PTEN mutation spectrum and genotype-phenotype correlations in Bannayan-Riley-Ruvalcaba syndrome suggest a single entity with Cowden syndrome

Germline mutations in the tumour suppressor gene PTEN have been implicated in two hamartoma syndromes that exhibit some clinical overlap, Cowden syndrome (CS) and Bannayan-Riley-Ruvalcaba syndrome (BRR). PTEN maps to 10q23 and encodes a dual specificity phosphatase, a substrate of which is phosphatidylinositol 3,4,5-triphosphate, a phospholipid in the phosphatidylinositol 3-kinase pathway. CS is characterized by multiple hamartomas and an increased risk of benign and malignant disease of the breast, thyroid and central nervous system, whilst the presence of cancer has not been formally documented in BRR. The partial clinical overlap in these two syndromes is exemplified by the hallmark features of BRR: macrocephaly and multiple lipomas, the latter of which occur in a minority of individuals with CS. Additional features observed in BRR, which may also occur in a minority of CS patients, include Hashimoto's thyroiditis, vascular malformations and mental retardation. Pigmented macules of the glans penis, delayed motor development and neonatal or infant onset are noted only in BRR. In this study, constitutive DNA samples from 43 BRR individuals comprising 16 sporadic and 27 familial cases, 11 of which were families with both CS and BRR, were screened for PTEN mutations. Mutations were identified in 26 of 43 (60%) BRR cases. Genotype-phenotype analyses within the BRR group suggested a number of correlations, including the association of PTEN mutation and cancer or breast fibroadenoma in any given CS, BRR or BRR/CS overlap family ( P = 0.014), and, in particular, truncating mutations were associated with the presence of cancer and breast fibroadenoma in a given family ( P = 0.024). Additionally, the presence of lipomas was correlated with the presence of PTEN mutation in BRR patients ( P = 0.028). In contrast to a prior report, no significant difference in mutation status was found in familial versus sporadic cases of BRR ( P = 0.113). Comparisons between BRR and a previously studied group of 37 CS families suggested an increased likelihood of identifying a germline PTEN mutation in families with either CS alone or both CS and BRR when compared with BRR alone ( P = 0.002). Among CS, BRR and BRR/CS overlap families that are PTEN mutation positive, the mutation spectra appear similar. Thus, PTEN mutation-positive CS and BRR may be different presentations of a single syndrome and, hence, both should receive equal attention with respect to cancer surveillance.

The Menkes/Wilson disease gene homologue in yeast provides copper to a ceruloplasmin-like oxidase required for iron uptake

The CCC2 gene of the yeast Saccharomyces cerevisiae is homologous to the human genes defective in Wilson disease and Menkes disease. A biochemical hallmark of these diseases is a deficiency of copper in ceruloplasmin and other copper proteins found in extracytosolic compartments. Here we demonstrate that disruption of the yeast CCC2 gene results in defects in respiration and iron uptake. These defects could be reversed by supplementing cells with copper, suggesting that CCC2 mutant cells were copper deficient. However, cytosolic copper levels and copper uptake were normal. Instead, CCC2 mutant cells lacked a copper-dependent oxidase activity associated with the extracytosolic domain of the FET3-encoded protein, a ceruloplasmin homologue previously shown to be necessary for high-affinity iron uptake in yeast. Copper restored oxidase activity both in vitro and in vivo, paralleling the ability of copper to restore respiration and iron uptake. These results suggest that the CCC2-encoded protein is required for the export of copper from the cytosol into an extracytosolic compartment, supporting the proposal that intracellular copper transport is impaired in Wilson disease and Menkes disease.

