R e s e a R c h a R t i c l e4 0 2 8 jci.org Volume 124 Number 9 September 2014 IntroductionPrimordial dwarfism denotes extreme linear growth impairment from early gestation. When severely reduced head size is also seen, primordial dwarfism is said to be microcephalic. This is generally taken to imply a global problem with either cellular or organismal growth and contrasts with primordial dwarfism in which the head size is relatively preserved, which is more likely to be accounted for by a more tissue-selective problem, affecting, for example, long bone growth (1). Many human genetic defects have now been described as underlying microcephalic primordial dwarfism. These may affect endocrine growth regulation by impairing production or action of IGF-1 (2, 3); however, most mutations reported to date affect cellautonomous growth. Some clustering of defects according to the cellular structure or process affected is apparent. For example, microcephalic primordial dwarfism can result from defects in genes involved in the licensing of DNA replication, in centrosome function, or in DNA damage response and repair pathways (reviewed in ref. 4).In keeping with the known roles of the defective genes, some syndromes feature a characteristic profile of chromosomal instability or impaired DNA damage repair. Some components of the syndromes cannot easily be explained by known functions of the affected gene, however. An example is the severe insulin resistance described in pericentrin deficiency (5). This usually appears during the first decade of life and is associated with severely fatty liver, elevated blood triglyceride levels, and diabetes, recapitulating in severe form the pandemic metabolic syndrome. Similar metabolic abnormalities are seen with milder growth impairment in both Bloom syndrome (6) and Werner syndrome (7), each caused by loss of a DNA helicase, and without growth impairment in patients with a heterozygous polymerase-inactivating mutation in DNA polymerase δ (8). However despite increasing evidence of a link between impaired DNA replication and/or damage responses and severe insulin resistance, the underlying mechanisms linking these remain unclear. ResultsA novel syndrome of primordial dwarfism. We identified 2 female patients with similar syndromes (details in Supplemental Material and Methods; supplemental material available online with this article; doi:10.1172/JCI73264DS1). The shared features are (a) severe primordial dwarfism, with facial dysmorphism, including Structural maintenance of chromosomes (SMC) complexes are essential for maintaining chromatin structure and regulating gene expression. Two the three known SMC complexes, cohesin and condensin, are important for sister chromatid cohesion and condensation, respectively; however, the function of the third complex, SMC5-6, which includes the E3 SUMO-ligase NSMCE2 (also widely known as MMS21) is less clear. Here, we characterized 2 patients with primordial dwarfism, extreme insulin resistance, and gonadal failure and identified compound heterozygou...

A homozygous mutational change in the Ataxia-Telangiectasia and RAD3 related (ATR) gene was previously reported in two related families displaying Seckel Syndrome (SS). Here, we provide the first identification of a Seckel Syndrome patient with mutations in ATRIP, the gene encoding ATR–Interacting Protein (ATRIP), the partner protein of ATR required for ATR stability and recruitment to the site of DNA damage. The patient has compound heterozygous mutations in ATRIP resulting in reduced ATRIP and ATR expression. A nonsense mutational change in one ATRIP allele results in a C-terminal truncated protein, which impairs ATR–ATRIP interaction; the other allele is abnormally spliced. We additionally describe two further unrelated patients native to the UK with the same novel, heterozygous mutations in ATR, which cause dramatically reduced ATR expression. All patient-derived cells showed defective DNA damage responses that can be attributed to impaired ATR–ATRIP function. Seckel Syndrome is characterised by microcephaly and growth delay, features also displayed by several related disorders including Majewski (microcephalic) osteodysplastic primordial dwarfism (MOPD) type II and Meier-Gorlin Syndrome (MGS). The identification of an ATRIP–deficient patient provides a novel genetic defect for Seckel Syndrome. Coupled with the identification of further ATR–deficient patients, our findings allow a spectrum of clinical features that can be ascribed to the ATR–ATRIP deficient sub-class of Seckel Syndrome. ATR–ATRIP patients are characterised by extremely severe microcephaly and growth delay, microtia (small ears), micrognathia (small and receding chin), and dental crowding. While aberrant bone development was mild in the original ATR–SS patient, some of the patients described here display skeletal abnormalities including, in one patient, small patellae, a feature characteristically observed in Meier-Gorlin Syndrome. Collectively, our analysis exposes an overlapping clinical manifestation between the disorders but allows an expanded spectrum of clinical features for ATR–ATRIP Seckel Syndrome to be defined.

