Establishment of an Efficient Immortalization Strategy Using HMEJ-Based bTERT Insertion for Bovine Cells.

Immortalized cell lines have been used in a wide range of applications in research on immune disorders and cellular metabolic regulation due to the stability and uniformity of their cellular characteristics. At present, the investigation into molecular functions and signaling pathways within bovine cells remains largely limited by the lack of immortalized model cells. Current methods for immortalizing bovine cells are mainly restricted to the ectopic expression of human telomerase reverse transcriptase (hTERT) through transient transfection or virus-mediated delivery, which have defects in efficiency and reliability. In this study, we identified bovine TERT (bTERT) as a novel potent biofactor for immortalizing bovine cells with great advantages over hTERT, and established an efficient and easily manipulated strategy for the immortalization of bovine primary cells. Through the homology-mediated end-joining-based insertion of bTERT at the ROSA26 locus, we successfully generated immortalized bovine fetal fibroblast cell lines with stable characteristics. The observed limitation of this strategy in immortalizing bovine bone marrow-derived macrophages was attributed to the post-translational modification of bTERT, causing inhibited nuclear localization and depressed activity of bTERT in this terminally differentiated cell. In summary, we constructed an innovative method to achieve the high-quality immortalization of bovine primary cells, thereby expanding the prospects for the future application of immortalized bovine model cell lines.

DAP kinase activates a p19ARF/p53-mediated apoptotic checkpoint to suppress oncogenic transformation.

DAP kinase is a pro-apoptotic calcium-regulated serine/threonine kinase, whose expression is frequently lost in human tumours. Here we show that DAP kinase counteracts oncogene-induced transformation by activating a p19ARF/p53-dependent apoptotic checkpoint. Ectopic expression of DAP kinase suppressed oncogenic transformation of primary embryonic fibroblasts by activating p53 in a p19ARF-dependent manner. Consequently, the fibroblasts underwent apoptosis, characterized by caspase activation and DNA fragmentation. In response to c-Myc or E2F-1, the endogenous DAP kinase protein was upregulated. Furthermore, functional or genetic inactivation of the endogenous DAP kinase reduced the extent of induction of p19ARF/p53 and weakened the subsequent apoptotic responses to c-Myc or E2F-1. These results establish a role for DAP kinase in an early apoptotic checkpoint designed to eliminate pre-malignant cells during cancer development.

TP53 mutations coincide with the ectopic expression of activation-induced cytidine deaminase in the fibroblast-like synoviocytes derived from a fraction of patients with rheumatoid arthritis.

Main features of rheumatoid arthritis (RA), hyperplasia of fibroblast-like synoviocytes (FLS) and joint destruction are caused by inflammatory cytokines produced in chronic autoimmune inflammation. Cell-intrinsic acquisition of tumour-like phenotypes of RA-FLS could also be responsible for the aggressive proliferation and invasion, which are supported by the fact that in some cases RA-FLS has mutations of a tumour suppressor gene TP53. However, the underlying molecular mechanism for TP53 mutations in RA-FLS has not yet been clarified. Recently it has been reported that the non-lymphoid cells in the inflammatory tissues express ectopically the activation-induced cytidine deaminase (AID) gene that induces somatic hypermutations, not only at the immunoglobulin (Ig) gene variable regions in germinal centre B lymphocytes but also at coding regions in TP53. Real-time polymerase chain reaction (PCR) analyses revealed more than half (five of nine) of the RA-FLS lines we established showed the markedly increased expression of AID. AID transcription in RA-FLS was augmented by tumour necrosis factor (TNF)-alpha and even by physiological concentration of beta-oestradiol that could not induce AID transcription in osteoarthritis-FLS. Furthermore, AID-positive RA-FLS presented a higher frequency of somatic mutations in TP53. Cytological and immunohistochemical analyses demonstrated clearly the ectopic expression of AID in the FLS at the RA synovium. These data suggested strongly a novel consequence of RA; the ectopic expression of AID in RA-FLS causes the somatic mutations and dysfunction of TP53, leading to acquisition of tumour-like properties by RA-FLS.