An operator at -280 base pairs that is required for repression of araBAD operon promoter: addition of DNA helical turns between the operator and promoter cyclically hinders repression

A site has been found that is required for repression of the Escherichia coli araBAD operon. This site was detected by the in vivo properties of deletion mutants. In vitro protection studies with DNase I and dimethylsulfate showed that araC protein can specifically bind in this area to nucleotides lying at position -265 to -294 with respect to the araBAD operon promoter (PBAD) transcription start point. The previously known sites of protein binding in the ara operon lie between +20 and -160. Since the properties of deletion strains show that all the sites required for araBAD induction lie between +20 and -110, the new site at -280 exerts its repressive action over an unusually large distance along the DNA. Insertions of -16, -8, 0, 5, 11, 15, 24, and 31 base pairs of DNA between the new site and PBAD were constructed. Repression was impaired in those cases in which half-integral turns of the DNA helix were introduced, but repression was nearly normal for the insertions of 0, +11, and +31 base pairs.

Hydroxylation of Saccharomyces cerevisiae ceramides requires Sur2p and Scs7p

The Saccharomyces cerevisiae SCS7 and SUR2 genes are members of a gene family that encodes enzymes that desaturate or hydroxylate lipids. Sur2p is required for the hydroxylation of C-4 of the sphingoid moiety of ceramide, and Scs7p is required for the hydroxylation of the very long chain fatty acid. Neither SCS7 nor SUR2 are essential for growth, and lack of the Scs7p- or Sur2p-dependent hydroxylation does not prevent the synthesis of mannosyldiinositolphosphorylceramide, the mature sphingolipid found in yeast. Deletion of either gene suppresses the Ca2+-sensitive phenotype of csg2Delta mutants, which arises from overaccumulation of inositolphosphorylceramide due to a defect in sphingolipid mannosylation. Characterization of scs7 and sur2 mutants is expected to provide insight into the function of ceramide hydroxylation.

An inositolphosphorylceramide synthase is involved in regulation of plant programmed cell death associated with defense in Arabidopsis

The Arabidopsis thaliana resistance gene RPW8 triggers the hypersensitive response (HR) to restrict powdery mildew infection via the salicylic acid-dependent signaling pathway. To further understand how RPW8 signaling is regulated, we have conducted a genetic screen to identify mutations enhancing RPW8-mediated HR-like cell death (designated erh). Here, we report the isolation and characterization of the Arabidopsis erh1 mutant, in which the At2g37940 locus is knocked out by a T-DNA insertion. Loss of function of ERH1 results in salicylic acid accumulation, enhanced transcription of RPW8 and RPW8-dependent spontaneous HR-like cell death in leaf tissues, and reduction in plant stature. Sequence analysis suggests that ERH1 may encode the long-sought Arabidopsis functional homolog of yeast and protozoan inositolphosphorylceramide synthase (IPCS), which converts ceramide to inositolphosphorylceramide. Indeed, ERH1 is able to rescue the yeast aur1 mutant, which lacks the IPCS, and the erh1 mutant plants display reduced ( approximately 53% of wild type) levels of leaf IPCS activity, indicating that ERH1 encodes a plant IPCS. Consistent with its biochemical function, the erh1 mutation causes ceramide accumulation in plants expressing RPW8. These data reinforce the concept that sphingolipid metabolism (specifically, ceramide accumulation) plays an important role in modulating plant programmed cell death associated with defense.

Tsc13p is required for fatty acid elongation and localizes to a novel structure at the nuclear-vacuolar interface in Saccharomyces cerevisiae

The TSC13/YDL015c gene was identified in a screen for suppressors of the calcium sensitivity of csg2Delta mutants that are defective in sphingolipid synthesis. The fatty acid moiety of sphingolipids in Saccharomyces cerevisiae is a very long chain fatty acid (VLCFA) that is synthesized by a microsomal enzyme system that lengthens the palmitate produced by cytosolic fatty acid synthase by two carbon units in each cycle of elongation. The TSC13 gene encodes a protein required for elongation, possibly the enoyl reductase that catalyzes the last step in each cycle of elongation. The tsc13 mutant accumulates high levels of long-chain bases as well as ceramides that harbor fatty acids with chain lengths shorter than 26 carbons. These phenotypes are exacerbated by the deletion of either the ELO2 or ELO3 gene, both of which have previously been shown to be required for VLCFA synthesis. Compromising the synthesis of malonyl coenzyme A (malonyl-CoA) by inactivating acetyl-CoA carboxylase in a tsc13 mutant is lethal, further supporting a role of Tsc13p in VLCFA synthesis. Tsc13p coimmunoprecipitates with Elo2p and Elo3p, suggesting that the elongating proteins are organized in a complex. Tsc13p localizes to the endoplasmic reticulum and is highly enriched in a novel structure marking nuclear-vacuolar junctions.

