Somatic, gain-of-function mutations in ras genes were the first specific genetic alterations identified in human cancer about 3 decades ago. Studies during the last quarter century have characterized the Ras proteins as essential components of signaling networks controlling cellular proliferation, differentiation, or survival. The oncogenic mutations of the H-ras, N-ras, or K-ras genes frequently found in human tumors are known to throw off balance the normal outcome of those signaling pathways, thus leading to tumor development. Oncogenic mutations in a number of other upstream or downstream components of Ras signaling pathways (including membrane RTKs or cytosolic kinases) have been detected more recently in association with a variety of cancers. Interestingly, the oncogenic Ras mutations and the mutations in other components of Ras/MAPK signaling pathways appear to be mutually exclusive events in most tumors, indicating that deregulation of Ras-dependent signaling is the essential requirement for tumorigenesis. In contrast to sporadic tumors, separate studies have identified germline mutations in Ras and various other components of Ras signaling pathways that occur in specific association with a number of different familial, developmental syndromes frequently sharing common phenotypic cardiofaciocutaneous features. Finally, even without being a causative force, defective Ras signaling has been cited as a contributing factor to many other human illnesses, including diabetes and immunological and inflammatory disorders. We aim this review at summarizing and updating current knowledge on the contribution of Ras mutations and altered Ras signaling to development of various tumoral and nontumoral pathologies.

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Ras in cancer and developmental diseases.

The prevalence of neurofibromatosis type 1 (NF1) is about 1/3,000. There are no known ethnic groups in which NF1 does not occur or is unusually common. The prevalence is somewhat higher in young children than in adults, a difference that probably results at least in part from the early death of some NF1 patients. NF1 is fully penetrant in adults, but many disease features increase in frequency or severity with age. The reproductive fitness of NF1 patients is reduced by about one-half. About half of all cases result from new mutations. The estimated rate of new NF1 mutations is unusually high, but the basis for this high mutation rate is not known. Am. J. Med. Genet. (Semin. Med. Genet.) 89:1-6, 1999.

Epidemiology of neurofibromatosis type 1

Neurofibromatosis type 1 (NF1) is one of the most common autosomal dominant disorders and is caused by mutations in the NF1 gene. Mutation detection is complex due to the large size of the NF1 gene, the presence of pseudogenes and the great variety of possible lesions. Although there is no evidence for locus heterogeneity in NF1, mutation detection rates rarely exceed 50%. We studied 67 unrelated NF1 patients fulfilling the NIH diagnostic criteria, 29 familial and 38 sporadic cases, using a cascade of complementary techniques. We performed a protein truncation test starting from puromycin-treated EBV cell lines and, if no mutation was found, continued with heteroduplex, FISH, Southern blot and cytogenetic analysis. We identified the germline mutation in 64 of 67 patients and 32 of the mutations are novel. This is the highest mutation detection rate reported in a study of typical NF1 patients. All mutations were studied at the genomic and RNA level. The mutational spectrum consisted of 25 nonsense, 12 frameshift, 19 splice mutations, six missense and/or small in-frame deletions, one deletion of the entire NF1 gene, and a translocation t(14;17)(q32;q11.2). Our data suggest that exons 10a-10c and 37 are mutation-rich regions and that together with some recurrent mutations they may account for almost 30% of the mutations in classical NF1 patients. We found a high frequency of unusual splice mutations outside of the AG/GT 5 cent and 3 cent splice sites. As some of these mutations form stable transcripts, it remains possible that a truncated neurofibromin is formed.

Exhaustive mutation analysis of the NF1 gene allows identification of 95% of mutations and reveals a high frequency of unusual splicing defects.

