Duplication of 7q34 is specific to juvenile pilocytic astrocytomas and a hallmark of cerebellar and optic pathway tumours.

BACKGROUND: Juvenile pilocytic astrocytomas (JPA), a subgroup of low-grade astrocytomas (LGA), are common, heterogeneous and poorly understood subset of brain tumours in children. Chromosomal 7q34 duplication leading to fusion genes formed between KIAA1549 and BRAF and subsequent constitutive activation of BRAF was recently identified in a proportion of LGA, and may be involved in their pathogenesis. Our aim was to investigate additional chromosomal unbalances in LGA and whether incidence of 7q34 duplication is associated with tumour type or location. METHODS AND RESULTS: Using Illumina-Human-Hap300-Duo and 610-Quad high-resolution-SNP-based arrays and quantitative PCR on genes of interest, we investigated 84 paediatric LGA. We demonstrate that 7q34 duplication is specific to sporadic JPA (35 of 53 - 66%) and does not occur in other LGA subtypes (0 of 27) or NF1-associated-JPA (0 of 4). We also establish that it is site specific as it occurs in the majority of cerebellar JPA (24 of 30 - 80%) followed by brainstem, hypothalamic/optic pathway JPA (10 of 16 - 62.5%) and is rare in hemispheric JPA (1 of 7 - 14%). The MAP-kinase pathway, assessed through ERK phosphorylation, was active in all tumours regardless of 7q34 duplication. Gain of function studies performed on hTERT-immortalised astrocytes show that overexpression of wild-type BRAF does not increase cell proliferation or baseline MAPK signalling even if it sensitises cells to EGFR stimulation. CONCLUSIONS AND INTERPRETATION: Our results suggest that variants of JPA might arise from a unique site-restricted progenitor cell where 7q34 duplication, a hallmark of this tumour-type in association to MAPK-kinase pathway activation, potentially plays a site-specific role in their pathogenesis. Importantly, gain of function abnormalities in components of MAP-Kinase signalling are potentially present in all JPA making this tumour amenable to therapeutic targeting of this pathway.

Inhibition of medulloblastoma cell invasion by Slit.

Invasion of brain tumor cells has made primary malignant brain neoplasms among the most recalcitrant to therapeutic strategies. We tested whether the secreted protein Slit2, which guides the projection of axons and developing neurons, could modulate brain tumor cell invasion. Slit2 inhibited the invasion of medulloblastoma cells in a variety of in vitro models. The effect of Slit2 was inhibited by the Robo ectodomain. Time-lapse videomicroscopy indicated that Slit2 reduced medulloblastoma invasion rate without affecting cell direction or proliferation. Both medulloblastoma and glioma tumors express Robo1 and Slit2, but only medulloblastoma invasion is inhibited by recombinant Slit2 protein. Downregulation of activated Cdc42 may contribute to this differential response. Our findings reinforce the concept that neurodevelopmental cues such as Slit2 may provide insights into brain tumor invasion.

Demonstration by transfection studies that mutations in the adrenocorticotropin receptor gene are one cause of the hereditary syndrome of glucocorticoid deficiency.

The hereditary syndrome of unresponsiveness to ACTH is a rare autosomal recessive disorder characterized by low levels of serum cortisol and high levels of plasma ACTH. There is no cortisol response to exogenous ACTH. Recent cloning of the human ACTH receptor gene has enabled us to study this gene in patients with glucocorticoid deficiency. By using the PCR to amplify the coding sequence of the ACTH receptor gene, we identified three mutations in two unrelated patients. One mutation present in homozygous form converted the negatively charged Asp107, located in the third transmembrane domain, to an uncharged Asn residue. The second patient was a compound heterozygote: the paternal allele contained a one-nucleotide insertion leading to a stop codon within the third extracellular loop, and the maternal allele contained a point mutation converting Cys251 to Phe, also in the third extracellular loop. Normal and mutant ACTH receptor genes were expressed in the M3 cell line, and intracellular cAMP production in response to ACTH was measured. For the mutant receptors, no response to physiological ACTH concentrations was detected, suggesting an impaired binding of ACTH to the receptors and/or an altered coupling to the adenylate cyclase effector.

[Mutations of ACTH receptor gene and familial syndrome of glucocorticoid deficiency]. / Mutations du gène du récepteur à l'ACTH et syndrome familial de déficit en glucocorticoïdes.

Familial isolated glucocorticoid deficiency syndrome is characterized by low cortisol plasma levels despite high ACTH levels without any stimulation of steroid production after ACTH administration. However, the mineralocorticoid function is well-preserved in this syndrome which indicates a specific resistance to ACTH. Recent cloning of the ACTH receptor allowed to study this receptor in this particular syndrome. After studying sixteen affected families, we have found three mutations in two patients from non-related families. One of these patients was a double heterozygote compound (C251F, G217fs) while the other one was homozygote for another mutation D107N. The mutant receptors were expressed in vitro in transfected M3 cells (S91 Cloudman cells) which represents a working expression system to express the ACTH receptor. Production of intracellular cyclic AMP was calculated in the presence of increasing concentrations of ACTH. The EC50 values were estimated (C251F: 3.5 +/- 0.9 x 10(-9) M, D107N: 3.0 +/- 0.9 x 10(-9) M, G217fs: 4.8 +/- 0.9 x 10(-9) M) and comparison with the value obtained for the wild type ACTH receptor (5.1 +/- 0.9 x 10(-10) M) indicates a clear 6 to 9 shift to the right due to an impaired function of these mutant receptors. Such results were expected for the G217fs mutation, and could be explained by a decrease in ligand affinity or an impaired coupling to adenylate cyclase in the case of amino acid substitutions. A total of twelve mutations has been described in the literature although eight of them have not been tested in vitro until now.