Familial schwannomatosis: exclusion of the NF2 locus as the germline event

BACKGROUND

Schwannomatosis is a recently recognized disorder, defined as multiple pathologically proven schwannomas without vestibular tumors diagnostic of neurofibromatosis 2 (NF2). Some investigators have questioned whether schwannomatosis is merely an attenuated form of NF2.

METHODS

The authors identified eight families in which a proband met their diagnostic criteria for schwannomatosis. Archived and prospectively acquired tumor specimens were studied by mutational analysis at the NF2 locus, loss of heterozygosity analysis along chromosome 22, and fluorescent in situ hybridization analysis of NF2 and the more centromeric probe BCR. Linkage analysis could be performed in six of eight families.

RESULTS

Clinical characterization of these kindreds showed that no affected family member harbored a vestibular tumor. Molecular analysis of 28 tumor specimens from 17 affected individuals in these kindreds revealed a pattern of somatic NF2 inactivation incompatible with our current understanding of NF2 as an inherited tumor suppressor gene syndrome. Linkage analysis excluded the NF2 locus in two kindreds, and showed a maximum lod score of 6.60 near the more centromeric marker D22S1174.

CONCLUSIONS

Schwannomatosis shows clinical and molecular differences from NF2 and should be considered a third major form of neurofibromatosis. Further work is needed to identify the inherited genetic element responsible for familial schwannomatosis.

Molecular study of frequency of mosaicism in neurofibromatosis 2 patients with bilateral vestibular schwannomas

Neurofibromatosis 2 (NF2) is a severe autosomal dominant disorder that predisposes to multiple tumours of the nervous system. About half of all patients are founders with clinically unaffected parents. The purpose of the present study was to examine the extent to which mosaicism is present in NF2 founders. A total of 233 NF2 founders with bilateral vestibular schwannomas (BVS) were screened by exon scanning. NF2 mutations were detected in the blood samples of 122 patients (52%). In 10 of the 122 cases, the ratio of mutant to normal alleles was obviously less than 1, suggesting mosaicism. Tumour specimens were available from 35 of the 111 subjects in whom no mutation could be detected in blood specimens. Mutational analysis by exon scanning detected typical NF2 mutations in 21 of the 35 tumours. In nine subjects, the alterations found in tumours could be confirmed to be the constitutional mutation based on finding of identical mutations in pathologically and/or anatomically distinct second tumours. In six other subjects with only a single tumour available, allelic loss of the NF2 gene was found in addition to the mutation in each tumour, suggesting that either the mutation or the deletion of the NF2 gene is probably the constitutional genetic alteration. Our results suggest that failure to find constitutional mutations in blood specimen from these 15 patients was not because of the limitation of the applied screening technique, but the lack of the mutations in their leucocytes, best explained by mosaicism. Extrapolating the rate (15/35 = 43%) of mosaicism in these 35 cases to the 111 NF2 founders with no constitutional NF2 mutations found in their blood, we inferred 48 mosaic subjects (111 x 0.429). Adding the 10 mosaic cases detected directly in blood specimens, we estimate the rate of mosaicism to be 24.8% (58/233) in our cohort of 233 NF2 founders with bilateral vestibular schwannomas.

The parental origin of new mutations in neurofibromatosis 2

Neurofibromatosis 2 (NF2) is an autosomal dominant disorder characterized by schwannomas and meningiomas that develop after inactivation of both copies of the NF2 gene. Approximately half of all patients with NF2 have unaffected parents and the disease results from new mutations at the NF2 locus. Loss of heterozygosity (LOH) in tumor specimens due to deletions covering the normal NF2 allele can be used to infer the haplotypes surrounding underlying mutations and determine the allelic origin of new mutations. We studied 71 sporadic NF2 patients using both LOH and pedigree analysis and compared the parental origin of the new mutation with the underlying molecular change. In the 45 informative individuals, 31 mutations (69%) were of paternal and 14 (31%) were of maternal origin (P=0.016). Comparison with corresponding constitutional mutations revealed no correlation between parental origin and the type or location of the mutations. However, in 4 of 6 patients with somatic mosaicism the NF2 mutation was of maternal origin. A slight parent of origin effect on severity of disease was found. Further clinical and molecular studies are needed to determine the basis of these unexpected observations.

