Search for ReCQL4 mutations in 39 patients genotyped for suspected Rothmund-Thomson/Baller-Gerold syndromes.

Three overlapping conditions, namely Rothmund-Thomson (RTS), Baller-Gerold (BGS) and RAPADILINO syndromes, have been attributed to RECQL4 mutations. Differential diagnoses depend on the clinical presentation, but the numbers of known genes remain low, leading to the widespread prescription of RECQL4 sequencing. The aim of our study was therefore to determine the best clinical indicators for the presence of RECQL4 mutations in a series of 39 patients referred for RECQL4 molecular analysis and belonging to the RTS (27 cases) and BGS (12 cases) spectrum. One or two deleterious RECQL4 mutations were found in 10/27 patients referred for RTS diagnosis. Clinical and molecular reevaluation led to a different diagnosis in 7/17 negative cases, including Clericuzio-type poikiloderma with neutropenia, hereditary sclerosing poikiloderma, and craniosynostosis/anal anomalies/porokeratosis. No RECQL4 mutations were found in the BGS group without poikiloderma, confirming that RECQL4 sequencing was not indicated in this phenotype. One chromosomal abnormality and one TWIST mutation was found in this cohort. This study highlights the search for differential diagnoses before the prescription of RECQL4 sequencing in this clinically heterogeneous group. The combination of clinically defined subgroups and next-generation sequencing will hopefully bring to light new molecular bases of syndromes with poikiloderma, as well as BGS without poikiloderma.

Renal insufficiency, a frequent complication with age in oral-facial-digital syndrome type I.

The oral-facial-digital syndrome type I (OFD I) is characterized by multiple congenital malformations of the face, oral cavity and digits. A polycystic kidney disease (PKD) is found in about one-third of patients but long-term outcome and complications are not well described in the international literature. Renal findings have been retrospectively collected in a cohort of 34 females all carrying a pathogenic mutation in the OFD1 gene with ages ranging from 1 to 65 years. Twelve patients presented with PKD - 11/16 (69%) if only adults were considered -with a median age at diagnosis of 29 years [IQR (interquartile range) = (23.5-38)]. Among them, 10 also presented with renal impairment and 6 were grafted (median age = 38 years [IQR = (25-48)]. One grafted patient under immunosuppressive treatment died from a tumor originated from a native kidney. The probability to develop renal failure was estimated to be more than 50% after the age of 36 years. Besides, neither genotype-phenotype correlation nor clinical predictive association with renal failure could be evidenced. These data reveal an unsuspected high incidence rate of the renal impairment outcome in OFD I syndrome. A systematic ultrasound (US) and renal function follow-up is therefore highly recommended for all OFD I patients.

Tissue-dependent enhancement of transgene expression by introns of replacement histone H3 genes of Arabidopsis.

Intron-bearing replacement histone H3 genes in Arabidopsis and other plants are highly and constitutively expressed. We demonstrate that the introns located within the 5'-untranslated regions (5'-UTR) of the two Arabidopsis replacement H3 genes will abolish the cell cycle dependence of an endogenous histone H4 promoter. We demonstrate that these introns, functionally combined with their endogenous promoters, could produce the high and constitutive expression of the replacement H3 genes observed in planta. They strongly increase gene expression whatever the promoter, from the strong 35S CaMV promoter to complete and resected promoters of cell cycle-dependent and replacement histone genes. Quantitative analysis of the extent of reporter gene enhancement in different parts of developing transgenic plantlets, ranging from 2-fold to 70-fold, supports the notion that trans-acting factors are responsible for this effect. Such factors appear most abundant in roots.

Plant A-type cyclins.

Although the basic mechanisms which control the progression through the cell cycle appear to be conserved in all higher eukaryotes, the unique features of the plant developmental programme must be somehow reflected in a plant-specific regulation of the factors which control cell division. In the past few years, considerable progress has been achieved in identifying the major components of the cell cycle machinery in plants, especially the cyclin-dependent kinases (CDKs) and their regulatory subunits, the cyclins. The question of how these components direct expression of specific genes at specific stages of the cell cycle, and how they are themselves regulated, constitutes a challenge for the present and for the years to come. This review summarizes our current knowledge of a particular class of plant cyclins, the A-type cyclins, which can be further subdivided into three structural groups. The putative functions of these A-type cyclins are discussed in relation to the presence of remarkable motifs in their amino acid sequences, and to their specific transcriptional regulation, protein amount and subcellular localization.

Cell cycle regulation of the tobacco ribonucleotide reductase small subunit gene is mediated by E2F-like elements.