A postprophase topoisomerase II-dependent chromatid core separation step in the formation of metaphase chromosomes.

Metaphase chromatids are believed to consist of loops of chromatin anchored to a central scaffold, of which a major component is the decatenatory enzyme DNA topoisomerase II. Silver impregnation selectively stains an axial element of metaphase and anaphase chromatids; but we find that in earlier stages of mitosis, silver staining reveals an initially single, folded midline structure, which separates at prometaphase to form two chromatid axes. Inhibition of topoisomerase II prevents this separation, and also prevents the contraction of chromatids that occurs when metaphase is arrested. Immunolocalization of topoisomerase II alpha reveals chromatid cores analogous to those seen with silver staining. We conclude that the chromatid cores in early mitosis form a single structure, constrained by DNA catenations, which must separate before metaphase chromatids can be resolved.

Specific association of human telomerase activity with immortal cells and cancer.

Synthesis of DNA at chromosome ends by telomerase may be necessary for indefinite proliferation of human cells. A highly sensitive assay for measuring telomerase activity was developed. In cultured cells representing 18 different human tissues, 98 of 100 immortal and none of 22 mortal populations were positive for telomerase. Similarly, 90 of 101 biopsies representing 12 human tumor types and none of 50 normal somatic tissues were positive. Normal ovaries and testes were positive, but benign tumors such as fibroids were negative. Thus, telomerase appears to be stringently repressed in normal human somatic tissues but reactivated in cancer, where immortal cells are likely required to maintain tumor growth.

An immortalized goat mammary epithelial cell line induced with human telomerase reverse transcriptase (hTERT) gene transfer.

Although mammary epithelial cell lines can provide a rapid and reliable indicator of gene expression efficiency of transgenic animals, their short lifespan greatly limits this application. To provide stable and long lifespan cells, goat mammary epithelial cells (GMECs) were transduced with pLNCX2-hTERT by retrovirus-mediated gene transfer. Transduced GMECs were evaluated by reverse transcriptase polymerase chain reaction (RT-PCR), proliferation assays, karyotype analysis, telomerase activity assay, western blotting, soft agar assay, and injection into nude mice. Non-transduced GMECs were used as a control. The hTERT-GMECs had higher telomerase activity and extended proliferative lifespan compared to non-transfected GMECs; even after Passage 50, hTERT-GMECs had a near diploid complement of chromosomes. Furthermore, they did not gain the anchorage-independent growth property and were not associated with a malignant phenotype in vitro or in vivo.

Transformation of human and murine fibroblasts without viral oncoproteins.

Murine embryo fibroblasts are readily transformed by the introduction of specific combinations of oncogenes; however, the expression of those same oncogenes in human cells fails to convert such cells to tumorigenicity. Using normal human and murine embryonic fibroblasts, we show that the transformation of human cells requires several additional alterations beyond those required to transform comparable murine cells. The introduction of the c-Myc and H-RAS oncogenes in the setting of loss of p53 function efficiently transforms murine embryo fibroblasts but fails to transform human cells constitutively expressing hTERT, the catalytic subunit of telomerase. In contrast, transformation of multiple strains of human fibroblasts requires the constitutive expression of c-Myc, H-RAS, and hTERT, together with loss of function of the p53, RB, and PTEN tumor suppressor genes. These manipulations permit the development of transformed human fibroblasts with genetic alterations similar to those found associated with human cancers and define specific differences in the susceptibility of human and murine fibroblasts to experimental transformation.

Different target range and cytotoxic specificity of adaphostin and 17-allylamino-17-demethoxygeldanamycin in imatinib-resistant and sensitive cell lines.