Eukaryotic initiation factor (eIF) 4B interacts with several components of the initiation pathway and is targeted for cleavage during apoptosis. In a cell-free system, cleavage of eIF4B by caspase-3 coincides with a general inhibition of protein synthetic activity. Affinity chromatography demonstrates that mammalian eIF4B interacts with the poly(A)-binding protein and that a region consisting of the N-terminal 80 amino acids of eIF4B is both necessary and sufficient for such binding. This interaction is lost when eIF4B is cleaved by caspase-3, which removes the N-terminal 45 amino acids. Similarly, the association of eIF4B with the poly(A)-binding protein in vivo is reduced when cells are induced to undergo apoptosis. Cleavage of the poly(A)-binding protein itself, using human rhinovirus 3C protease, also eliminates the interaction with eIF4B. Thus, disruption of the association between mammalian eIF4B and the poly(A)-binding protein can occur during both apoptosis and picornaviral infection and is likely to contribute to the inhibition of translation observed under these conditions.Binding of mRNA to eukaryotic ribosomes involves numerous protein-RNA and protein-protein interactions at both ends of the mRNA (reviewed in Refs. 1-4). The 40 S ribosome and associated initiation factors (5-8) binds to eIF4G, 1 which acts as a scaffold molecule (1, 3, 9 -11). eIF4G associates with the mRNA cap structure via eIF4E (3, 12, 13), as well as with eIF4A (3) and the poly(A)-binding protein (PABP) linked to the poly(A) tail (2, 14 -18). This complex facilitates the ability of the 40 S ribosomal subunit to "scan" along the mRNA until it reaches a suitable initiation codon, whereupon the 60 S subunit joins to form the 80 S initiation complex (3, 19). eIF4B, which can also interact with the cap (20, 21), stimulates the RNA helicase activity of eIF4A (13,(22)(23)(24)(25) and is required to mediate mRNA binding to ribosomes (26 -28). It possesses three potential regulatory domains: an RNA binding domain (RNA recognition motif), a hydrophilic region (DRYG) that mediates its binding to eIF3p170 (29), and a serine-rich region at the C terminus (22).Both the induction of apoptosis and viral infection result in a rapid but incomplete inhibition of protein synthesis, accompanied by the proteolytic cleavage of certain initiation factors (reviewed in Refs. 3, 4, and 30). During apoptosis, degradation of both forms of eIF4G (eIF4GI and eIF4GII) occurs in a variety of cell types (31-38). Selective cleavages of eIF4B, eIF3p35, a population of eIF2␣, and the eIF4E-binding protein 4E-BP1 have also been observed (33, 35, 36, 39 -41).In plants, eIF4B can interact with PABP, and this leads to enhancement both of the RNA binding activity of PABP (42) and of the RNA helicase activity of the [eIF4A⅐eIF4B] complex (43). It has remained unclear whether such an interaction occurs in mammalian cells and, if so, whether it might be affected by the cleavage of eIF4B that occurs in apoptotic cells. PABP itself can also be a target for proteolytic cleavag...

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Eukaryotic initiation factor (eIF) 4G is an integral member of the translation initiation machinery. The molecule serves as a scaffold for several other initiation factors, including eIF4E, eIF4AI, the eIF3 complex, and poly(A)-binding protein (PABP). Previous work indicates that complexes between these proteins exhibit enhanced mRNA cap-binding and RNA helicase activities relative to the respective individual proteins, eIF4E and eIF4A. The eIF4G-PABP interaction has been implicated in enhancing the formation of 48 S and 80 S initiation complexes and ribosome recycling through mRNA circularization. The eIF3-eIF4GI interaction is believed to forge the link between the 40 S subunit and the mRNA. Here we have investigated the behavior in vitro and in intact cells of eIF4GIf molecules lacking either the PABP-binding site, the eIF3-binding site, the middle domain eIF4A-binding site, or the C-terminal segment that includes the second eIF4A-binding site. Although in some cases the mutant forms were recruited more slowly, all of these eIF4G variants could form complexes with eIF4E, enter 48 S complexes and polysomes in vivo and in vitro, and partially rescue translation in cells targeted with eIF4GI short interfering RNA. In the reticulocyte lysate, eIF4G unable to interact directly with PABP showed little impairment in its ability to support translation, whereas loss of either of the eIF4A-binding sites or the eIF3-binding site resulted in a marked decrease in activity. We conclude that there is considerable redundancy in the mechanisms forming initiation complexes in mammalian cells, such that many individual interactions have regulatory rather than essential roles.