Ribonucleotide reductase (RNR) is a key enzyme involved in the DNA synthesis pathway. The RNR-encoded genes are cell cycle regulated and specifically expressed in S phase. The promoter of the RNR2 gene encoding for the small subunit was isolated from tobacco. Both in vivo and in vitro studies of the DNA-protein interactions in synchronized BY2 tobacco cells showed that two E2F-like motifs were involved in multiple specific complexes, some of which displayed cell cycle-regulated binding activities. Moreover, these two elements could specifically interact with a purified tobacco E2F protein. Involvement of the E2F elements in regulating the RNR2 promoter was checked by functional analyses in synchronized transgenic BY2 cells transformed with various RNR2 promoter constructs fused to the luciferase reporter gene. The two E2F elements were involved in upregulation of the promoter at the G1/S transition and mutation of both elements prevented any significant induction of the RNR promoter. In addition, one of the E2F elements sharing homology with the animal E2F/cell cycle-dependent element motif behaved like a repressor when outside of the S phase. These data provide evidence that E2F elements play a crucial role in cell cycle regulation of gene transcription in plants.

Cell-cycle modulation of CK2 activity in tobacco BY-2 cells.

Protein kinase CK2 is an ubiquitous Ser/Thr kinase essential for cell growth. We have used the highly synchronizable tobacco BY-2 cell line to investigate whether CK2 activity and expression are regulated in a cell cycle phase-dependent manner in higher plants. Specific cDNA probes for tobacco CK2alpha and beta subunits, respectively, and polyclonal antibodies recognising alpha and beta subunits separately, were obtained to determine mRNA and protein levels of both subunits. Our results show that CK2 activity oscillates throughout the cell cycle, peaking at G1/S and M phases, due to a post-translational regulation of the tetrameric enzyme. Additional levels of control of CK2 expression operate in relation to the proliferative state of the cells, including differential accumulation of alpha and beta transcripts and post-transcriptional regulation of protein levels (beta subunit). Moreover, in vivo inhibition of CK2 activity corroborates the requirement of the functional CK2 to progress through the cell division cycle, and suggests that CK2 might play an important role at the G2/M checkpoint.

Distinct cyclin D genes show mitotic accumulation or constant levels of transcripts in tobacco bright yellow-2 cells.

The commitment of eukaryotic cells to division normally occurs during the G1 phase of the cell cycle. In mammals D-type cyclins regulate the progression of cells through G1 and therefore are important for both proliferative and developmental controls. Plant CycDs (D-type cyclin homologs) have been identified, but their precise function during the plant cell cycle is unknown. We have isolated three tobacco (Nicotiana tabacum) CycD cyclin cDNAs: two belong to the CycD3 class (Nicta;CycD3;1 and Nicta;CycD3;2) and the third to the CycD2 class (Nicta;CycD2;1). To uncouple their cell-cycle regulation from developmental control, we have used the highly synchronizable tobacco cultivar Bright Yellow-2 in a cell-suspension culture to characterize changes in CycD transcript levels during the cell cycle. In cells re-entering the cell cycle from stationary phase, CycD3;2 was induced in G1 but subsequently remained at a constant level in synchronous cells. This expression pattern is consistent with a role for CycD3;2, similar to mammalian D-type cyclins. In contrast, CycD2;1 and CycD3;1 transcripts accumulated during mitosis in synchronous cells, a pattern of expression not normally associated with D-type cyclins. This could suggest a novel role for plant D-type cyclins during mitosis.

Study of phase-specific gene expression in synchronized tobacco cells.

Although the basic mechanisms which control the progression through the cell cycle appear to be conserved in all higher eukaryotes, the unique features of the plant developmental programme must be somehow reflected in a plant-specific regulation of the factors which control cell division. In the last few years, considerable progress has been achieved in identifying the major components of the cell cycle in plants. The question of how these components direct expression of specific genes at specific stages of the cell cycle, and how they are themselves regulated, constitutes a challenge for the present and the next years. This review summarizes our current knowledge at molecular and biochemical levels of cell cycle-regulated expression in the model system, the synchronized tobacco BY2 cell suspension, and discusses the results in comparison to those obtained by different methods and in other plant systems.

Multilevel regulation of histone gene expression during the cell cycle in tobacco cells.

The respective involvement of transcriptional and post-transcriptional mechanisms in coupling H3 and H4 histone gene expression to the S phase of the cell cycle has been studied in synchronized tobacco cells. Induction of histone gene expression at the G1/S transition is shown to be essentially directed by an increase in the transcription rate in response to cellular signals occurring at the initiation step of DNA replication. Histone gene induction thus precedes the burst of DNA synthesis. However, when the elongation step of DNA replication is ineffective or artificially arrested, feedback mechanisms apparently act at the translation level to avoid overproduction of histone proteins from their mRNAs. At the end of S phase, post-transcriptional mechanisms ensure a rapid degradation of histone mRNAs. Transcription factors are bound to the cis -elements of histone promoters throughout the cell cycle, thus suggesting a post-translational modification of some of them to trigger promoter activation at the G1/S transition. Based on these results, a model is proposed for histone gene transcriptional induction in connection with the components of the cell cycle machinery.