Imatinib mesylate is a selective inhibitor of the oncogenic tyrosine kinase, Bcr-Abl, and is widely used as a first-line treatment for chronic myeloid leukaemia (CML). Prolonged monotherapy is frequently associated with patients becoming refractory to imatinib. Therefore, there is considerable interest in small molecule inhibitors which may be used either as replacements or as adjuncts to existing imatinib therapy. For this purpose, it is most likely that drugs which do not share imatinib's mechanism of action will be most valuable. We compared two such compounds with different modes of action, adaphostin and 17-allylamino-17-demethoxygeldanamycin (17-AAG), for their cytotoxic effect and ability to induce the downregulation of cellular proteins in a murine haemopoietic cell line transformed with human p210(Bcr-Abl), and two subclones resistant to imatinib owing to an Abl-kinase domain mutation (E255K) or amplification of the BCR-ABL gene, respectively. We found that, whereas 17-AAG selectively killed Bcr-Abl-positive cells and inhibited proteins dependent on heat-shock protein 90 for their stability (p210(Bcr-Abl) and Akt), adaphostin induced the downregulation of multiple cell-signalling proteins (p210(Bcr-Abl), Akt, Bcr, Abl and STAT5a) and was cytotoxic to both Bcr-Abl-positive and -negative cells. We suggest that both compounds may prove useful in the treatment of CML but caution that undesirable side-effects may result from the inhibition of multiple cell signalling proteins.

Regulation of insulin receptor signaling by the protein tyrosine phosphatase TCPTP.

The human protein tyrosine phosphatase TCPTP exists as two forms: an endoplasmic reticulum-targeted 48-kDa form (TC48) and a nuclear 45-kDa form (TC45). Although targeted to the nucleus, TC45 can exit in response to specific stimuli to dephosphorylate cytoplasmic substrates. In this study, we investigated the downregulation of insulin receptor (IR) signaling by TCPTP. In response to insulin stimulation, the TC48-D182A and TC45-D182A "substrate-trapping" mutants formed stable complexes with the endogenous tyrosine-phosphorylated IR beta-subunit in 293 cells. Moreover, in response to insulin stimulation, the TC45-D182A mutant accumulated in the cytoplasm of cells overexpressing the IR and in part colocalized with the IR beta-subunit at the cell periphery. These results indicate that the IR may serve as a cellular substrate for both TC48 and TC45. In immortalized TCPTP(-/-) murine embryo fibroblasts, insulin-induced IR beta-subunit tyrosine phosphorylation and protein kinase PKB/Akt activation were enhanced relative to the values in TCPTP(+/+) cells. Importantly, the expression of TC45 or TC48 to physiological levels suppressed the enhanced insulin-induced signaling in TCPTP(-/-) cells. These results indicate that the differentially localized variants of TCPTP may dephosphorylate the IR and downregulate insulin-induced signaling in vivo.

RanBPM associates with CD39 and modulates ecto-nucleotidase activity.

CD39/ecto-NTPDase 1 (nucleoside triphosphate diphosphohydrolase 1) is an ecto-nucleotidase that influences P2 receptor activation to regulate vascular and immune cell adhesion and signalling events pivotal in inflammation. Whether CD39 interacts with other membrane or cytoplasmic proteins has not been established to date. Using the yeast two-hybrid system, we note that the N-terminus of CD39 binds to RanBPM (Ran binding protein M; also known as RanBP9), a multi-adaptor scaffolding membrane protein originally characterized as a binding protein for the small GTPase Ran. We confirm formation of complexes between CD39 and RanBPM in transfected mammalian cells by co-immunoprecipitation studies. Endogenous CD39 and RanBPM are also found to be co-expressed and abundant in cell membranes of B-lymphocytes. NTPDase activity of recombinant CD39, but not of N-terminus-deleted-CD39 mutant, is substantially diminished by RanBPM co-expression in COS-7 cells. The conserved SPRY [repeats in splA and RyR (ryanodine receptor)] moiety of RanBPM is insufficient alone for complete physical and functional interactions with CD39. We conclude that CD39 associations with RanBPM have the potential to regulate NTPDase catalytic activity. This intermolecular interaction may have important implications for the regulation of extracellular nucleotide-mediated signalling.