An introduction to plant sphingolipids and a review of recent advances in understanding their metabolism and function

Sphingolipids are ubiquitous constituents of eukaryotic cells, and have been intensively investigated in mammals and yeast for decades. Aspects of sphingolipid biochemistry in plants have been explored only recently. To date, progress has been made in determining the structure and occurrence of sphingolipids in plant tissues; in characterizing the enzymatic steps involved in production and turnover of sphingolipids (and, in some cases, the genes encoding the relevant enzymes); and in identifying a variety of biological functions for sphingolipids in plants. Given that these efforts are far from complete and much remains to be learned, this review represents a status report on the burgeoning field of plant sphingolipid biochemistry. Contents Summary 677 I. Introduction 678 II. Plant sphingolipid structure 678 III. Sphingolipid metabolism in plants 683 IV. Sphingolipid functions in plants 693 V. Conclusions 696 Acknowledgements 696 References 696.

The Saccharomyces cerevisiae TSC10/YBR265w gene encoding 3-ketosphinganine reductase is identified in a screen for temperature-sensitive suppressors of the Ca2+-sensitive csg2Delta mutant

Saccharomyces cerevisiae csg2Delta mutants accumulate the sphingolipid inositolphosphorylceramide, which renders the cells Ca2+-sensitive. Temperature-sensitive mutations that suppress the Ca2+ sensitivity of csg2Delta mutants were isolated and characterized to identify genes that encode sphingolipid synthesis enzymes. These temperature-sensitive csg2Delta suppressors (tsc) fall into 15 complementation groups. The TSC10/YBR265w gene was found to encode 3-ketosphinganine reductase, the enzyme that catalyzes the second step in the synthesis of phytosphingosine, the long chain base found in yeast sphingolipids. 3-Ketosphinganine reductase (Tsc10p) is essential for growth in the absence of exogenous dihydrosphingosine or phytosphingosine. Tsc10p is a member of the short chain dehydrogenase/reductase protein family. The tsc10 mutants accumulate 3-ketosphinganine and microsomal membranes isolated from tsc10 mutants have low 3-ketosphinganine reductase activity. His6-tagged Tsc10p was expressed in Escherichia coli and isolated by nickel-nitrilotriacetic acid column chromatography. The recombinant protein catalyzes the NADPH-dependent reduction of 3-ketosphinganine. These data indicate that Tsc10p is necessary and sufficient for catalyzing the NADPH-dependent reduction of 3-ketosphinganine to dihydrosphingosine.

Tsc3p is an 80-amino acid protein associated with serine palmitoyltransferase and required for optimal enzyme activity

Serine palmitoyltransferase catalyzes the first step of sphingolipid synthesis, condensation of serine and palmitoyl CoA to form the long chain base 3-ketosphinganine. The LCB1/TSC2 and LCB2/TSC1 genes encode homologous proteins of the alpha-oxoamine synthase family required for serine palmitoyltransferase activity. The other alpha-oxoamine synthases are soluble homodimers, but serine palmitoyltransferase is a membrane-associated enzyme composed of at least two subunits, Lcb1p and Lcb2p. Here, we report the characterization of a third gene, TSC3, required for optimal 3-ketosphinganine synthesis in Saccharomyces cerevisiae. S. cerevisiae cells lacking the TSC3 gene have a temperature-sensitive lethal phenotype that is reversed by supplying 3-ketosphinganine, dihydrosphingosine, or phytosphingosine in the growth medium. The tsc3 mutant cells have severely reduced serine palmitoyltransferase activity. The TSC3 gene encodes a novel 80-amino acid protein with a predominantly hydrophilic amino-terminal half and a hydrophobic carboxyl terminus that is membrane-associated. Tsc3p coimmunoprecipitates with Lcb1p and/or Lcb2p but does not bind as tightly as Lcb1p and Lcb2p bind to each other. Lcb1p and Lcb2p remain tightly associated with each other and localize to the membrane in cells lacking Tsc3p. However, Lcb2p is unstable in cells lacking Lcb1p and vice versa.