Although neurofibromatosis 1 (NF1) is a relatively common autosomal dominant condition, information about its effect on mortality is limited. We used Multiple-Cause Mortality Files, compiled from U.S. death certificates by the National Center for Health Statistics, for 1983 through 1997. We identified 3,770 cases of presumed NF1 among 32,722,122 deaths in the United States, a frequency of 1/8,700, which is one-third to one-half the estimated prevalence. Mean and median ages at death for persons with NF1 were 54.4 and 59 years, respectively, compared with 70.1 and 74 years in the general population. Results of proportionate mortality ratio (PMR) analyses showed that persons with NF1 were 34 times more likely (PMR=34.3, 95% confidence interval [CI] 30.8–38.0) to have a malignant connective or other soft-tissue neoplasm listed on their death certificates than were persons without NF1. Overall, persons with NF1 were 1.2 times more likely than expected (PMR=1.21, 95% CI 1.14–1.28) to have a malignant neoplasm listed on their death certificates, but the PMR was 6.07 (95% CI 4.88–7.45) for persons who died at 10–19 years of age and was 4.93 (95% CI 4.14–5.82) for those who died at 20–29 years of age. Similarly, vascular disease was recorded more often than expected on death certificates of persons with NF1 who died at <30 years of age (PMR=3.26, 95% CI 1.31–6.71 at age <10 years; PMR=2.68, 95% CI 1.38–4.68 at age 10–19 years; and PMR=2.25, 95% CI 1.46–3.32 at 20–29 years) but not in older persons. This study supports previous findings of decreased life expectancy for persons with NF1 and, within the limitations of death certificates, provides population-based data about NF1 morbidity and mortality that are useful to clinicians caring for patients with NF1.

Mortality in neurofibromatosis 1: an analysis using U.S. death certificates.

BACKGROUND

The development of malignant and benign tumors in patients with neurofibromatosis type 1 (NF1) was investigated in a long term follow-up study of 70 adult NF1 patients living in Goteborg, Sweden, on January 1, 1978. Their mean age at that time was 44 years (range, 20-81 years). The 70 NF1 patients had previously been investigated in a population-based study.



METHODS

The first part of this study involved a cancer registry study. The authors compared the number of tumors in the 70 NF1 patients reported to the Swedish Cancer Registry during the period 1978-1989 with the number of tumors expected in the general population by matching the incidence rates of the two populations specific to age, time of follow-up, and gender. The 95% confidence interval for the risk quotient between the risk to the patients and the risk to the general population was estimated. The second part of the study was a clinical pathologic follow-up study. All living patients were offered a clinical reexamination in 1990. All hospital records for all the NF1 patients were reviewed, and death certificates were also reviewed when available.



RESULTS

Malignant tumors were reported to the Cancer Registry four times as often in the NF1 patient group as in the general population (95% confidence interval, 2.1-7.6) during the follow-up period 1978-1989. Before 1978, 5 of 70 patients (7%) had 6 malignant tumors; these patients were not included in the Cancer Registry study. Using all available clinical data on the 70 NF1 patients from their birth up to 1990, the authors found that 17 of 70 patients (24%) had developed a total of 19 malignant tumors, namely, 5 sarcomas (in 7% of patients), 13 carcinomas (in 16%), and 1 malignant melanoma (in 1%). Four pheochromocytomas (in 6% of patients), 2 adenomas, and 1 C-cell hyperplasia were diagnosed. Five gastrointestinal stromal tumors (in 7% of patients) were also diagnosed.



CONCLUSIONS

Malignant tumors were reported to the Swedish Cancer Registry significantly more often in the NF1 patients than was expected in the general population matched for age, gender, and time of follow-up. The development of tumors is part of the NF1 disease process, and this deserves attention both in the clinical setting and in family counseling dealing with complications of NF1 in adulthood. Cancer 1997; 79:2125-31. © 1997 American Cancer Society.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/%28SICI%291097-0142%2819970601%2979%3A11%3C2125%3A%3AAID-CNCR9%3E3.0.CO%3B2-N

Malignant and benign tumors in patients with neurofibromatosis type 1 in a defined Swedish population

Neurofibromatosis type 1 (NF1), also called von Recklinghausen disease or peripheral neurofibromatosis, is a common autosomal dominant disorder characterised by multiple neurofibromas, cafe au lait spots, and Lisch nodules of the iris, with a variable clinical expression. The gene responsible for this condition, NF1, has been isolated by positional cloning. It spans over 350 kb of genomic DNA in chromosomal region 17q11.2 and encodes an mRNA of 11-13 kb containing at least 59 exons. NF1 is widely expressed in a variety of human and rat tissues. Four alternatively spliced NF1 transcripts have been identified. Three of these transcript isoforms (each with an extra exon: 9br, 23a, and 48a, respectively) show differential expression to some extent in various tissues, while the fourth isoform (2.9 kb in length) remains to be examined. The protein encoded by NF1, neurofibromin, has a domain homologous to the GTPase activating protein (GAP) family, and downregulates ras activity. The identification of somatic mutations in NF1 from tumour tissues strongly supports the speculation that NF1 is a member of the tumour suppressor gene family. Although the search for mutations in the gene has proved difficult, germline mutation analysis has shown that around 82% of all the fully characterised NF1 specific mutations so far predict severe truncation of neurofibromin. Further extensive studies are required to elucidate the gene function and the mutation spectrum. This should then facilitate the molecular diagnosis and the development of new therapy for the disease.

https://jmg.bmj.com/content/jmedgenet/33/1/2.full.pdf

Molecular genetics of neurofibromatosis type 1 (NF1).