Schwann cell-onion bulb tumor of the trigeminal nerve: hyperplasia, dysplasia or neoplasia?

Onion bulbs are concentric lamellar structures formed by Schwann or perineurial cells, which may be seen in several generalized or localized diseases of the peripheral nerve. There is debate regarding the pathogenesis of localized tumefactions displaying these microscopic structures. We report the fifth case, to our knowledge, of a Schwann cell-onion bulb tumor, which arose in the trigeminal nerve of a child with an unclassifiable, probably distinct, neurocutaneous syndrome; we also provide evidence for a neoplastic or hamartomatous origin. Molecular studies failed to establish an abnormality in the NF1, NF2, PMP22, or Connexin 32 genes. Similar and previously reported cases are discussed, as well as other onion bulb-forming entities.

Clonal analysis of a case of multiple meningiomas using multiple molecular genetic approaches: pathology case report

OBJECTIVE

Multiple meningiomas are uncommon brain tumors occurring concurrently in several intracranial locations in the same patient. In the present study, we determined the clonality, methylation status of deoxyribonucleic acid, and relationship of genetic alterations in eight meningiomas from one female patient.

METHODS

Six molecular genetic techniques, including two methylation-based clonality assays and one transcription-based clonality assay, methylation analysis of CpG islands by methylation-specific polymerase chain reaction, loss of heterozygosity, microsatellite instability, and mutational analysis of the NF2 gene on chromosome 22, were used in comparative investigations on clonality and genetic alterations.

RESULTS

The presence of clonal tumor cells was demonstrated by 1) loss of the same copy of chromosome 22 in all eight tumors; 2) transcription of the human AR gene from the same allele in six of eight tumors; 3) a common unmethylated allele at the AR locus in all eight tumors; and 4) the identical single-basepair insertion mutation in exon 9 of the NF2 gene in six of eight tumors. In addition, loss of a copy of the X chromosome in one tumor nodule and microsatellite instability in another nodule were observed.

CONCLUSION

Taken together, this case of multiple meningiomas was most likely monoclonal in origin. Loss of chromosome 22 was an early event during the development of multiple meningiomas and was followed by mutations at the NF2 locus. Later events, including loss of the X chromosome, variation of AR gene expression, or microsatellite instability, may also have played a role in the development of multiple meningiomas in this patient.

Allelic expression of the NF2 gene in neurofibromatosis 2 and schwannomatosis

Neurofibromatosis type 2 (NF2) is a genetic disorder characterized by formation of multiple schwannomas and meningiomas due to inactivating mutations in the NF2 tumor suppressor gene on chromosome 22. We describe a polymorphism in the 3' untranslated region of the NF2 gene that is informative in about one-third of individuals. This polymorphism permitted an assessment of the relative expression of NF2 transcripts in lymphoblastoid cell RNA from 22 unrelated NF2 patients heterozygous for a germline NF2 mutation, along with 6 schwannomatosis patients, and 14 unaffected controls. Unequal allelic expression (1.8- to 20-fold) was detected in 15 of the NF2 cases, but in none of the schwannomatosis or control individuals. Underexpression of the NF2 mutant allele was documented for all 6 nonsense or frameshift mutations, 3 of 6 splice mutations, and 1 of 4 missense mutations, which, unexpectedly, was shown to alter the NF2 transcript and create a premature stop codon. In contrast, equal expression or slight overexpression of NF2 mutant alleles was observed for 2 in-frame deletions, 2 splice alterations, and 3 missense mutations. In the remaining 5 cases, the allele representing the mutant transcript was not known. Thus, truncating NF2 mutations, which are the most frequent alterations in NF2 patients and NF2-associated tumors, were associated with underexpression of the mutant allele, whereas the less common in-frame alterations usually showed normal or slight overexpression of the mutant transcript.