Metabolic response to iron deficiency in Saccharomyces cerevisiae

Iron is an essential cofactor for enzymes involved in numerous cellular processes, yet little is known about the impact of iron deficiency on cellular metabolism or iron proteins. Previous studies have focused on changes in transcript and proteins levels in iron-deficient cells, yet these changes may not reflect changes in transport activity or flux through a metabolic pathway. We analyzed the metabolomes and transcriptomes of yeast grown in iron-rich and iron-poor media to determine which biosynthetic processes are altered when iron availability falls. Iron deficiency led to changes in glucose metabolism, amino acid biosynthesis, and lipid biosynthesis that were due to deficiencies in specific iron-dependent enzymes. Iron-sulfur proteins exhibited loss of iron cofactors, yet amino acid synthesis was maintained. Ergosterol and sphingolipid biosynthetic pathways had blocks at points where heme and diiron enzymes function, whereas Ole1, the essential fatty acid desaturase, was resistant to iron depletion. Iron-deficient cells exhibited depletion of most iron enzyme activities, but loss of activity during iron deficiency did not consistently disrupt metabolism. Amino acid homeostasis was robust, but iron deficiency impaired lipid synthesis, altering the properties and functions of cellular membranes.

Sequence, mapping and disruption of CCC2, a gene that cross-complements the Ca(2+)-sensitive phenotype of csg1 mutants and encodes a P-type ATPase belonging to the Cu(2+)-ATPase subfamily

We have isolated, sequenced, mapped and disrupted a gene, CCC2, from Saccharomyces cerevisiae. This gene displays non-allelic complementation of the Ca(2+)-sensitive phenotype conferred by the csg1 mutation. Analysis of the CCC2p amino acid sequence reveals that it encodes a member of the P-type ATPase family and is most similar to a subfamily thought to consist of Cu2+ transporters, including the human genes that mutate to cause Wilson disease and Menkes disease. The ability of this gene, in two or more copies, to reverse the csg1 defect suggests that Ca(2+)-induced death of csg1 mutant cells is related to Cu2+ metabolism. Cells without CCC2 require increased Cu2+ concentrations for growth. Therefore CCC2p may function to provide Cu2+ to a cellular compartment rather than in removal of excess Cu2+.

Yeast genome-wide drug-induced haploinsufficiency screen to determine drug mode of action

Methods to systematically test drugs against all possible proteins in a cell are needed to identify the targets underlying their therapeutic action and unwanted effects. Here, we show that a genome-wide drug-induced haploinsufficiency screen by using yeast can reveal drug mode of action in yeast and can be used to predict drug mode of action in human cells. We demonstrate that dihydromotuporamine C, a compound in preclinical development that inhibits angiogenesis and metastasis by an unknown mechanism, targets sphingolipid metabolism. The systematic, unbiased and genome-wide nature of this technique makes it attractive as a general approach to identify cellular pathways affected by drugs.

