No aberrant methylation of neurofibromatosis 1 gene (NF1) promoter in pilocytic astrocytoma in childhood

The molecular biology of WHO grade I astrocytomas

World Health Organization (WHO) grade I astrocytomas include pilocytic astrocytoma (PA) and subependymal giant cell astrocytoma (SEGA). As technologies in pharmacologic neo-adjuvant therapy continue to progress and as molecular characteristics are progressively recognized as potential markers of both clinically significant tumor subtypes and response to therapy, interest in the biology of these tumors has surged. An updated review of the current knowledge of the molecular biology of these tumors is needed. We conducted a Medline search to identify published literature discussing the molecular biology of grade I astrocytomas. We then summarized this literature and discuss it in a logical framework through which the complex biology of these tumors can be clearly understood. A comprehensive review of the molecular biology of WHO grade I astrocytomas is presented. The past several years have seen rapid progress in the level of understanding of PA in particular, but the molecular literature regarding both PA and SEGA remains nebulous, ambiguous, and occasionally contradictory. In this review we provide a comprehensive discussion of the current understanding of the chromosomal, genomic, and epigenomic features of both PA and SEGA and provide a logical framework in which these data can be more readily understood.

Is double inactivation of the Nf1 gene responsible for the development of congenital pseudarthrosis of the tibia associated with NF1?

The pathogenic mechanism responsible for congenital pseudarthrosis of the tibia (CPT) is not well understood although the possibility of double inactivation of the neurofibromatosis type 1 (Nf1) gene has been suggested. In the present study, loss of heterozygosity was investigated in fibrous hamartoma tissues harvested from 16 patients with CPT associated with NF1 using four genetic markers that span the Nf1 gene. Based on the assumption that a single cell with double inactivation of Nf1 would undergo clonal growth and cause fibrous hamartoma, we investigated clonality in fibrous hamartoma tissues by analyzing X-chromosome inactivation patterns in 11 female patients. Loss of Nf1 heterozygosity in fibrous hamartoma tissues was observed at one or two genetic markers in 4 out of the 16 patients tested. In clonality assays, 3 of 11 patients showed a clonal growth pattern, 5 a non-clonal pattern, and 3 were non-informative. These findings support that double inactivation of the Nf1 gene and subsequent clonal growth could be a pathogenic feature of the fibrous hamartoma tissue at least in some of the CPT but might not be essential requirements of CPT development.

Monozygotic Twins With Neurofibromatosis Type 1 (NF1) Display Differences in Methylation ofNF1Gene Promoter Elements, 5' Untranslated region, Exon and Intron 1

Neurofibromatosis type 1 (NF1) is a common autosomal dominant disorder caused by heterozygotic inactivation of the NF1 tumor suppressor gene at 17q11.2. The associated phenotypes are highly variable, and modifying genes have been proposed to explain at least in part the intriguing expressivity. Given that haploinsufficiency of the NF1 gene product neurofibromin is responsible for some of the clinical manifestations, variations in expression of the wildtype NF1 allele might modify the phenotype. We therefore investigated epigenetic molecular modifications that could result in variable expression of the normal NF1 allele. To exclude confounding by DNA sequence variations, we analyzed monozygotic twin pairs with NF1 who presented with several discordant features. We fine-mapped the methylation pattern of a nearly 1 kb NF1 promoter region in lymphocytes of 8 twin pairs. All twin pairs showed significant intra-pair differences in methylation, especially of specific promoter subregions such as 5'UTR, exon 1 and intron 1 (+7 to +622), transcription factor binding sites and promoter elements like NF1HCS. Furthermore, we detected significant intra-pair differences in cytosine methylation for the region from -249 to -234 with regard to discordance for optic glioma with a higher grade of methylation in glioma cases. In conclusion, our findings of epigenetic differences of the NF1 promoter in leukocytes within mono zygotic twin pairs may serve as a proof of principle for other tissues. The results point towards a role of methylation patterns of the normal NF1 allele for expression differences and for modification of the NF1 phenotype.

MAPK pathway activation and the origins of pediatric low-grade astrocytomas

Low-grade astrocytomas (LGAs) are the most common type of brain tumor in children. Until recently, very little was known about the underlying biology and molecular genetics of these tumors. However, within the past year a number of studies have shown that the MAPK pathway is constitutively activated in a high proportion of LGAs. Several genetic aberrations which generate this deregulation of the MAPK pathway have been identified, most notably gene fusions between KIAA1549 and BRAF. In this review we summarize these findings, discuss how these gene fusions may arise and consider possible implications for diagnosis and treatment.

Molecular diagnosis of neurofibromatosis type 1: 2 years experience

Our experience of providing an NF1 gene diagnostic mutation detection service as part of the U.K. Genetic Testing Network (UKGTN) is presented. A total of 169 unrelated individuals suspected of having neurofibromatosis type I (NF1) were referred for NF1 diagnostic testing over a 2 year period. Mutation analysis of the entire NF1 coding region and the flanking splice sites was carried out, and included the use of a combination of FISH, dHPLC and MLPA. Possible disease causing mutations were identified in 109 (64%) cases. These comprised 88 different sequence alterations, of which 57 were novel. Out of the 169 cases referred, there were 102 patients with reliable clinical data, of whom 78 satisfied the NIH diagnostic criteria for NF1. Within this better defined cohort of NF1 patients, NF1 mutations were identified in 61 individuals (78%), showing the importance of clinical selection on overall test sensitivity, and highlighting the problem of full clinical data collection in the audit of routine services. As mutation detection technologies advance, facilitating direct sequencing of all coding and flanking non-coding regions of the NF1 gene, the development of an even more cost-effective, quick and sensitive diagnostic test for future testing of NF1 is discussed.