Improving preimplantation genetic diagnosis for Fragile X syndrome: two new powerful single-round multiplex indirect and direct tests.

Fragile X syndrome (FraX) is caused by the expansion of an unstable CGG repeat located in the Fragile X mental retardation 1 gene (FMR1) gene. Preimplantation genetic diagnosis (PGD) can be proposed to couples at risk of transmitting the disease, that is, when the female carries a premutation or a full mutation. We describe two new single-cell, single-round multiplex PCR for indirect and direct diagnosis of FraX on biopsied embryos. These tests include five unpublished, highly heterozygous simple sequence repeats, and the co-amplification of non-expanded CGG repeats for the direct test. Heterozygosity of the new markers ranged from 69 to 81%. The mean rate of non-informative marker included in the tests was low (26% and 23% for the new indirect and direct tests, respectively). This strategy allows offering a PGD for FraX to 96% of couples requesting it in our centre. A conclusive genotype was obtained in all cells with a rate of cells presenting an allele dropout ranging from 17% for the indirect test to 26% for the direct test. The new indirect test was applied for eight PGD cycles: 32 embryos were analysed, 9 were transferred and 3 healthy babies were born. By multiplexing these highly informative markers, robustness of the diagnosis is improved and the loss of potentially healthy embryos (because they are non-diagnosed or misdiagnosed) is limited. This may increase the chances of success of couples requesting a PGD for FraX, in particular, when premature ovarian insufficiency in premutated women leads to a reduced number of embryos available for analysis.

The words of the regulatory code are arranged in a variable manner in highly conserved enhancers.

The cis-regulatory regions of many developmental regulators and transcription factors are believed to be highly conserved in the genomes of vertebrate species, suggesting specific regulatory mechanisms for these gene classes. We functionally characterized five notochord enhancers, whose sequence is highly conserved, and systematically mutated two of them. Two subregions were identified to be essential for expression in the notochord of the zebrafish embryo. Synthetic enhancers containing the two essential regions in front of a TATA-box drive expression in the notochord while concatemerization of the subregions alone is not sufficient, indicating that the combination of the two sequence elements is required for notochord expression. Both regions are present in the five functionally characterized notochord enhancers. However, the position, the distance and relative orientation of the two sequence motifs can vary substantially within the enhancer sequences. This suggests that the regulatory grammar itself does not dictate the high evolutionary conservation between these orthologous cis-regulatory sequences. Rather, it represents a less well-conserved layer of sequence organization within these sequences.

Birth after pre-implantation genetic diagnosis (PGD) of spinocerebellar ataxia 2 (Sca2).

BACKGROUND: Spinocerebellar ataxia 2 (SCA2) is an autosomal-dominant neurodegenerative disease caused by an extended polyglutamine sequence in the ATXN2 protein. We describe the development of a new single-cell multiplex PCR protocol for pre-implantation genetic diagnosis (PGD) of SCA2 and its successful clinical application. METHODS: Three duplex tests have been developed, one, which combines the detection of the CAG repeats in addition to the D12S821 microsatellite, another, the amplification of the CAG repeats and the D12S1333 microsatellite and the last, the combination of both microsatellites D12S821 and D12S1333. RESULTS: PCR conditions were established using 226 single lymphoblasts or patient lymphocysts. Amplification was obtained in an average of 99.6%, a complete genotype in 86%, a conclusive result in 96% and an allelic drop-out (ADO) rate of 10.7% was observed. PGD for SCA2 was performed for a couple with a paternal risk of transmitting the pathology. Two cycles were done from which 18 embryos were biopsied, 8 were diagnosed as unaffected, 9 as affected and 1 gave no results. In both cycles 2 embryos were transferred, with no pregnancy at the first attempt, and a twin pregnancy at the second attempt. The patient delivered one girl and one boy at 36 weeks and 3 days.

Conserved and acquired features of neurogenin1 regulation.

The telencephalon shows vast morphological variations among different vertebrate groups. The transcription factor neurogenin1 (ngn1) controls neurogenesis in the mouse pallium and is also expressed in the dorsal telencephalon of the evolutionary distant zebrafish. The upstream regions of the zebrafish and mammalian ngn1 loci harbour several stretches of conserved sequences. Here, we show that the upstream region of zebrafish ngn1 is capable of faithfully recapitulating endogenous expression in the zebrafish and mouse telencephalon. A single conserved regulatory region is essential for dorsal telencephalic expression in the zebrafish, and for expression in the dorsal pallium of the mouse. However, a second conserved region that is inactive in the fish telencephalon is necessary for expression in the lateral pallium of mouse embryos. This regulatory region, which drives expression in the zebrafish diencephalon and hindbrain, is dependent on Pax6 activity and binds recombinant Pax6 in vitro. Thus, the regulatory elements of ngn1 appear to be conserved among vertebrates, with certain differences being incorporated in the utilisation of these enhancers, for the acquisition of more advanced features in amniotes. Our data provide evidence for the co-option of regulatory regions as a mechanism of evolutionary diversification of expression patterns, and suggest that an alteration in Pax6 expression was crucial in neocortex evolution.