A panel of 200 unrelated NF1 individuals has been screened for mutations using a panel of specific clones for the entire gene. DNA analysis on conventional Southern blots indicated that (20) 10% of NF1 patients showed aberrant bands. Small lesions involving nucleotide alterations were detected in a further 10 patients; 5 of these alterations have been fully characterised and are the novel mutations in the NF1 gene. A number of mutations were identified in exon 2. Identical mutations in this exon in two unrelated individuals involved an insertion of cytosine into codon 5662 and resulted in an inappropriate stop codon. This mutation also created a new MnlI site. Another novel mutation in exon 2 resulted from the insertion of thymidine at nucleotide 5678, which also created an inappropriate stop codon. We have so far completed the screen of exons 1-9 of the NF1 gene for the identification of mutations and have found no evidence of clustering of such mutations in the gene.

Analysis of mutations at the neurofibromatosis 1 (NF1) locus.

Despite the extensive search for disease causing mutations in exons 28-36 of the neurofibromatosis type 1 (NF1) gene, the NF1 specific mutations so far documented account for only a small proportion of all NF1 cases. In this study, we have used 8 sets of new primers to amplify sequences throughout the NF1 gene, including 10 different exons and their flanking intron sequences. The derived PCR products from 150 independent NF1 patients and 50 normal controls were examined by heteroduplex analysis on Hydrolink gels. Three novel mutations were identified and characterised. Two of these mutations include the same 3-bp deletion (AAT) within exon 17 with a silent codon change from ACA (threonine) to ACG (threonine) and a loss of the codon ATG (methionine). The third mutation is a 10-bp deletion (TTCTCTTGGA) within exon 44 resulting in the formation of an inappropriate stop codon. These results should be useful for the further elucidation of the molecular basis of NF1.

Neurofibromatosis type 1 (NF1): the search for mutations by PCR-heteroduplex analysis on Hydrolink gels

We have screened a total of 105 unrelated patients with neurofibromatosis type l (NF1) for mutations in exon 28 of the NF1 gene using heteroduplex analysis and single strand conformation polymorphism analysis. One novel mutation has been identified and characterised. This mutation involves a 13-bp deletion (AAACTGGCTGAGC or AACTGGCTGAGCA) from base position 5077 (or 5078) to 5089 (or 5090) of the cDNA coding sequence. This alteration leads to a reading frame shift with a premature amber termination signal (TAG) at codon 1694. In addition, there is a change from lysine to threonine at codon 1693. The truncated gene product is estimated to be 1125 amino acid residues shorter than the predicted normal protein (2818 amino acids).

A de novo nonsense mutation in exon 28 of the neurofibromatosis type l (NF1) gene

We have characterized two intragenic polymorphisms in the neurofibromatosis type 1 (NF1) gene by direct sequencing of PCR products. The variants for these polymorphisms were initially detected on Hydrolink gels. One of the polymorphisms involves a G to A transition in intron 41 at the 28th base upstream of exon 42 with an observed {open_quote}G{close_quote}/{open_quote}A{close_quote} heterozygosity of 0.42. The other polymorphism is a T to C transition in intron 16 at the 16th base upstream of exon 17 with an observed {open_quote}T{close_quote}/{open_quote}C{close_quote} heterozygosity of 0.09. In combination with other documented polymorphisms in the NF1 gene, these variants should assist in genetic analysis of NF1 families. 24 refs., 3 figs.

Ming Hong Shen

Papers

Molecular genetics of neurofibromatosis type 1 (NF1).

Analysis of mutations at the neurofibromatosis 1 (NF1) locus.

Neurofibromatosis type 1 (NF1): the search for mutations by PCR-heteroduplex analysis on Hydrolink gels

A de novo nonsense mutation in exon 28 of the neurofibromatosis type l (NF1) gene

Two single base polymorphisms in introns 41 and 16 of the NF1 gene