Intracranial and Spinal Melanotic Schwannoma in the Same Patient

Melanotic schwannoma is a nerve sheath tumor composed of melanin - producing cells with ultrastructural features of Schwann cells, which is very rare. These tumors can appear intracranially and less common in the spinal canal. The prognosis is usually good although cases with metastases and death are described in the literature. We present the unusual and rare case of a young man with a spinal melanotic schwannoma at L5/S1 who presented 30 months later with an intracranial melanotic schwannoma.

Sulindac derivatives inhibit cell growth and induce apoptosis in primary cells from malignant peripheral nerve sheath tumors of NF1-patients

BACKGROUND: Malignant peripheral nerve sheath tumors (MPNSTs) are neoplasms leading to death in most cases. Patients with Neurofibromatosis type 1 have an increased risk of developing this malignancy. The metabolites of the inactive prodrug Sulindac, Sulindac Sulfide and Sulindac Sulfone (Exisulind) are new chemopreventive agents that show promising results in the treatment of different cancer types. In this study we examined the antineoplastic effect of these compounds on primary cells derived from two MPNSTs of Neurofibromatosis type 1 patients. RESULTS: Exisulind and Sulindac Sulfide showed a dramatic time- and dose-dependent growth inhibitory effect with IC50-values of 120 microM and 63 microM, respectively. The decrease in viability of the tested cells correlated with induction of apoptosis. Treatment with 500 microM Exisulind and 125 microM Sulindac Sulfide for a period of 2 days increased the rate of apoptosis 21-27-fold compared to untreated cells. Reduced expression of RAS-GTP and phosphorylated ERK1/2 was detected in treated MPNST cells. Moreover, elevated levels of phosphorylated SAPK/JNK were found after drug treatment, and low activation of cleaved caspase-3 was seen. CONCLUSIONS: Our results suggest that this class of compounds may be of therapeutic benefit for Neurofibromatosis type 1 patients with MPNST.

Autosomal dominant lateral temporal epilepsy: clinical spectrum, new epitempin mutations, and genetic heterogeneity in seven European families

PURPOSE

[corrected] To describe the clinical and genetic findings of seven additional pedigrees with autosomal dominant lateral temporal epilepsy (ADLTE).

METHODS

A personal and family history was obtained from each affected and unaffected member, along with a physical and neurologic examination. Routine and sleep EEGs, computed tomography (CT), or magnetic resonance imaging (MRI) were performed in almost all the patients. DNAs from family members were typed with several microsatellite markers localized on either side of LGI1 at 10q24 and screened for LGI1 mutations.

RESULTS

The seven families included a total of 34 affected individuals (10 deceased). The age at onset ranged between 8 and 50 years (average, 22 years). Twenty-six patients had clear-cut focal (elementary, complex, or secondarily generalized) seizures, characterized by prominent auditory auras in 68% of the cases. Less frequent ictal symptoms were visual, psychic, or aphasic seizures, the latter occurring in isolation in one family. The attacks were rare and well controlled by antiepileptic drug treatment but recurred after drug discontinuation. Interictal EEGs were usually unrevealing. MRI or CT scans were negative. Analysis of LGI1/Epitempin exons failed to show mutations in three pedigrees. Linkage analysis strongly suggested exclusion of linkage in one of these families. We found two novel missense mutations, a T-->C substitution in exon 6 at position 598, and a T-->A transition in exon 8 at position 1295, the latter being detected in a family with aphasic seizures.

CONCLUSIONS

Our data confirm the inclusion of aphasic seizures within the ADLTE clinical spectrum, suggest the existence of locus heterogeneity in ADLTE, and provide new familial cases with LGI1 missense mutations associated with the disease.

