases, and the patients with a common deletion suggest that the majority of the clinical features in SMS result from RAI1 mutation, although phenotypic variability exists even among the individuals with RAI1 point mutations. Bioinformatics analyses of RAI1 and comparative genomics between human and mouse orthologues revealed a zinc finger-like plant homeo domain (PHD) at the carboxyl terminus that is conserved in the trithorax group of chromatin-based transcription regulators. These findings suggest RAI1 is involved in transcriptional control through a multi-protein complex whose function may be altered in individuals with SMS.

No abstract

Hematopoietic stem cells (HSCs) maintain tissue homeostasis by rapidly responding to environmental changes. Although this function is well understood, the molecular mechanisms governing this characteristic are largely unknown. We used a sequenced-based strategy to explore the role of both transcriptional and post-transcriptional regulation in HSC biology. We characterized the gene expression differences between HSCs, both quiescent and proliferating, and their differentiated progeny. This analysis revealed a large fraction of sequence tags aligned to intronic sequences, which we showed were derived from unspliced transcripts. A comparison of the biological properties of the observed spliced versus unspliced transcripts in HSCs showed that the unspliced transcripts were enriched in genes involved in DNA binding and RNA processing. In addition, levels of unspliced message decreased in a transcriptspecific fashion after HSC activation in vivo. This change in unspliced transcript level coordinated with increases in gene expression of splicing machinery components. Combined, these results suggest that post-transcriptional regulation is important in HSC activation in vivo. STEM CELLS 2006;24:662-670

Genomic disorders refer to a group of syndromes caused by DNA rearrangements, such as deletions and duplications, which result in an alteration of normal gene dosage. The chromosomal rearrangements are usually relatively small and often difficult to detect cytogenetically. In a subset of such conditions the rearrangements comprise multiple unrelated contiguous genes that are physically linked and thus have been referred to as contiguous gene syndromes (CGS). In general, each syndrome presents a complex clinical phenotype that has been attributed generally to dosage sensitive gene(s) present in the responsible chromosomal interval. A common mechanism for CGS resulting from interstitial deletion/duplication has recently been elucidated. The DNA rearrangements result from nonallelic homologous recombination (NAHR) utilizing flanking low-copy repeats (LCRs) as recombination substrates. The resulting rearrangements often involve the same genomic region, a common deletion or duplication, making it difficult to assign a specific phenotype or endophenotype to a single responsible gene. The human and mouse genome sequencing projects, in conjunction with the ability to engineer mouse chromosome rearrangements, have enabled the production of mouse models for CGS and genomic disorders. In this review we present an overview of different techniques utilized to generate mouse models for selected genomic disorders. These models foment novel insights into the specific genes that convey the phenotype by dosage and/or position effects and provide opportunities to explore therapeutic options.