Up-regulation of lysyl oxidase in spontaneous revertants of H-ras-transformed rat fibroblasts.

Neoplastic transformation mediated by ras oncogenes is associated with down-regulation of gene expression. We have constructed a subtracted complementary DNA library from preneoplastic rat 208F fibroblasts by hybridizing with mRNA from a ras-transformed subclone. One of the complementary DNA clones identified by this approach encodes the 3' end of lysyl oxidase, the homologue of the mouse ras recision gene. Expression of lysyl oxidase was almost completely down-regulated in two clones of H-ras-transformed 208F cells (FE-8 and FE-56). We isolated a set of spontaneous phenotypic revertants of FE-8 cells (designated FSR) by cloning at limiting dilution. FSR revertant clones expressed high levels of lysyl oxidase and H-ras mRNA but grew only poorly in semisolid agar medium as opposed to anchorage-independent parental FE-8 cells. We obtained subclones of FSR cells which displayed again the transformed morphology of FE-8 cells but required anchorage for growth and continued to express high levels of lysyl oxidase mRNA. Thus, expression of lysyl oxidase correlated with the suppression of anchorage-independent growth rather than with flat morphology. Lysyl oxidase might be a useful marker to distinguish between different aspects of reversion and transformation.

The different biological effects of telomestatin and TMPyP4 can be attributed to their selectivity for interaction with intramolecular or intermolecular G-quadruplex structures.

Demonstration of the existence of G-quadruplex structures in telomeres of Stylonychia macronuclei and in the promoter of c-myc in human cells has validated these secondary DNA structures as potential targets for drug design. The next important issue is the selectivity of G-quadruplex-interactive agents for the different types of G-quadruplex structures. In this study, we have taken an important step in associating specific biological effects of these drugs with selective interaction with either intermolecular or intramolecular G-quadruplex structures formed in telomeres. Telomestatin is a natural product isolated from Streptomyces anulatus 3533-SV4 and has been shown to be a very potent telomerase inhibitor through its G-quadruplex interaction. We have demonstrated that telomestatin interacts preferentially with intramolecular versus intermolecular G-quadruplex structures and also has a 70-fold selectivity for intramolecular G-quadruplex structures over duplex DNA. Telomestatin is able to stabilize G-quadruplex structures that are formed from duplex human telomeric DNA as well as from single-stranded DNA. Importantly, telomestatin stabilizes these G-quadruplex structures in the absence of monovalent cations, which is a unique characteristic among G-quadruplex-interactive compounds. At noncytotoxic concentrations, telomestatin suppresses the proliferation of telomerase-positive cells within several weeks. In contrast, TMPyP4, a compound that preferentially facilitates the formation of intermolecular G-quadruplex structures, suppresses the proliferation of alternative lengthening of telomeres (ALT)-positive cells as well as telomerase-positive cells. We have also demonstrated that TMPyP4 induces anaphase bridges in sea urchin embryos, whereas telomestatin did not have this effect, leading us to conclude that the selectivity of telomestatin for intramolecular G-quadruplex structures and TMPyP4 for intermolecular G-quadruplex structures is important in mediating different biological effects: stabilization of intramolecular G-quadruplex structures produces telomerase inhibition and accelerated telomere shortening, whereas facilitation of the formation of intermolecular G-quadruplex structures induces the formation of anaphase bridges.

Melittin-induced hyperactivation of phospholipase A2 activity and calcium influx in ras-transformed cells.