Cochlear Implantation after Acoustic Tumour Resection in Neurofibromatosis Type 2: Impact of Intra- and Postoperative Neural Response Telemetry Monitoring

The present paper reports about a 16-year-old male with neurofibromatosis type 2 (NF-2) of the Wishart type with bilateral deafness who had undergone cochlear implantation after resection of the acoustic neuroma (AN) of the same side. Neural response telemetry (NRT) recordings are essential in those patients during cochlear implantation where no stapedial reflexes can be electrically elicited due to the resection of the AN. In the present case, amplitude growth function and a type II pattern of the NRT waveforms could be well established. The comparison of the N(1) response intra-operatively and after 2 years showed a decline in latency by 50% and an increase in absolute amplitude by 10 times at the same current level of electrical stimulation. This improved auditory nerve transduction suggested a change to a 'faster' encoding strategy to improve speech understanding. The change from SPEAK to ACE 18 months after the operation led to an increase in the open-set sentence recognition test from 52 to 88%. Thus, NRT recordings monitor the intra-operative success of electrode placement and help to assess the integrity of the auditory pathway. Moreover, they can reliably be used in programming the speech processor postoperatively as objective tool. In patients with NF-2, the restoration of hearing can be successfully achieved in several ways. The indications for hearing implants (auditory brain stem and cochlear implants) should be carefully considered with respect to the remaining, functional integrity of the auditory nerve and the technical possibilities to monitor the success of these procedures.

Malignant peripheral nerve sheath tumours in neurofibromatosis type 1: MRI supports the diagnosis of malignant plexiform neurofibroma

Plexiform neurofibroma (PNF) is a typical feature of neurofibromatosis 1 (NF1). About 10% of patients with NF1 develop malignant peripheral nerve-sheath tumours (MPNST), usually arising from PNF, and this is the major cause of poor survival. A better prognosis can be achieved if the tumours are diagnosed at an early stage. Our objective was to establish MRI criteria for MPNST and to test their usefulness in detecting early malignant change in PNF. MRI was performed on 50 patients with NF1 and nerve-sheath tumours, of whom seven had atypical pain, tumour growth or neurological deficits indicative of malignancy; the other 43 were asymptomatic. On MRI all seven symptomatic patients had inhomogeneous lesions, due to necrosis and haemorrhage and patchy contrast enhancement. In one patient, the multiplicity of confluent tumours with inhomogeneous areas in addition to central lesions did not allow exclusion of malignancy. Only three of the 43 asymptomatic patients had comparable changes; the other 40 patients had tumours being of relatively homogeneous structure on T1- and T2-weighted images before and after contrast enhancement. All three asymptomatic patients with inhomogeneous lesions were shown to have MPNST.

Multiple unilateral schwannomas: segmental neurofibromatosis type 2 or schwannomatosis?

Schwannomas are benign solitary tumours of the peripheral nerve sheaths. The occurrence of multiple schwannomas usually implies hereditary disease. The most frequent syndrome associated with multiple schwannomas is neurofibromatosis type 2 (NF2), which is defined by bilateral vestibular schwannomas. Schwannomatosis is a distinct disease characterized by multiple pathologically proven schwannomas in the absence of vestibular schwannomas. It is not currently known if the presence of multiple schwannomas confined to a limb may represent a mosaic form of NF2 or a distinct disease, because mutation analysis of these tumours is not routinely performed. We report a 31-year-old patient who presented with multiple slowly growing subcutaneous tumours on his left arm. His family history was negative for cutaneous tumours or central nervous system disease, and he did not have additional features of NF2. Magnetic resonance tomography and ophthalmological examination excluded vestibular schwannoma and eye stigmata of NF2. After resection of three tumours, histological analysis confirmed the diagnosis of benign schwannomas. Molecular genetic analysis by temperature gradient gel electrophoresis and microsatellite marker analysis demonstrated two distinct mutations of the NF2 gene (NF2) in two different schwannomas, with concomitant loss of heterozygosity in both tumours. In contrast, neither normal skin nor peripheral blood lymphocytes revealed mutations of NF2. The clinical and molecular genetic findings suggest that the diagnosis in our patient is schwannomatosis rather than segmental NF2 because the mutations found in different tumours were not identical. The possibility of a localized predisposition for the acquisition of NF2 mutations is discussed.