Merlin: the neurofibromatosis 2 tumor suppressor

In recent years, it has become clear that the ERMs occupy a crucial position as protein linkers that both respond to and participate in reorganization of membrane-cytoskeletal interactions. With the identification of new binding partners, the ERMs are also implicated in linked regulation of the activities of particular membrane proteins. Thus, they reside at a junction in a complex web of interactions that must respond to stimuli from both outside and inside the cell. As expected from its structural motifs, merlin behaves in a manner similar to the ERM proteins, but with some notable differences. Chief among these is the absence of intramolecular interaction to mask intermolecular interaction domains in isoform 2. The full range of merlin's intermolecular interactions remains to be delineated, but it can be expected from the comparison to ERMs that merlin also sits within a web of interactions that may involve multiple partners and signaling pathways, some of which it shares with the ERMs. Defining merlin's tumor suppressor function will likely require identifying those differences that are peculiarly important in the target cell types of NF2. However, the fact that inactivation of merlin in the mouse by targeted mutagenesis produces a variety of malignant tumors with a high rate of metastasis [33] suggests that merlin's suppression of tumor formation may involve different partners and pathways in different cell types and genetic backgrounds. Consequently, the disruptions due to merlin inactivation in the progression of malignant mesothelioma may represent a tumor suppressor role operating by a different pathway than that in schwannoma or meningioma.

Loss of the NF2 gene and merlin occur by the tumorlet stage of schwannoma development in neurofibromatosis 2

Loss of the neurofibromatosis 2 (NF2) gene-encoded protein merlin is a universal finding in sporadic and NF2-associated schwannomas. Certain NF2 patients may develop numerous minute Schwann cell tumorlets of the spinal nerve roots in addition to larger, frank schwannomas and thereby provide an opportunity to investigate the timing of NF2 gene/merlin loss in Schwann cell tumorigenesis. We studied an NF2 patient with a germline NF2 gene frameshift mutation who had many Schwann cell tumorlets and schwannomas. Loss of heterozygosity studies of DNA from microdissected specimens showed allelic loss of the NF2 region of chromosome 22q in tumorlets as well as schwannomas. Immunohistochemistry further demonstrated loss of merlin expression in tumorlets as well as schwannomas, with intact expression in adjacent nerve. Thus, loss of both NF2 alleles and merlin occur early in Schwann cell tumorigenesis, before the tumorlet stage. The study of tumorlets and schwannomas in such patients may also provide an opportunity to elucidate mechanisms responsible for the subsequent growth of Schwann cell lesions into symptomatic tumors.

Molecular analysis of the NF2 tumor-suppressor gene in schwannomatosis

Patients with multiple schwannomas without vestibular schwannomas have been postulated to compose a distinct subclass of neurofibromatosis (NF), termed "schwannomatosis." To compare the molecular-genetic basis of schwannomatosis with NF2, we examined the NF2 locus in 20 unrelated schwannomatosis patients and their affected relatives. Tumors from these patients frequently harbored typical truncating mutations of the NF2 gene and loss of heterozygosity of the surrounding region of chromosome 22. Surprisingly, unlike patients with NF2, no heterozygous NF2-gene changes were seen in normal tissues. Examination of multiple tumors from the same patient revealed that some schwannomatosis patients are somatic mosaics for NF2-gene changes. By contrast, other individuals, particularly those with a positive family history, appear to have an inherited predisposition to formation of tumors that carry somatic alterations of the NF2 gene. Further work is needed to define the pathogenetics of this unusual disease mechanism.

Universal absence of merlin, but not other ERM family members, in schwannomas

NF2 (neurofibromatosis 2, encoding the merlin protein) gene mutations and chromosome 22q loss have been demonstrated in the majority of sporadic and NF2-associated schwannomas, but many schwannomas fail to demonstrate genetic evidence of biallelic NF2 gene inactivation. In addition, the role of the merlin-related ERM family members (ezrin, radixin, and moesin) remains unclear in these tumors. We therefore studied expression of NF2-encoded merlin as well as ezrin, radixin, and moesin in 22 vestibular and peripheral schwannomas that had been evaluated for NF2 mutations and chromosome 22q loss. Western blotting and immunohistochemistry with antibodies directed against the amino and carboxy termini of merlin demonstrated loss of merlin expression in all studied schwannomas, including 12 tumors lacking genetic evidence of biallelic NF2 gene inactivation. Western blotting with antibodies directed against ezrin, radixin, and moesin, however, showed expression of these proteins in all schwannomas. In addition, immunohistochemistry with an antibody to moesin revealed widespread expression in tumor and endothelial cells. These data indicate that the specific loss of merlin is universal to schwannomas and is not linked to loss of ezrin, radixin, or moesin expression.