The activated ras oncogene is a key mediator of cellular transformation and is present in a wide variety of primary human neoplasms. The biochemical role of the ras oncogene in cellular transformation is at present unclear, and hence approaches to control its activities in transformed cells have met with limited success. Previous studies have demonstrated the ability of melittin, a 26 amino acid amphipathic peptide from bee venom, to specifically counterselect for cells in culture that express high levels of the ras oncogene product. The biochemical basis for this counterselection is currently unknown. This study demonstrates the ability of melittin to hyperactivate phospholipase A2 (PLA2) in ras-transformed cells by the mediation of enhanced influx of calcium ions (Ca2+). This hyperactivation of PLA2 and Ca2+ mobilization in ras-transformed cells by melittin is mimicked by the calcium ionophore, A23187. Both melittin- and A23187-mediated PLA2 hyperactivation require Ca2+. However, the action of melittin is strongly dependent on extracellular Ca2+, whereas that of A23187 is not. Melittin-induced Ca2+ influx and PLA2 hyperactivation is inhibited by manganese ions (Mn2+). These studies reveal a close correlation between the extent of PLA2 hyperactivation and Ca2+ mobilization, suggesting a causal relationship.

The ETV6-NTRK3 gene fusion encodes a chimeric protein tyrosine kinase that transforms NIH3T3 cells.

The congenital fibrosarcoma t(12;15)(p13;q25) rearrangement splices the ETV6 (TEL) gene on chromosome 12p13 in frame with the NTRK3 (TRKC) neurotrophin-3 receptor gene on chromosome 15q25. Resultant ETV6-NTRK3 fusion transcripts encode the helix - loop - helix (HLH) dimerization domain of ETV6 fused to the protein tyrosine kinase (PTK) domain of NTRK3. We show here that ETV6-NTRK3 homodimerizes and is capable of forming heterodimers with wild-type ETV6. Moreover, ETV6-NTRK3 has PTK activity and is autophosphorylated on tyrosine residues. To determine if the fusion protein has transforming activity, NIH3T3 cells were infected with recombinant retroviral vectors carrying the full-length ETV6-NTRK3 cDNA. These cells exhibited a transformed phenotype, grew macroscopic colonies in soft agar, and formed tumors in severe combined immunodeficient (SCID) mice. We hypothesize that chimeric proteins mediate transformation by dysregulating NTRK3 signal transduction pathways via ligand-independent dimerization and PTK activation. To test this hypothesis, we expressed a series of ETV6-NTRK3 mutants in NIH3T3 cells and assessed their transformation activities. Deletion of the ETV6 HLH domain abolished dimer formation with either ETV6 or ETV6-NTRK3, and cells expressing this mutant protein were morphologically non-transformed and failed to grow in soft agar. An ATP-binding mutant failed to autophosphorylate and completely lacked transformation activity. Mutants of the three NTRK3 PTK activation-loop tyrosines had variable PTK activity but had limited to absent transformation activity. Of a series of signaling molecules well known to bind to wild-type NTRK3, only phospholipase-Cgamma (PLCgamma) associated with ETV6-NTRK3. However, a PTK active mutant unable to bind PLCgamma did not show defects in transformation activity. Our studies confirm that ETV6-NTRK3 is a transforming protein that requires both an intact dimerization domain and a functional PTK domain for transformation activity. Oncogene (2000) 19, 906 - 915.

Increased manganese superoxide dismutase (SOD-2) is part of the mechanism for prostate tumor suppression by Mac25/insulin-like growth factor binding-protein-related protein-1.

Increased expression of mac25/insulin-like growth factor binding-protein related protein-1 (IGFBP-rP1) in human breast and prostate epithelial cell lines results in the suppression of tumor growth. CDNA expression array analysis revealed increased manganese superoxide dismutase (SOD-2) expression in the mac25/IGFBP-rP1-transfected M12 human prostate cancer cell line compared to M12 control cells. SOD-2 has been postulated to be a tumor suppressor. SOD-2 was also increased in LNCaP cells stably transfected with mac25/IGFBP-rP1, but not in mac25/IGFBP-rP1-transfected PC-3 cells. Mac25 LNCaP cells had a marked decrease in tumor growth in nude mice compared to controls, but there was no difference in tumor growth in mac25 PC-3 cells compared to control. Phosphorylated Erk and Akt were increased in the M12 and LNCaP transfected mac25/IGFBP-rP1 cells but not PC-3 mac25. Inhibition of PI-3 kinase results in a marked decrease in viability of the M12-mac25 cells compared to M12 controls. Cells treated with H(2)O(2) result in an increase in phospho-ERK. Transfection of SOD-2 in M12 cells markedly decreased tumor growth, apoptosis, G1 delay in the cell cycle, and expression of senescence associated beta-galactosidase. These results suggest that one of the downstream mediators of the senescence-associated tumor suppression effect of mac25/IGFBP-rP1 is SOD-2.