Growth type of plexiform neurofibromas in NF1 determined on magnetic resonance images

This article describes some types of growth in plexiform neurofibroma (PNF) on magnetic resonance images (MRI). This tumor is almost exclusively associated with NF1. On MRI, the tumor is depicted as a hyperintensive area on T2-weighted images. We distinguished 3 patterns of tumor growth: first, the superficial and non-invasive tumors, that are restricted to the cutis and subcutis, only eventually having outgrowth to the muscles beneath and are slow growing. Second, the displacing PNF that develop in deeper layers of the skin or within the body. They can grow to a large extent but do not invade adjacent muscles or skin. Thirdly, the invasive type with no visible margins that cannot be resected without adjacent structures or organs. A combination of these tumors can sometimes be noted, e.g. a displacing tumorous nerve developing in a large lumpy, non-invasive PNF. These categories might be used as a current guideline for medical advice, surgical treatment planning and medication trials.

Ki-67 proliferation-index (MIB-1) of neurofibromas in neurofibromatosis type 1 patients

OBJECTIVE

The aim of this study was to analyse the proliferation rate of neurofibromas, in neurofibromatosis type 1 (NF1) patients in order to find out whether tumor growth can be correlated with the different subtypes, size and localisation of tumors, or gender. Large tumors and those localisations that do not allow a complete resection, e.g. the trigeminal branch plexiform neurofibromas, often require repeated surgical interventions. Therefore, the question whether partial resection is associated with alterations of the tumor type and proliferation is of great interest.

MATERIALS AND METHODS

We investigated 317 specimens of 96 patients. Twenty-five specimens were identified as local recurrences, all of them being consecutive resections in the previously operated area. All patients were NF 1-affected individuals who fulfilled the US National Institute of Health consensus criteria for defining the disease. The proliferation index (PI) was assessed on formalin-fixed, paraffin-embedded tissue stained with the MIB- 1 antibody (Ki-67 antigen). The PI was evaluated in three high-power fields (0.1 square millimeter) in the area with the highest proliferative activity. The correlations were calculated according to Spearman-Rho.

RESULTS

Men were more often surgically treated in the head and neck than women (p < 0.02). Plexiform neurofibromas were more frequently operated on in the head and neck than in other regions (p < 0.01). Older patients were more often treated for the diffuse cutaneous type of neurofibromas (p < 0.0001). The type of tumor did not differ from primaries to recurrent tumors. The MIB- 1 PI showed no association with any of the clinical parameters. In particular, there was no difference of the MIB-1 index between primaries and recurrent tumors.

DISCUSSION

This study showed, for the first time, that proliferation in neurofibromas is not enhanced in previously partially resected neurofibromas. Hence, the argument that trauma or surgery for neurofibromas might promote proliferation, especially in the plexiform neurofibroma, is not supported by the results of the present study. Further this analysis demonstrated interdependencies between tumor type, localisation, age and gender indicative of the social difficulties encountered by the NF1 patients which may be helpful for the advising practitioner.

Intrafamilial correlation of clinical manifestations in neurofibromatosis 2 (NF2)

Measuring correlation in clinical traits among relatives is important to our understanding of the causes of variable expressivity in Mendelian diseases. Random effects models are widely used to estimate intrafamilial correlations, but such models have limitations. We incorporated survival techniques into a random effects model so that it can be used to estimate intrafamilial correlations in continuous variables with right censoring, such as age at onset. We also describe a negative-binomial gamma mixture model to determine intrafamilial correlations of discrete (e.g., count) data. We demonstrate the utility of these methods by analyzing intrafamilial correlations among patients with neurofibromatosis 2 (NF2), an autosomal-dominant disease caused by mutations of the NF2 tumor-suppressor gene. We estimated intrafamilial correlations in age at first symptom of NF2, age at onset of hearing loss, and number of intracranial meningiomas in 390 NF2 nonprobands from 153 unrelated families. A significant intrafamilial correlation was observed for each of the three features: age at onset (0.35; 95% confidence interval (CI) 0.23-0.47), age at onset of hearing loss (0.51; 95% CI, 0.35-0.64), and number of meninginomas (0.29; 95% CI, 0.15-0.43). Significant correlations were also observed for age at first symptom within NF2 families with truncating mutations (0.41; 95% CI, 0.06-0.68) or splice-site mutations (0.29; 95% CI, 0.03-0.51), for age at onset of hearing loss within families with missense mutations (0.67; 95% CI, 0.18-0.89), and for number of meningiomas within families with splice-site mutations (0.39; 95% CI, 0.13-0.66). Our findings are consistent with effects of both allelic and nonallelic familial factors on the clinical variability of NF2.