CD44 expression is aberrant in benign Schwann cell tumors possessing mutations in the neurofibromatosis type 2, but not type 1, gene

Atypical expression of CD44 splice variants has been implicated in the progression of numerous tumors. This abnormal CD44 expression is presumed to result from gene alterations that cause tumorigenic transformation. Two tumor types that have been linked to specific gene alterations are schwannomas, which have mutations in the neurofibromatosis (NF) type 2 (NF2) gene, and neurofibromas, which characteristically possess NF type 1 (NF1) gene mutations. We examined CD44 expression in normal sciatic nerves, in schwannomas with confirmed NF2 mutations, and in neurofibromas and malignant peripheral nerve sheath tumor tissue and cell lines from NF1 patients. Compared to normal nerves, schwannomas express higher total levels of CD44 and additional splice variants, whereas CD44 expression in neurofibromas is unaltered. Malignant peripheral nerve sheath tumor tissue and cell lines express the CD44v6 epitope, which is not expressed by normal Schwann cells or by other Schwann cell tumors. These data indicate that altered CD44 expression correlates strictly with mutations in the NF2 but not NF1 gene and suggest that CD44v6 might be a marker for the malignant transformation of Schwann cells.

Frequency and distribution of NF2 mutations in schwannomas

Sporadic and inherited schwannomas were scanned for the nature, frequency, and distribution of mutations in the NF2 locus encoding the merlin tumor suppressor protein on 22q. Of 58 tumors, 47% displayed loss of heterozygosity for NF2, leaving a total of 89 NF2 alleles to be examined. Pathogenic alterations were identified in 62 of these alleles, including 36 frameshifts with premature termination, 14 nonsense mutations, and 12 changes presumed to affect splicing. Effects of ten of the latter were confirmed in the NF2 transcript and indicated that activation of cryptic splice sites in coding sequence is another frequent mechanism leading to truncation of merlin. The mutations were relatively evenly distributed across both the protein 4.1 superfamily (exons 1-9) and the alpha-helical (exons 10-15) domains of merlin, but they did not occur at all in exons 16 and 17, which encode the protein's alternative COOH-termini. The data support the "two-hit" tumor suppressor model for formation of schwannomas and indicate that loss of merlin function can be achieved by truncation at various locations in the protein. However, the absence of mutations in exons 16 and 17 suggests that an inactivating mutation affecting only one of the merlin's alternative termini may not be sufficient to eliminate tumor suppressor function.

Neurofibromatosis 2: loss of merlin's protective spell

Schwannomas and meningiomas occur as multiple tumors in sufferers of neurofibromatosis 2 (NF2) and as solitary tumors in the general population due to the inactivation of a gene at chromosome 22q12. In 1993, a location cloning approach revealed this tumor suppressor, dubbed merlin, as a novel member of a family of proteins that link elements of the cytoskeleton and the cell membrane. Subsequent investigations have confirmed merlin's role in tumor formation, but have yet to reveal its mechanism of action.