The essential nature of sphingolipids in plants as revealed by the functional identification and characterization of the Arabidopsis LCB1 subunit of serine palmitoyltransferase

Serine palmitoyltransferase (SPT) catalyzes the first step of sphingolipid biosynthesis. In yeast and mammalian cells, SPT is a heterodimer that consists of LCB1 and LCB2 subunits, which together form the active site of this enzyme. We show that the predicted gene for Arabidopsis thaliana LCB1 encodes a genuine subunit of SPT that rescues the sphingolipid long-chain base auxotrophy of Saccharomyces cerevisiae SPT mutants when coexpressed with Arabidopsis LCB2. In addition, homozygous T-DNA insertion mutants for At LCB1 were not recoverable, but viability was restored by complementation with the wild-type At LCB1 gene. Furthermore, partial RNA interference (RNAi) suppression of At LCB1 expression was accompanied by a marked reduction in plant size that resulted primarily from reduced cell expansion. Sphingolipid content on a weight basis was not changed significantly in the RNAi suppression plants, suggesting that plants compensate for the downregulation of sphingolipid synthesis by reduced growth. At LCB1 RNAi suppression plants also displayed altered leaf morphology and increases in relative amounts of saturated sphingolipid long-chain bases. These results demonstrate that plant SPT is a heteromeric enzyme and that sphingolipids are essential components of plant cells and contribute to growth and development.

Plant sphingolipids: function follows form

Plant sphingolipids are structurally diverse molecules that are important as membrane components and bioactive molecules. An appreciation of the relationship between structural diversity and functional significance of plant sphingolipids is emerging through characterization of Arabidopsis mutants coupled with advanced analytical methods. It is increasingly apparent that modifications such as hydroxylation and desaturation of the sphingolipid nonpolar long-chain bases and fatty acids influence their metabolic routing to particular complex sphingolipid classes and their functions in signaling pathways and other cellular processes, such as membrane protein targeting. Here, we review recent reports investigating some of the more prevalent sphingolipid structural modifications and their functional importance in plants.

SUR1 (CSG1/BCL21), a gene necessary for growth of Saccharomyces cerevisiae in the presence of high Ca2+ concentrations at 37 degrees C, is required for mannosylation of inositolphosphorylceramide

Saccharomyces cerevisiae cells require two genes, CSG1/SUR1 and CSG2, for growth in 50 mM Ca2+, but not 50 mM Sr2+. CSG2 was previously shown to be required for the mannosylation of inositolphosphorylceramide (IPC) to form mannosylinositolphosphorylceramide (MIPC). Here we demonstrate that SUR1/CSG1 is both genetically and biochemically related to CSG2. Like CSG2, SUR1/CSG1 is required for IPC mannosylation. A 93-amino acid stretch of Csg1p shows 29% identity with the alpha-1, 6-mannosyltransferase encoded by OCH1. The SUR1/CSG1 gene is a dose-dependent suppressor of the Ca(2+)-sensitive phenotype of the csg2 mutant, but overexpression of CSG2 does not suppress the Ca2+ sensitivity of the csg1 mutant. The csg1 and csg2 mutants display normal growth in YPD, indicating that mannosylation of sphingolipids is not essential. Increased osmolarity of the growth medium increases the Ca2+ tolerance of csg1 and csg2 mutant cells, suggesting that altered cell wall synthesis causes Ca(2+)-induced death. Hydroxylation of IPC-C to form IPC-D requires CCC2, a gene encoding an intracellular Cu2+ transporter. Increased expression of CCC2 or increased Cu2+ concentration in the growth medium enhances the Ca2+ tolerance of csg1 mutants, suggesting that accumulation of IPC-C renders csg1 cells Ca2+ sensitive.

Members of the Arabidopsis FAE1-like 3-ketoacyl-CoA synthase gene family substitute for the Elop proteins of Saccharomyces cerevisiae