[Abnormalities of the maxillary sinus in type 1 neurofibromatosis]

AIM

The aim of this study was to determine the malformations of the maxillary sinus in neurofibromatosis type 1 patients (NF1).

MATERIAL AND METHODS

Twenty-two patients with NF1 were investigated clinically and radiologically: 11 had an unilateral trigeminal plexiform neurofibroma and 11 had multiple cutaneous neurofibromas. The histological type of NF was ascertained in all cases following tumor resections. The malformation of the maxillary sinus was assessed on plain radiographs and computed or magnetic resonance tomograms. Intraindividual side comparison was used to judge the size of the sinus and its position in the midface.

RESULTS

In patients with cutaneous neurofibromas the maxillary sinus appeared symmetrical in size and position. The pneumatization of the sinus had no abnormalities on the radiographs. Malformations of the maxillary sinus were restricted to plexiform neurofibromas. On the side affected by a plexiform NF, the sinus appeared hypoplastic and caudally displaced due to an enlarged ipsilateral orbit. The expansion of the sinus to the lateral side was impaired, obviously due to tumor masses. Consecutively, the alveolar process of the affected side was also displaced leading to a complex malocclusion.

DISCUSSION

Malformations of the face are frequently presented as case reports in the literature. Emphasis is given to the elephantiasis-like tumor growth of the face in certain patients with NF1. The underlying pathology has not yet been fully understood. This report provides evidence that in the midfacial region the overgrowth is predominantly caused by the plexiform neurofibroma itself and that the bones can even be hypoplastic and show scoliosis-like malformation compared to the nonaffected side. These findings are relevant when debulking procedures of the face are planned for NF1 patients.

Presymptomatic diagnosis for children of sporadic neurofibromatosis 2 patients: a method based on tumor analysis

PURPOSE

To provide presymptomatic diagnosis for children of sporadic neurofibromatosis 2 patients in whom no NF2-mutations were found by screening their blood-DNA.

METHODS

Tumors of four patients were analyzed for NF2 allele losses and mutations.

RESULTS

Nonsense NF2 mutations and NF2 allele losses were found in all tumors. None of these alterations was found in any of eight children examined, suggesting that these children did not inherit the disease.

CONCLUSIONS

Finding two genetic alterations of a tumor suppressor gene in associated tumors is useful for presymptomatic diagnosis. Identification of the lost allele in tumors alone also enables exclusion of disease transmission in 50% of cases.

Loss of NF1 alleles distinguish sporadic from NF1-associated pilocytic astrocytomas

Pilocytic astrocytomas classified as WHO grade I typically arise in childhood and upon complete surgical removal carry a favorable prognosis. Children with neurofibromatosis 1 (NF1) have a vastly increased risk for pilocytic astrocytomas, especially for those of the optic nerve. Using 4 intragenic NF1 microsatellite markers, we examined losses of NF1 alleles on the long arm of chromosome 17 in 12 NF1-associated and 25 sporadic pilocytic astrocytomas. The TP53 gene region on the short arm of chromosome 17 was also examined in these tumors using 3 markers. Loss of 1 NF1 allele was detected in 11 of 12 (92%) informative NF1-associated pilocytic astrocytomas. In contrast, only 1 of 24 informative (4%) sporadic pilocytic astrocytomas exhibited allelic loss in the NF1 region. Among the 11 NF1-associated tumors with NF1 loss, 5 had also lost alleles on 17p. The high rate of NF1 allele loss in NF1-associated pilocytic astrocytomas suggests a tumor initiating or promoting action of the NF1 gene in these patients. On the other hand, the much lower rate of NF1-allele loss in sporadic pilocytic astrocytomas argues for only minor importance of NF1 in that patient group. The present data support different mechanisms in the formation of NF1-associated and sporadic pilocytic astrocytomas.