Differential regulation of human RecQ family helicases in cell transformation and cell cycle.

Three human RecQ DNA helicases, WRN, BLM and RTS, are involved in the genetic disorders associated with genomic instability and a high incidence of cancer. RecQL1 and RecQL5 also belong to the human RecQ helicase family, but their correlation with genetic disorders, if any, is unknown. We report here that in human B cells transformed by Epstein-Barr virus (EBV), human fibroblasts and umbilical endothelial cells transformed by simian virus 40, the expression of WRN, BLM, RTS and RecQL1 was sharply up-regulated. In B cells this expression was stimulated within 5-40 h by the tumor promoting agent phorbol myristic acetate (PMA). Interestingly, RecQL5beta, an alternative splicing product of RecQL5 with a nuclear localization signal, is expressed in resting B cells without significant modulation of its synthesis by EBV or PMA, suggesting it has a role in resting cells. We also roughly determined the number of copies per cell for the five RecQ helicase in B cells. In addition, levels of the different RecQ helicases are modulated in different ways during the cell cycle of actively proliferating fibroblasts and umbilical endothelial cells. Our results support the view that the levels of WRN, BLM, RTS and RecQL1 are differentially up-regulated to guarantee genomic stability in cells that are transformed or actively proliferating.

Functional roles of Akt signaling in mouse skin tumorigenesis.

The mouse skin carcinogenesis protocol is a unique model for understanding the molecular events leading to oncogenic transformation. Mutations in the Ha-ras gene, and the presence of functional cyclin D1 and the EGF receptor, have proven to be important in this system. However, the signal transduction pathways connecting these elements during mouse skin carcinogenesis are poorly understood. This paper studies the relevance of the Akt and ERK pathways in the different stages of chemically induced mouse skin tumors. Akt activity increases throughout the entire process, and its early activation is detected prior to increased cyclin D1 expression. ERK activity rises only during the later stages of malignant conversion. The observed early increase in Akt activity appears to be due to raised PI-3K activity. Other factors acting on Akt such as ILK activation and decreased PTEN phosphatase activity appear to be involved at the conversion stage. To further confirm the involvement of Akt in this process, PB keratinocytes were transfected with Akt and subsequently injected into nude mice. The expression of Akt accelerates tumorigenesis and contributes to increased malignancy of these keratinocytes as demonstrated by the rate of appearance, the growth and the histological characteristics of the tumors. Collectively, these data provide evidence that Akt activation is one of the key elements during the different steps of mouse skin tumorigenesis.

Differential sensitivity of cancer cells to inhibitors of the epidermal growth factor receptor family.