Several 3-keto-synthases have been studied, including the soluble fatty acid synthases, those involved in polyketide synthesis, and the FAE1-like 3-ketoacyl-CoA synthases. All of these condensing enzymes have a common ancestor and an enzymatic mechanism that involves a catalytic triad consisting of Cys, His, and His/Asn. In contrast to the FAE1-like family of enzymes that mediate plant microsomal fatty acid elongation, the condensation step of elongation in animals and in fungi appears to be mediated by the Elop homologs. Curiously these proteins bear no resemblance to the well characterized 3-keto-synthases. There are three ELO genes in yeast that encode the homologous Elo1p, Elo2p, and Elo3p proteins. Elo2p and Elo3p are required for synthesis of the very long-chain fatty acids, and mutants lacking both Elo2p and Elo3p are inviable confirming that the very long-chain fatty acids are essential for cellular functions. In this study we show that heterologous expression of several Arabidopsis FAE1-like genes rescues the lethality of an elo2Deltaelo3Delta yeast mutant. We further demonstrate that FAE1 acts in conjunction with the 3-keto and trans-2,3-enoyl reductases of the elongase system. These studies indicate that even though the plant-specific FAE1 family of condensing enzymes evolved independently of the Elop family of condensing enzymes, they utilize the same reductases and presumably dehydratase that the Elop proteins rely upon.

Childhood amyotrophic lateral sclerosis caused by excess sphingolipid synthesis

Amyotrophic lateral sclerosis (ALS) is a progressive, neurodegenerative disease of the lower and upper motor neurons with sporadic or hereditary occurrence. Age of onset, pattern of motor neuron degeneration and disease progression vary widely among individuals with ALS. Various cellular processes may drive ALS pathomechanisms, but a monogenic direct metabolic disturbance has not been causally linked to ALS. Here we show SPTLC1 variants that result in unrestrained sphingoid base synthesis cause a monogenic form of ALS. We identified four specific, dominantly acting SPTLC1 variants in seven families manifesting as childhood-onset ALS. These variants disrupt the normal homeostatic regulation of serine palmitoyltransferase (SPT) by ORMDL proteins, resulting in unregulated SPT activity and elevated levels of canonical SPT products. Notably, this is in contrast with SPTLC1 variants that shift SPT amino acid usage from serine to alanine, result in elevated levels of deoxysphingolipids and manifest with the alternate phenotype of hereditary sensory and autonomic neuropathy. We custom designed small interfering RNAs that selectively target the SPTLC1 ALS allele for degradation, leave the normal allele intact and normalize sphingolipid levels in vitro. The role of primary metabolic disturbances in ALS has been elusive; this study defines excess sphingolipid biosynthesis as a fundamental metabolic mechanism for motor neuron disease.

Detection of hypoglycemia with the GlucoWatch biographer

OBJECTIVE

Hypoglycemia is a common acute complication of diabetes therapy. The GlucoWatch biographer provides frequent and automatic glucose measurements with an adjustable low-glucose alarm. We have analyzed the performance of the biographer low-glucose alarm relative to hypoglycemia as defined by blood glucose < or = 3.9 mmol/l.

RESEARCH DESIGN AND METHODS

The analysis was based on 1,091 biographer uses from four clinical trials. which generated 14,487 paired (biographer and blood glucose) readings.

RESULTS

The results show that as the low-glucose alert level of the biographer is increased, the number of true positive alerts (alarm sounds and blood glucose < or = 3.9 mmol/l) and false positive alerts (alarm sounds but blood glucose >3.9 mmol/l) increased. When analyzed as a function of varying low-glucose alert levels, the results show receiver operator characteristic curves consistent with a highly useful diagnostic tool. Setting the alert level from 1.1 to 1.7 mmol/l above the level of concern is likely to optimize the trade-off between true positives and false positives for each user. When the same blood glucose data are analyzed for typical monitoring practices (two or four measurements per day), the results show that fewer hypoglycemic events are detected than those detected with the biographer.