Supporting evidence of a gene for partial epilepsy on 10q

A four-generation family with nine individuals with temporal partial epilepsy was studied. Detailed epilepsy history was investigated by structured interview. All putatively affected family members underwent a standardized electroencephalographic examination. The phenotype in the family was characterized by a short acoustic aura followed by rapid secondary generalization. To examine if the trait is linked to a region on 10q (interval D10S185-D10S1671), which has been reported in two other epilepsy families with similar phenotypes, linkage analysis was performed using nine markers covering the previously reported region. A maximum two-point LOD score of 2.1 at a recombination fraction of zero was obtained. All living affected individuals shared the same haplotype, while three unaffected at-risk adults did not. This result presents supporting evidence of a gene for partial epilepsy on 10q.

Allelic losses in neurofibromatosis 2-associated meningiomas

More than 50% of patients with neurofibromatosis 2 (NF2) develop meningiomas. Recently, a higher proliferative activity, more mitotic figures, and greater nuclear pleomorphism have been described for NF2-associated meningiomas compared with sporadic ones. To analyze whether such histological differences could reflect underlying genetic differences, we examined 30 meningiomas from 22 NF2 patients for allelic losses on those chromosome arms that are frequently affected by deletions in sporadic meningiomas. In addition, we assessed the proliferative activity of the tumors and studied NF2 germline mutations. Twenty-three meningiomas corresponded to WHO grade I (10 fibrous, 6 psammomatous, 4 transitional, 3 meningothelial) and 7 to WHO grade II. The average MIB-1 index was 1.60 +/- 0.85 (WHO grade I: 1.41 +/- 0.80, WHO grade II: 2.13 +/- 0.82). When compared with several published studies of sporadic meningiomas, the MIB-1 index in NF2-associated meningiomas was not higher. Loss of heterozygosity (LOH) flanking or within the NF2 locus at 22q12 was detected in 100% of the tumors. LOH on 1p was the second most frequent abnormality (40%), followed by losses on 10q (27%), 6q and 14q (24%), 18q (23%), and 9p (17%). LOH on 19q and 17p, which is not commonly seen in sporadic meningiomas, was also only rarely detected in NF2-associated meningiomas. NF2 gene mutations were detected in 8 of 15 patients analyzed and were located in exons 2, 5, 6, 7, and 8. We conclude that sporadic and NF2-associated meningiomas share a common spectrum and frequency of allelic deletions as well as, in contrast to previous observations, a similar proliferative activity.

Mutations and allelic loss of the NF2 gene in neurofibromatosis 2-associated skin tumors

Schwannomas in the skin are frequently observed in neurofibromatosis 2 patients. In about one-quarter of the cases, skin tumors are the first clinical symptoms of this disease. Recognizing neurofibromatosis-2-related skin tumors is therefore important for early diagnosis of neurofibromatosis 2, especially in pediatric patients. In this study, we examined 40 skin tumors (36 schwannomas and four neurofibromas) from 20 neurofibromatosis 2 patients for NF2 mutations and allelic loss. NF2 mutations have been identified in blood from 15 (75%) of the 20 patients. We found NF2 mutations in five (13%) and NF2 allelic loss in 18 (45%) of the 40 analyzed tumors. Genetic alterations (allelic loss or mutation) were thus found in 50 (63%) out of the total of 80 examined alleles. In 17 (43%) tumors, alterations were found on both NF2 alleles. These results suggest that, as in the case of vestibular schwannomas and meningiomas, loss of functional NF2 gene product is also the critical event in the development of skin schwannomas. Identification of genetic alterations of the NF2 gene in skin tumors may help to identify neurofibromatosis-2-associated skin tumors, thus assisting in the diagnosis of neurofibromatosis 2 in ambiguous cases, and excluding neurofibromatosis 1 in unclear cases. We also report that the detection rate of constitutional mutations was higher in patients with skin tumors (65%) than in patients without skin tumors (40%).