NF2 gene analysis distinguishes hemangiopericytoma from meningioma

The histogenesis of dural-based or "central" hemangiopericytomas (cHPCs) remains controversial. Some authors consider these tumors variants of meningiomas while others consider them akin to peripheral hemangiopericytomas (pHPCs). Meningiomas frequently have mutations in the neurofibromatosis 2 (NF2) gene, providing a molecular marker for meningiomas and other NF2-related tumors. We therefore analyzed the NF2 gene in cHPCs, pHPCs, and meningiomas to determine whether cHPCs are more similar at the molecular genetic level to meningiomas or pHPCs. Using paraffin-embedded archival material from 28 cHPCs (including three primary and recurrent tumors), 10 pHPCs, and 26 meningiomas, we scanned all 17 exons of the NF2 gene and flanking intronic sequences for mutations with single strand conformation polymorphism analysis and DNA sequencing. No NF2 mutations were found in either cHPCs or pHPCs, whereas 35% of meningiomas had NF2 gene alterations (P < 0.001). The NF2 gene mutations in meningiomas were all truncating mutations, consistent with previous studies. Our findings suggest that cHPCs are distinct from meningiomas at the molecular genetic level and support prior clinico-pathological data that distinguish these tumor-entities.

Mutational analysis of patients with neurofibromatosis 2

Neurofibromatosis 2 (NF2) is a genetic disorder characterized by the development of multiple nervous-system tumors in young adulthood. The NF2 gene has recently been isolated and found to encode a new member of the protein 4.1 family of cytoskeletal associated proteins, which we have named merlin. To define the molecular basis of NF2 in affected individuals, we have used SSCP analysis to scan the exons of the NF2 gene from 33 unrelated patients with NF2. Twenty unique SSCP variants were seen in 21 patients; 10 of these individuals were known to be the only affected person in their kindred, while 7 had at least one other known affected relative. In all cases in which family members were available, the SSCP variant segregated with the disease; comparison of sporadic cases with their parents confirmed the de novo variants. DNA sequence analysis revealed that 19 of the 20 variants observed are predicted to lead to a truncated protein due to frameshift, creation of a stop codon, or interference with normal RNA splicing. A single patient carried a 3-bp deletion removing a phenylalanine residue. We conclude that the majority of NF2 patients carry an inactivating mutation of the NF2 gene and that neutral polymorphism in the gene is rare.

Exon scanning for mutation of the NF2 gene in schwannomas

Family studies and tumor analyses have combined to indicate that neurofibromatosis 2 (NF2), a disorder characterized by multiple benign tumors of the nervous system, and sporadic non-inherited forms of the same tumor types are both caused by inactivation of a tumor suppressor gene located in 22q12. Recently, the gene encoding merlin, a novel member of a family of cytoskeleton-associated proteins, was identified as the NF2 tumor suppressor. To facilitate the search for merlin mutations, we have defined the exon-intron boundaries for all 17 NF2 exons, including one subject to alternative splicing. We have developed polymerase chain reaction assays to amplify each exon from genomic DNA, and used these assays to perform single-strand conformation polymorphism analysis of DNA from 30 sporadic and eight NF2-derived schwannomas, the hallmark tumor type in this disorder. Of a maximum of 60 alleles scanned, 32 showed mutations affecting expression of the merlin protein. Thirty of these mutations are predicted to lead to a truncated protein due to frameshift, creation of a stop codon, or interference with normal splicing, while two are missense mutations. Thus, inactivation of merlin is a common feature underlying both inherited and sporadic forms of schwannoma.

Analysis of the neurofibromatosis 2 gene in human ependymomas and astrocytomas

Ependymomas and astrocytomas commonly have allelic losses of chromosome 22q, which suggests the presence of a glioma tumor suppressor gene on 22q. A candidate tumor suppressor gene on 22q is the neurofibromatosis 2 (NF2) gene since NF2 patients have an increased susceptibility to ependymomas and astrocytomas. Using single strand conformation polymorphism analysis and direct DNA sequencing, we screened 8 ependymomas and 30 fibrillary astrocytomas from non-NF2 patients for mutations in the coding sequence and portions of the 3' untranslated region of the NF2 gene. Only one mutation was detected, a single base deletion in NF2 exon 7 from a spinal ependymoma, which had also lost the wild-type allele. These results suggest that the NF2 gene may be important in the formation of some ependymomas but the NF2 gene is probably not the critical chromosome 22q tumor suppressor gene involved in astrocytoma tumorigenesis.

