r in pseudouridine levels in mature rRNA. We did observe slightly decreased expression of H/ACA snoRNA and scaRNA in mutant iPS cells. It is no surprise that severe knockdown of DKC1 expression affects ribosome biogenesis but why do pathogenic mutations, when genocopied in mouse cells, affect ribosome biogenesis while the same mutations in humans do not A likely explanation is that in humans only mutations that allow a certain level of ribosome production are viable. In this way mutations that preferentially affect telomere maintenance and leave ribosome biogenesis intact are selected, since they rarely affect embryonic development. The identical mutations in mouse dyskerin, which has 90% identity with the human protein, will likely have a more disruptive effect, if indeed human mutations have been selected as we suggest, and 14 / 20 Dyskeratosis Congenita iPS Cells Fig 5. Expression of WT dyskerin can rescue the expression of LGR5, WLS and PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19666110 FRZB. A: Real-Time RT/PCR experiments showed that, in A353V iPS cells, the mRNA expression of LGR5, FRZB and WLS was 15 / 20 Dyskeratosis Congenita iPS Cells significantly increased after expressing WT dyskerin protein. Of about 41 DKC1 mutations that have been described most are present in single, or a handful of, families but the A353V mutation is recurrent and accounts for 40 percent of all Xlinked DC patients. The phenotype of patients with the A353V mutation varies but tends to be severe, with some children showing the signs of classical DC and others showing the severe features of HH. The lines of iPS cells with this mutation are therefore particularly promising if therapeutic approaches, such as gene correction, are pursued. Because DKC1 is X-linked only one allele is expressed in either male or female cells so whether or not 
the mutations show a dominant negative effect has not been important. However when MedChemExpress AZ-6102 contemplating adding back the WT gene it is important to know if the mutations show a dominant negative effect. The A353V mutation clearly shows a dominant negative effect with respect to telomerase activity and telomere maintenance, likely because two molecules of dyskerin are present in each telomerase RNP particle. Interestingly no dominant negative effect was seen with respect to the decreased WNT signaling. Perhaps the partial recovery of telomerase is enough to restore WNT signaling. The canonical WNT signaling pathway plays a prominent role in development, stem cell renewal and cancer. In the absence of WNT signaling, -catenin cannot accumulate in the cytoplasm since a destruction complex, including AXIN, APC, PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19667314 PP2A, GSK3b and CK1 degrades -catenin via the ubiquitin/proteasome pathway. However, when WNT binds the frizzled receptor at the cell surface, the destruction complex function is disrupted and -catenin accumulates in the nucleus and induces a cellular response through the TCF/LEF transcription factors. It has recently been shown that TERT transcription responds to WNT signaling in this way. Our finding that DKC1 mutations lead to decreased expression of frizzled receptors was unexpected. Perhaps telomerase levels stimulate transcription of frizzled receptors in proliferating cells to maintain high levels of WNT signaling and the telomerase needed for growth and proliferation via a self-maintaining cyclic mechanism. If this is disturbed by decreased telomerase levels then the transcription of receptor genes may decrease. It is interesting in this respect that the receptor most