Clinical responses to the HER1 (EGF receptor) inhibitors and HER2/neu/ErbB2 inhibitors correlate with high levels of receptor expression. However, a significant subset of patients with high receptor levels appear to be refractory to treatment. We have observed similar results in the 60 cell lines of the NCI Anti-Cancer Drug Screen using a panel of 11 selective HER1 inhibitors. As expected, low HER1-expressing cell lines were insensitive to HER1 inhibitors. In cell lines with high HER1 expression, low concentrations of HER1 inhibitors potently inhibit both HER1 phosphorylation and the mitogen-activated protein kinase (MAPK) pathway. However, this inhibition did not always correlate with cellular arrest. High HER1-expressing cell lines can be subdivided into two groups based on their sensitivity to HER1 inhibitors. In the sensitive group, receptor and growth inhibition was concordant and occurred at sub-micromolar concentrations of HER1 inhibitors. In the insensitive group, receptor inhibition occurred at a low concentration (< 1 microM) but concentrations that were ten times or higher were required for growth inhibition. Also, neither induction of p21 and cyclin D1 nor p53 status could explain the difference between sensitive and insensitive cells. Although EGF activated the MAPK pathway in all cell lines, only drug-sensitive cell lines responded to EGF (accelerated entry from G1 to S) and to HER1 inhibitors (G1 arrest) by changes in cell cycling. Furthermore, an EGF-dependent immortalized mammary epithelial cell line was extremely sensitive to a panel of HER1 inhibitors. We infer that independence from mitogen-mediated signaling confers insensitivity to HER1 inhibitors in a large subset of cancer cell lines.

Role for RhoB and PRK in the suppression of epithelial cell transformation by farnesyltransferase inhibitors.

Recent genetic investigations have established that RhoB gain-of-function is sufficient to mediate the antitransforming effects of farnesyltransferase inhibitors (FTIs) in H-Ras-transformed fibroblast systems. In this study, we addressed the breadth and mechanism of RhoB action in epithelial cells transformed by oncoproteins which are themselves insensitive to FTI inactivation. Rat intestinal epithelial (RIE) cells transformed by activated K-Ras or Rac1 were highly sensitive to FTI-induced actin reorganization and growth inhibition, despite the inability of FTI to block prenylation of either K-Ras or Rac1. Ectopic expression of the geranylgeranylated RhoB isoform elicited in cells by FTI treatment phenocopied these effects. Analysis of RhoB effector domain mutants pointed to a role for PRK, a Rho effector kinase implicated in the physiological function of RhoB in intracellular receptor trafficking, and these findings were supported further by experiments in a fibroblast system. We propose that FTIs recruit the antioncogenic RhoB protein in the guise of RhoB-GG to interfere with signaling by pro-oncogenic Rho proteins, possibly by sequestering common exchange factors or effectors such as PRK that are important for cell transformation.

cblE type of homocystinuria due to methionine synthase reductase deficiency: functional correction by minigene expression.

The cblE type of homocystinuria is a rare autosomal recessive disorder caused by impaired reductive activation of methionine synthase. Although earlier biochemical studies proposed that the methionine synthase enzyme might be activated by two different reducing systems, mutations were reported in only the methionine synthase reductase gene (MTRR) in cblE patients. The pathogenicity of MTRR mutations, however, has not yet been tested functionally. We report on nine patients of European origin affected by the cblE type of homocystinuria. They presented between 2 weeks and 3 years of age (median age 4 weeks) with anemia, which was macrocytic in only three patients, and with neurological involvement in all but two cases. Bone marrow examination performed in seven patients showed megaloblastic changes in all but one of them. All patients exhibited moderate to severe hyperhomocysteinemia (median plasma total homocysteine [Hcy] 92 mumol/L, range 44-169), while clearly reduced methionine was observed only in four cases. Pathogenic mutations were identified in both parental alleles of the MTRR gene in all patients. Five known (c.903+469T>C, c.1361C>T, c.1459G>A, c.1557-4_1557+3del7, and c.1622_1623dupTA) and three novel mutations (c.7A>T, c.1573C>T, and c.1953-6_1953-2del5) were detected. Importantly, transfection of fibroblasts of cblE patients with a wild-type MTRR minigene expression construct resulted in a significant approximately four-fold increase of methionine synthesis, indicating correction of the enzyme defect. Our study shows a link between a milder predominantly hematological presentation and homozygosity for the c.1361C>T mutation, but no other obvious genotype-phenotype correlation. The identification of mutations in the MTRR gene, together with restoration of methionine synthesis following MTRR minigene expression in cblE cells confirms that this disease is caused by defects in the MTRR gene.