Holoprosencephaly

Holoprosencephaly (HPE) is a complex brain malformation resulting from incomplete cleavage of the prosencephalon, occurring between the 18th and the 28th day of gestation and affecting both the forebrain and the face. It is estimated to occur in 1/16,000 live births and 1/250 conceptuses. Three ranges of increasing severity are described: lobar, semi-lobar and alobar HPE. Another milder subtype of HPE called middle interhemispheric variant (MIHF) or syntelencephaly is also reported. In most of the cases, facial anomalies are observed in HPE, like cyclopia, proboscis, median or bilateral cleft lip/palate in severe forms, ocular hypotelorism or solitary median maxillary central incisor in minor forms. These latter midline defects can occur without the cerebral malformations and then are called microforms. Children with HPE have many medical problems: developmental delay and feeding difficulties, epilepsy, instability of temperature, heart rate and respiration. Endocrine disorders like diabetes insipidus, adrenal hypoplasia, hypogonadism, thyroid hypoplasia and growth hormone deficiency are frequent. To date, seven genes have been positively implicated in HPE: Sonic hedgehog (SHH), ZIC2, SIX3, TGIF, PTCH, GLI2 and TDGF1. A molecular diagnosis can be performed by gene sequencing and allele quantification for the four main genes SHH, ZIC2, SIX3 and TGIF. Major rearrangements of the subtelomeres can also be identified by multiplex ligation-dependent probe amplification (MLPA). Nevertheless, in about 70% of cases, the molecular basis of the disease remains unknown, suggesting the existence of several other candidate genes or environmental factors. Consequently, a "multiple-hit hypothesis" of genetic and/or environmental factors (like maternal diabetes) has been proposed to account for the extreme clinical variability. In a practical approach, prenatal diagnosis is based on ultrasound and magnetic resonance imaging (MRI) rather than on molecular diagnosis. Treatment is symptomatic and supportive, and requires a multidisciplinary management. Child outcome depends on the HPE severity and the medical and neurological complications associated. Severely affected children have a very poor prognosis. Mildly affected children may exhibit few symptoms and may live a normal life

Genome Res

Despite numerous studies on specific sumoylated transcriptional regulators, the global role of SUMO on chromatin in relation to transcription regulation remains largely unknown. Here, we determined the genome-wide localization of SUMO1 and SUMO2/3, as well as of UBC9 (encoded by UBE2I) and PIASY (encoded by PIAS4), two markers for active sumoylation, along with Pol II and histone marks in proliferating versus senescent human fibroblasts together with gene expression profiling. We found that, whereas SUMO alone is widely distributed over the genome with strong association at active promoters, active sumoylation occurs most prominently at promoters of histone and protein biogenesis genes, as well as Pol I rRNAs and Pol III tRNAs. Remarkably, these four classes of genes are up-regulated by inhibition of sumoylation, indicating that SUMO normally acts to restrain their expression. In line with this finding, sumoylation-deficient cells show an increase in both cell size and global protein levels. Strikingly, we found that in senescent cells, the SUMO machinery is selectively retained at histone and tRNA gene clusters, whereas it is massively released from all other unique chromatin regions. These data, which reveal the highly dynamic nature of the SUMO landscape, suggest that maintenance of a repressive environment at histone and tRNA loci is a hallmark of the senescent state. The approach taken in our study thus permitted the identification of a common biological output and uncovered hitherto unknown functions for active sumoylation at chromatin as a key mechanism that, in dynamically marking chromatin by a simple modifier, orchestrates concerted transcriptional regulation of a network of genes essential for cell growth and proliferation