The chemical basis of serine palmitoyltransferase inhibition by myriocin

Sphingolipids (SLs) are essential components of cellular membranes formed from the condensation of L-serine and a long-chain acyl thioester. This first step is catalyzed by the pyridoxal-5'-phosphate (PLP)-dependent enzyme serine palmitoyltransferase (SPT) which is a promising therapeutic target. The fungal natural product myriocin is a potent inhibitor of SPT and is widely used to block SL biosynthesis despite a lack of a detailed understanding of its molecular mechanism. By combining spectroscopy, mass spectrometry, X-ray crystallography, and kinetics, we have characterized the molecular details of SPT inhibition by myriocin. Myriocin initially forms an external aldimine with PLP at the active site, and a structure of the resulting co-complex explains its nanomolar affinity for the enzyme. This co-complex then catalytically degrades via an unexpected 'retro-aldol-like' cleavage mechanism to a C18 aldehyde which in turn acts as a suicide inhibitor of SPT by covalent modification of the essential catalytic lysine. This surprising dual mechanism of inhibition rationalizes the extraordinary potency and longevity of myriocin inhibition.

Sphingolipids in the root play an important role in regulating the leaf ionome in Arabidopsis thaliana

Sphingolipid synthesis is initiated by condensation of Ser with palmitoyl-CoA producing 3-ketodihydrosphinganine (3-KDS), which is reduced by a 3-KDS reductase to dihydrosphinganine. Ser palmitoyltransferase is essential for plant viability. Arabidopsis thaliana contains two genes (At3g06060/TSC10A and At5g19200/TSC10B) encoding proteins with significant similarity to the yeast 3-KDS reductase, Tsc10p. Heterologous expression in yeast of either Arabidopsis gene restored 3-KDS reductase activity to the yeast tsc10Δ mutant, confirming both as bona fide 3-KDS reductase genes. Consistent with sphingolipids having essential functions in plants, double mutant progeny lacking both genes were not recovered from crosses of single tsc10A and tsc10B mutants. Although the 3-KDS reductase genes are functionally redundant and ubiquitously expressed in Arabidopsis, 3-KDS reductase activity was reduced to 10% of wild-type levels in the loss-of-function tsc10a mutant, leading to an altered sphingolipid profile. This perturbation of sphingolipid biosynthesis in the Arabidopsis tsc10a mutant leads an altered leaf ionome, including increases in Na, K, and Rb and decreases in Mg, Ca, Fe, and Mo. Reciprocal grafting revealed that these changes in the leaf ionome are driven by the root and are associated with increases in root suberin and alterations in Fe homeostasis.

Mutations in the yeast LCB1 and LCB2 genes, including those corresponding to the hereditary sensory neuropathy type I mutations, dominantly inactivate serine palmitoyltransferase

It was recently demonstrated that mutations in the human SPTLC1 gene, encoding the Lcb1p subunit of serine palmitoyltransferase (SPT), cause hereditary sensory neuropathy type I . As a member of the subfamily of pyridoxal 5'-phosphate enzymes known as the alpha-oxoamine synthases, serine palmitoyltransferase catalyzes the committed step of sphingolipid synthesis. The residues that are mutated to cause hereditary sensory neuropathy type I reside in a highly conserved region of Lcb1p that is predicted to be a catalytic domain of Lcb1p on the basis of alignments with other members of the alpha-oxoamine synthase family. We found that the corresponding mutations in the LCB1 gene of Saccharomyces cerevisiae reduce serine palmitoyltransferase activity. These mutations are dominant and decrease serine palmitoyltransferase activity by 50% when the wild-type and mutant LCB1 alleles are coexpressed. We also show that serine palmitoyltransferase is an Lcb1p small middle dotLcb2p heterodimer and that the mutated Lcb1p proteins retain their ability to interact with Lcb2p. Modeling studies suggest that serine palmitoyltransferase is likely to have a single active site that lies at the Lcb1p small middle dotLcb2p interface and that the mutations in Lcb1p reside near the lysine in Lcb2p that is expected to form the Schiff's base with the pyridoxal 5'-phosphate cofactor. Furthermore, mutations in this lysine and in a histidine residue that is also predicted to be important for pyridoxal 5'-phosphate binding to Lcb2p also dominantly inactivate SPT similar to the hereditary sensory neuropathy type 1-like mutations in Lcb1p.