Neurofibromatosis type 1 gene mutations in neuroblastoma

The introduction of human chromosome 17 suppresses the tumourigenicity of a neuroblastoma cell line in the absence of any effects on in vitro growth and the neurofibromatosis type 1 (NF1) gene may be responsible. Here we report that 4 out of 10 human neuroblastoma lines express little or no neurofibromin and that two of these lines show evidence of NF1 mutations, providing further proof that NF1 mutations occur in tumours that are not commonly found in NF1 patients. We also show that NF1 deficient neuroblastomas show only moderately elevated ras-GTP levels, in contrast to NF1 tumour cells, indicating that neurofibromin contributes differently to the negative regulation of ras in different cell types.

Clonal analysis of human meningiomas and schwannomas

Meningiomas and schwannomas are two of the most common tumors of the human nervous system. To determine whether these tumors arise from a single cell or from multiple cells, we used molecular genetic techniques to study X chromosome inactivation in meningiomas and schwannomas isolated from females including one who had neurofibromatosis type 2. The tumors were also screened for loss of heterozygosity at several loci on chromosome 22 using polymorphic DNA markers. Among nine meningiomas, at least three of which showed loss of alleles on chromosome 22 and five of which retained heterozygosity for the chromosome 22 alleles examined, all nine tumors were monoclonal. Among eight schwannomas, at least seven of which retained heterozygosity for chromosome 22 loci, seven were monoclonal. We conclude that human meningiomas and schwannomas arise from a single cell.

Clonal origin of pituitary adenomas

Benign pituitary adenomas are among the most common neurosurgical tumors and account for a diversity of clinical syndromes due to their hormone content and release. To determine whether these tumors arise from a single cell or multiple cells, the authors studied X chromosome inactivation in deoxyribonucleic acid (DNA) isolated from pituitary adenomas in women. Tumors of three different hormonal subtypes were examined. One tumor contained cells immunoreactive for prolactin and human growth hormone; one tumor contained foci immunoreactive for the beta-subunits of luteinizing hormone and follicle-stimulating hormone; and the third tumor had no immunoreactive prolactin, human growth hormone, beta-subunits of thyroid-stimulating hormone, luteinizing hormone, or follicle-stimulating hormone, or the alpha-subunit. Analysis of the DNA revealed that, in each of the three pituitary tumors, one X chromosome was active in all cells and one X chromosome was inactive, indicating that each of these tumors was monoclonal in origin. It is concluded that clinically evident pituitary tumors arise from a genetic mutation in a single cell.

Relative argininosuccinate synthetase mRNA levels and gene copy number in canavanine-resistant lymphoblasts

Mutants resistant to the arginine analogue, canavanine, have been isolated from two normal lymphoblast lines, MGL8B2 and MGL33. These mutants constitutively express up to 200-fold higher amounts of structurally normal argininosuccinate synthetase, the urea cycle enzyme that converts citrulline to argininosuccinate. Relative levels of argininosuccinate synthetase mRNA were compared among normal and canavanine-resistant lines using in vitro translation of poly(adenylic acid) RNA and blot hybridization of total cytoplasmic RNA to an argininosuccinate synthetase cDNA. Both of these approaches indicated that the canavanine-resistant lines contain increased steady-state levels of synthetase-specific mRNA relative to their sensitive parents and that these were roughly correlated with levels of enzyme activity. Blot hybridization of EcoRI-digested genomic DNA preparations revealed no detectable differences in argininosuccinate synthetase structural gene copy number between normal and canavanine-resistant lymphoblasts, demonstrating that the canavanine-resistant phenotype is not caused by gene amplification.

Variation in argininosuccinate synthetase activity in amniotic fluid cell cultures: implications for prenatal diagnosis of citrullinemia

Argininosuccinate synthetase activity was measured in amniotic fluid cells cultured from 25 normal fetuses and from 2 fetuses at risk for citrullinemia. Among the 25 control lines, argininosuccinate synthetase was low in epithelial-like cultures and high in more fibroblast-like cultures. The radioactivity incorporated from 14C-citrulline into protein was proportional to argininosuccinate synthetase activity. The activity of argininosuccinate lyase, the next enzyme in the urea cycle, was unaffected by a predominance of one or the other cell type. Argininosuccinate synthetase was low but detectable in cells from the two fetuses at risk for citrullinemia. The pregnancies were continued and both resulted in clinically normal females. Thus factors such as predominant cell type may affect area cycle enzyme activity in cultured amniotic fluid cells and should be taken into consideration in attempts to diagnose citrullinemia prenatally.