[Long-term outcome of gliomas of the visual pathway in type 1 neurofibromatosis]

BACKGROUND

Optic gliomas are frequently associated with neurofibromatosis type 1 (NF1) and belong to the diagnostic criteria of NF1. Different growth rates require differentiated strategies for screening and observation.

PATIENTS AND METHODS

29 patients (25 children < 11 years, 4 adults > 18 years) with visual pathway tumors and NF1 were examined neurologically, ophthalmogically, by means of MRI and VEP. Results were set into context with preceding investigations (mean follow up time 6.5 years).

RESULTS

11 children showed a stable condition, 14 an unfavorable process with substantial loss of vision. Children with an unfavorable process showed a lower age at diagnosis (3.2 versus 5.8 years; p < 0.05), more frequently strabism (11/14 versus 1/11; p < 0.05), optic atrophy (12/14 versus 1/11; p < 0.05), pathological VEP (9/9 versus 2/10; p = 0.001), visual field defects (9/9 versus 1/9) and involvement of the optic chiasm (11/14 versus 3/11; p < 0.05) than children with a stable condition. 3 of 4 adults had no visual symptoms despite involvement of the optic chiasm.

CONCLUSIONS

The crucial prognostic factor is the patient's age at the time of diagnosis. Optic gliomas which become symptomatic in early childhood (< 6 years) grow rapidly and require frequent ophthalmologic investigations and MRI. Tumors diagnosed in late childhood (> 6 years) do not progress, allowing for gradual extension of intervals between ophthalmological investigations.

Spinal tumours in neurofibromatosis type 1: an MRI study of frequency, multiplicity and variety

In neurofibromatosis type 1 (NF1) spinal tumours cause neurological symptoms in about 2 % of patients. Among over 1400 patients with NF1 we saw symptomatic spinal tumours in 23 (1.6 %). MRI of the entire spinal canal was obtained in 54 patients aged 5-56 years with NF1. The number, site, morphology and signal characteristics of the spinal tumours were recorded and analysed. There were 24 patients with symptoms such as sensory impairment or paralysis; 30 patients had no neurological deficits. Of the 24 symptomatic patients, 23 (96 %) had spinal tumours, while we saw spinal tumours in 12 (40 %) of the 30 patients without neurological deficits. No spinal segment was preferred in symptomatic or asymptomatic patients. Most intraspinal extramedullary tumours were primarily extradural and intraforaminal. MRI showed intramedullary tumours in 3 patients (6 %), intraspinal extramedullary tumours in 18 (33 %) and intraforaminal tumours in 31 (57 %). Only neurological deficits in patients with NF1 should prompt further diagnostic clarification. In patients with neurological symptoms there may be a multiplicity of masses in the spinal canal, which can lead to difficulties in attaching symptoms to a certain tumour. In patients who do not satisfy the NIH criteria, it can be a helpful observation that spinal tumours in NF1 are primarily intraforaminal, extending into the spinal canal, while in NF2 they are mostly intraspinal intradural tumours.

Allelic loss of the NF1 gene in NF1-associated plexiform neurofibromas

Neurofibromatosis 1 (NF1) is an autosomal dominant disorder with a complex variety of clinical symptoms. Genetic alteration of the NF1 gene on 17q11.2 is the disease. Neurofibromas of the peripheral nervous system are one main manifestation. A variant of neurofibroma is the plexiform neurofibroma which can be found in about 30% of NF1-patients, often causing severe clinical symptoms. In this study, we examined 14 such tumors from 10 NF1-patients for allele loss of the NF1 gene (LOH: loss of heterozygosity) using four intragenic polymorphic markers. Loss of heterozygosity was found in eight tumors from five patients, and suspected in one additional tumor from another patient. This finding suggests that loss of the second allele, and thus inactivation of both alleles of the NF1 gene, is associated with the development of plexiform neurofibromas. The 14 plexiform neufibromas were also examined for mutation in the TP53 gene by screening exons 5 through 8 using temperature gradient gel electrophoresis. No mutation was found in any of the tumors.