A disease-causing mutation in the active site of serine palmitoyltransferase causes catalytic promiscuity

The autosomal dominant peripheral sensory neuropathy HSAN1 results from mutations in the LCB1 subunit of serine palmitoyltransferase (SPT). Serum from patients and transgenic mice expressing a disease-causing mutation (C133W) contain elevated levels of 1-deoxysphinganine (1-deoxySa), which presumably arise from inappropriate condensation of alanine with palmitoyl-CoA. Mutant heterodimeric SPT is catalytically inactive. However, mutant heterotrimeric SPT has approximately 10-20% of wild-type activity and supports growth of yeast cells lacking endogenous SPT. In addition, long chain base profiling revealed the synthesis of significantly more 1-deoxySa in yeast and mammalian cells expressing the heterotrimeric mutant enzyme than in cells expressing wild-type enzyme. Wild-type and mutant enzymes had similar affinities for serine. Surprisingly, the enzymes also had similar affinities for alanine, indicating that the major affect of the C133W mutation is to enhance activation of alanine for condensation with the acyl-CoA substrate. In vivo synthesis of 1-deoxySa by the mutant enzyme was proportional to the ratio of alanine to serine in the growth media, suggesting that this ratio can be used to modulate the relative synthesis of sphinganine and 1-deoxySa. By expressing SPT as a single-chain fusion protein to ensure stoichiometric expression of all three subunits, we showed that GADD153, a marker for endoplasmic reticulum stress, was significantly elevated in cells expressing mutant heterotrimers. GADD153 was also elevated in cells treated with 1-deoxySa. Taken together, these data indicate that the HSAN1 mutations perturb the active site of SPT resulting in a gain of function that is responsible for the HSAN1 phenotype.

Expression of stem-cell factor and its receptor c-kit protein in normal testicular tissue and malignant germ-cell tumours

The proto-oncogene c-kit and its ligand stem-cell factor (SCF) may play an important role in the development of normal and malignant testicular tissue. This study investigates the presence of SCF and c-kit protein in 32 orchiectomy specimens of patients with testicular cancer, in 5 specimens of normal testicular tissue and in three established non-seminomatous germ-cell cancer cell lines (H12.1, H32, 577ML) by an immunohistochemical approach. Out of 9 testicular cancer specimens classified as pure seminomas, 7 (78%) showed a strong immunohistochemical reaction for both SCF and c-kit protein on the surface of the tumour cells. Fourteen non-seminomatous germ-cell tumours composed of embryonal carcinoma were completely negative for both SCF and c-kit proteins and only faint positivity was found in 6 tumours (26%). Differentiated teratomatous structures within the specimens on non-seminomatous tumours showed a strong immunohistochemical reaction for SCF and c-kit protein in 8 of 11 (73%) cases. All three testicular cancer cell lines showed only faint staining reactions for c-kit protein and none for SCF. No secretion of SCF by the three lines in vitro was detected. The addition of high concentrations of SCF (100 ng/ml) to the testicular cancer cell lines in culture conditions without fetal calf serum resulted in a 1.4 to 3-fold growth stimulation compared to cell growth in serum-free medium alone. This effect was not detectable when the cells were cultured in serum-containing media. In the normal testicular tissue the germ-cells displayed a strong immunohistochemical reaction for c-kit protein while SCF positivity was found at the tubular membrane and on the surface of Sertoli cells. The SCF/c-kit system may possess a regulatory function in normal testicular tissue by possibly providing the microenvironment necessary for spermatogenesis. With the development of testicular cancer, this regulatory system seems to be lost, particularly in non-seminomatous germ-cell tumours. A growth-stimulatory effect of high concentrations of SCF on non-seminomatous testicular cancer cell lines can be detected only in culture conditions with serum-free media. The effects achievable by the combination of SCF with other growth factors need to be further studied, as well as the role of the c-kit/SCF regulatory system for normal spermatogenesis and its possible implications for the understanding and treatment of male infertility.