Expression of the human argininosuccinate synthetase gene in hamster transferents

The structural gene for human argininosuccinate synthetase [L-citrulline:L-aspartate ligase (AMP-forming), EC 6.3.4.5] was transferred to argininosuccinate synthetase-deficient Chinese hamster cells via metaphase chromosomes isolated from human lymphoblast line MGL8D1, a constitutive overproducer of argininosuccinate synthetase, and from its repressible parent, MGL8B2. Argininosuccinate synthetase expression was selected for in citrulline-containing medium, and the human origin of the argininosuccinate synthetase expressed by seven transferents was identified by isoelectric focusing. Stable transferents expressing MGL8D1 argininosuccinate synthetase fell into two classes: (i) those whose argininosuccinate synthetase activity was reduced to 10-50% by arginine, similar to the repression of argininosuccinate synthetase synthesis observed in normal human lymphoblasts, and (ii) those that constitutively expressed argininosuccinate synthetase when grown in the presence of arginine or citrulline. Two transferents from the MGL8B2 donor constitutively expressed human arginonosuccinate synthetase. Three hamster revertants were isolated that constitutively expressed hamster argininosuccinate synthetase. Transferents and revertants exhibited growth-dependent changes in argininosuccinate synthetase activity, in contrast to the constant synthetase activity levels in donor lymphoblasts during growth. The isolation of stable transferents that constitutively or repressibly express argininosuccinate synthetase makes possible the analysis of regulatory signals influencing expression of the argininosuccinate synthetase gene.

Canavanine-resistant variants of human lymphoblasts

Variants resistant to canavanine, an arginine analogue, have been isolated from two long-term human lymphoblastoid cell lines. They are 20-fold more resistant to canavanine than the parental lines and this phenotype is stable in the absence of canavanine for more than 100 generations. The specific activity of argininosuccinate synthetase, the first of two enzymes necessary for the conversion of citrulline to arginine, is elevated in variants from both cell lines. Furthermore, this enzyme activity is refractory to the repression caused by arginine in normal lymphoblasts. The specific activity of argininosuccinate lyase, the second enzyme in the pathway from citrulline to arginine, is not appreciably changed. Arginine uptake appears normal in the variants since they grow as well as the parental lines in media containing a wide range of arginine concentrations. Arginyl-tRNA synthetase activity is also unchanged. Thus the canavanine-resistant variants have altered control of at least one urea cycle enzyme and appear to be regulatory mutants of human cells.

Control of argininosuccinate synthetase by arginine in human lymphoblasts

Human lymphoblasts in long-term culture have the enzyme activities necessary to convert citrulline to arginine: argininosuccinate synthetase and argininosuccinate lyase. Upon transfer from arginine-supplemented to citrulline-supplemented medium, lymphoblasts exhibit a lag period before resuming exponential growth. During this lag the specific activity of argininosuccinate synthetase increases an average of 60-fold. Argininosuccinate lyase activity remains unchanged. If normal lymphoblasts are starved in arginine-deficient medium without citrulline or if argininosuccinate lyase--deficient lymphoblasts are transferred to citrulline-containing medium, argininosuccinate synthetase activity increases linearly for several days and reaches even higher levels. Cycloheximide blocks the increase in enzyme activity. Cells grown in citrulline medium and pulse labeled with 35S-methionine incorporate more 35S-methionine into argininosuccinate synthetase protein than cells grown in arginine; the rate of disappearance of radioactively labeled enzyme is the same in citrulline- and arginine-grown cells. Arginine or a closely related metabolite thus appears to repress the synthesis of argininosuccinate synthetase of human lymphoblasts in culture.