V(D)J and immunoglobulin class switch recombinations: a paradigm to study the regulation of DNA end-joining

The immune system is the site of intense DNA damage/modification, which occur during the development and maturation of B and T lymphocytes. V(D)J recombination is initiated by the Rag1 and Rag2 proteins and the formation of a DNA double-strand break (DNA dsb). This DNA lesion is repaired through the use of the non-homologous end-joining (NHEJ) pathway, several factors of which have been identified through the survey of immunodeficient conditions in humans and mice. Upon antigenic recognition in secondary lymphoid organs, mature B cells further diversify their repertoire through class switch recombination (CSR). CSR is a region-specific rearrangement process triggered by the activation-induced cytidine deaminase factor and also proceeds through the introduction of DNA dsb. However, unlike V(D)J recombination, CSR does not rely strictly on NHEJ for the repair of the DNA lesion. Instead, CSR, but not V(D)J recombination, requires the major factors of the DNA damage response. V(D)J recombination and CSR thus represent an interesting paradigm to study the regulation among the various DNA repair pathways

Sugar signaling modulates SHOOT MERISTEMLESS expression and meristem function in Arabidopsis

In plants, development of all above-ground tissues is controlled by the shoot apical meristem (SAM) which balances cell proliferation and differentiation to allow life-long growth. To maximize fitness and survival, meristem activity is adjusted to the prevailing conditions through a poorly understood integration of developmental signals with environmental and nutritional information. Here, we show that sugar signals influence SAM function by altering the protein levels of SHOOT MERISTEMLESS (STM), a key regulator of meristem maintenance. STM is less abundant in the inflorescence meristems of plants grown or treated under limiting light conditions, with lower STM levels correlating with lower sugar content in these meristems. Additionally, sucrose but not light is sufficient to sustain STM accumulation in excised inflorescences. Plants overexpressing the α1-subunit of SUCROSE-NON-FERMENTING1-RELATED KINASE 1 (SnRK1) accumulate less STM protein under optimal light conditions, despite higher sugar accumulation in the meristem. Furthermore, SnRK1α1 interacts physically with STM, suggesting a direct local repression. Surprisingly, silencing SnRK1α in the meristem leads to reduced STM expression and severe developmental phenotypes previously associated with STM loss-of-function. Altogether, we demonstrate that sugars promote STM accumulation and that the SnRK1 sugar sensor plays a dual role in the SAM, limiting STM abundance under unfavorable conditions but being required for overall meristem organization and integrity. This highlights the importance of sugars and SnRK1 signaling for the proper coordination of meristem activities

Cernunnos, a novel V(D)J recombination /non homologus end-joining factor, is mutated in human T and B cell immunodeficiency associated with microcephaly.

DNA double-strand breaks (DSBs) occur at random upon genotoxic stresses and represent obbligatory intermediates during physiological DNA rearrangement events such as the V(D)J recombination in the immune system. DSBs, which are among the most toxic DNA lesions, are preferentially repaired by the nonhomologous end-joining (NHEJ) pathway in higher eukaryotes. Failure to properly repair DSBs results in genetic instability, developmental delay, and various forms of immunodeficiency. Here we describe five patients with growth retardation, microcephaly, and immunodeficiency characterized by a profound T + B lymphocytopenia. An increased cellular sensitivity to ionizing radiation, a defective V(D)J recombination, and an impaired DNA-end ligation process both in vivo and in vitro are indicative of a general DNA repair defect in these patients. All five patients carry mutations in the Cernunnos gene, which was identified through cDNA functional complementation cloning. Cernunnos/XLF represents a novel DNA repair factor essential for the NHEJ pathway