Quantitative analysis of NF1 and OMGP gene transcripts in sporadic gliomas, sporadic meningiomas and neurofibromatosis type 1-associated plexiform neurofibromas

The close association of neurofibromatosis type 1 (NF1) with gliomas raises the question of whether the NF1 gene may be involved in the pathogenesis of sporadic astrocytic brain tumors. However, no frequent mutations within NF1 have been described in these tumors. Recent data on a limited series of gliomas indicate that NF1 expression may even be increased, thereby questioning the role of NF1 as a tumor suppressor in astrocytomas. In the present study, we examined the expression of NF1 in a series of 96 tumors including astrocytomas, meningiomas and plexiform neurofibromas. NF1 RNA transcription levels were compared to those of the reference genes B2M, ACTB and GAPD. The expression of OMGP, which is interposed in the NF1 gene, served as an additional control. NF1 expression did not significantly diverge among different malignancy stages of astrocytomas. As expected, the plexiform neurofibromas showed only very low NF1 expression. A striking finding was the highly variable expression of those genes selected to serve as references. While B2M and ACTB exhibited comparable levels of expression within different grades of astrocytomas and meningiomas, GAPD showed an inverse pattern in these tumors. In conclusion, NF1 expression is strongly reduced in NF1-associated plexiform neurofibromas but not in astrocytic tumors. The significant differences between B2M, ACTB and GAPD transcript levels brings into question the common practice of defining gene expression as a ratio between the transcripts of interest and those of these reference genes.

Loss of NF1 allele in Schwann cells but not in fibroblasts derived from an NF1-associated neurofibroma

Neurofibromas, the hallmark of neurofibromatosis 1, are composed mainly of Schwann cells and fibroblasts. Inactivation of both NF1 alleles is the cause of these benign tumors, but it is unknown which cell type is the progenitor. In this study, we selectively cultured Schwann cells from an NF1-associated neurofibroma. Fibroblasts were also obtained by culturing the tumor cells under standard conditions. Using four intragenic markers, we genotyped the NF1 locus in the original tumor and in the derived Schwann cells and fibroblasts. Loss of heterozygosity for two informative markers, which indicates loss of one NF1 allele, was found in Schwann cells but not in fibroblasts. This result suggests that genetic alterations of the NF1 gene in Schwann cells are responsible for the development of neurofibromas.

Mosaicism in sporadic neurofibromatosis 2 patients

More than half of neurofibromatosis 2 (NF2) patients represent de novo mutations which could have occurred at either pre-zygotic or post-zygotic stages. A post-zygotic mutation can result in mosaicism. In four sporadic NF2 patients, we found NF2 mutations in only a portion of corresponding leukocytes. In two other sporadic patients, no mutations were found in leukocytes but constitutional NF2 mutations were suggested by identical mutations in different tumors from each patient. We screened leukocyte DNA from a total of 16 inherited and 91 sporadic NF2 patients, and found NF2 mutations in 13 (81%) of the former and in 46 (51%) of the latter cases. The 30% difference in the rate of detection of mutations ( P = 0.051) might be partially explained by mosaicism in a portion of sporadic NF2 patients who carry the mutations in such a fashion that their leukocytes are unaffected. Among sporadic cases, we found mutations more frequently in patients with severe phenotypes (59%) than in patients with mild phenotypes (23%) (difference of 36%, P = 0.007). Mosaicism might be more common in the latter patient group since small populations of mutation-bearing cells can in some cases result in mild phenotypes and can also lead to difficulties in identifying mutations. No mutations were found in eight patients suspected of having NF2. Mosaicism with an extremely small population of affected cells may explain the incomplete phenotypes in some of these patients and the lack of mutations in their leukocytes. These findings suggest that mosaicism is relatively common in NF2 and may have important implications for diagnosis, prognosis and genetic counseling.