The roles of trace elements in foetal and neonatal development

Manganese, zinc and copper are essential for normal prenatal and neonatal development. Manganese deficiency causes skeletal abnormalities, congenital ataxia due to abnormal inner ear development, and abnormal brain function. Depression of mucopolysaccharide synthesis and manganese superoxide dismutase activity may be fundamental to ultrastructural and other defects. In copper deficiency, neurological and skeletal abnormalities are due to impairment of phospholipid synthesis and collagen crosslinking, and possibly to low activity of copper metalloenzymes. The fundamental defect leading to the extremely teratogenic effects of zinc deficiency is related to depressed synthesis of DNA. In the neonatal period, poor survival and growth and depressed function of the immune system are salient features. Developmental patterns of trace element concentrations in various tissues suggest that important changes in metabolic regulation of trace elements may occur during the neonatal period. This hypothesis is being investigated by studies of molecular localization of trace elements in certain neonatal tissues, in conjunction with similar observations in milk.     

Fetal drug therapy.

Fetal drug therapy encompasses several areas, including the prevention of external genital masculinization in 21-hydroxylase deficiency syndrome (congenital adrenal hyperplasia), biochemical amelioration of methylmalonic acidemia, and biotin-responsive multiple carboxylase deficiency. The correction of cardiac arrhythmias has become relatively commonplace, and a reduction in the risks of neural tube defects is now possible with the use of preconceptual and early conceptual folic acid. Similarly, fetal function can be altered by the induction of fetal lung maturity using a number of agents; corticosteroids are the most common fetal pharmaceutic agent, and a number of other agents have also been tried. The most common route of administering pharmaceutic agents is through the mother and the placenta, although the direct administration of certain agents is becoming more common.     

UV Radiation–Sensitive Norin 1 Rice Contains Defective Cyclobutane Pyrimidine Dimer Photolyase

Norin 1, a progenitor of many economically important Japanese rice strains, is highly sensitive to the damaging effects of UVB radiation (wavelengths 290 to 320 nm). Norin 1 seedlings are deficient in photorepair of cyclobutane pyrimidine dimers. However, the molecular origin of this deficiency was not known and, because rice photolyase genes have not been cloned and sequenced, could not be determined by examining photolyase structural genes or upstream regulatory elements for mutations. We therefore used a photoflash approach, which showed that the deficiency in photorepair in vivo resulted from a functionally altered photolyase. These results were confirmed by studies with extracts, which showed that the Norin 1 photolyase–dimer complex was highly thermolabile relative to the wild-type Sasanishiki photolyase. This deficiency results from a structure/function alteration of photolyase rather than of nonspecific repair, photolytic, or regulatory elements. Thus, the molecular origin of this plant DNA repair deficiency, resulting from a spontaneously occurring mutation to UV radiation sensitivity, is defective photolyase.     

UV radiation-sensitive norin 1 rice contains defective cyclobutane pyrimidine dimer photolyase

Norin 1, a progenitor of many economically important Japanese rice strains, is highly sensitive to the damaging effects of UVB radiation (wavelengths 290 to 320 nm). Norin 1 seedlings are deficient in photorepair of cyclobutane pyrimidine dimers. However, the molecular origin of this deficiency was not known and, because rice photolyase genes have not been cloned and sequenced, could not be determined by examining photolyase structural genes or upstream regulatory elements for mutations. We therefore used a photoflash approach, which showed that the deficiency in photorepair in vivo resulted from a functionally altered photolyase. These results were confirmed by studies with extracts, which showed that the Norin 1 photolyase-dimer complex was highly thermolabile relative to the wild-type Sasanishiki photolyase. This deficiency results from a structure/function alteration of photolyase rather than of nonspecific repair, photolytic, or regulatory elements. Thus, the molecular origin of this plant DNA repair deficiency, resulting from a spontaneously occurring mutation to UV radiation sensitivity, is defective photolyase.     

Therapeutic Exploitation of Checkpoint Defects in Cancer Cells Lacking p53 Function

Cytotoxic agents form the basis of most cancer therapies. These agents primarily affect rapidly proliferating cells, so their use incurs morbidity associated with damage to tissues such as bone marrow and gastrointestinal mucosa. Clinical outcome would be improved if it were possible to develop therapeutics with more specific activity against p53-deficient cancers, which account for over 50% of all cases. p53 deficiency alters the cellular response to DNA damage in that it leaves cells with attenuated DNA damage checkpoint controls and a reduced propensity for apoptotic cell death. Thus, the DNA repair capacity of these cells is reduced but survival is increased. This promotes genomic instability and contributes to the resistance of p53-deficient cells to cytotoxic agents. Disabling the residual G2 checkpoint function of p53-deficient cells may favour cell death following DNA damage. Several potential strategies for G2 checkpoint abrogation show promise for the specific sensitization of cancer cells. Here we detail how the G2 DNA damage checkpoint is influenced by p53 status and how the loss of p53 function in cancer cells can be exploited to enhance the cytotoxicity of anti-cancer agents.     

Therapeutic exploitation of checkpoint defects in cancer cells lacking p53 function

Cytotoxic agents form the basis of most cancer therapies. These agents primarily affect rapidly proliferating cells, so their use incurs morbidity associated with damage to tissues such as bone marrow and gastrointestinal mucosa. Clinical outcome would be improved if it were possible to develop therapeutics with more specific activity against p53-deficient cancers, which account for over 50% of all cases. p53 deficiency alters the cellular response to DNA damage in that it leaves cells with attenuated DNA damage checkpoint controls and a reduced propensity for apoptotic cell death. Thus, the DNA repair capacity of these cells is reduced but survival is increased. This promotes genomic instability and contributes to the resistance of p53-deficient cells to cytotoxic agents. Disabling the residual G(2) checkpoint function of p53-deficient cells may favour cell death following DNA damage. Several potential strategies for G(2) checkpoint abrogation show promise for the specific sensitization of cancer cells. Here we detail how the G(2) DNA damage checkpoint is influenced by p53 status and how the loss of p53 function in cancer cells can be exploited to enhance the cytotoxicity of anti-cancer agents.     

Human mutations in glucose 6-phosphate dehydrogenase reflect evolutionary history.

Glucose 6-phosphate dehydrogenase (G6PD) is a cytosolic enzyme encoded by a housekeeping X-linked gene whose main function is to produce NADPH, a key electron donor in the defense against oxidizing agents and in reductive biosynthetic reactions. Inherited G6PD deficiency is associated with either episodic hemolytic anemia (triggered by fava beans or other agents) or life-long hemolytic anemia. We show here that an evolutionary analysis is a key to understanding the biology of a housekeeping gene. From the alignment of the amino acid (aa) sequence of 52 glucose 6-phosphate dehydrogenase (G6PD) species from 42 different organisms, we found a striking correlation between the aa replacements that cause G6PD deficiency in humans and the sequence conservation of G6PD: two-thirds of such replacements are in highly and moderately conserved (50-99%) aa; relatively few are in fully conserved aa (where they might be lethal) or in poorly conserved aa, where presumably they simply would not cause G6PD deficiency. This is consistent with the notion that all human mutants have residual enzyme activity and that null mutations are lethal at some stage of development. Comparing the distribution of mutations in a human housekeeping gene with evolutionary conservation is a useful tool for pinpointing amino acid residues important for the stability or the function of the corresponding protein. In view of the current explosive increase in full genome sequencing projects, this tool will become rapidly available for numerous other genes.     

Rnr4p, a novel ribonucleotide reductase small-subunit protein.

Ribonucleotide reductases catalyze the formation of deoxyribonucleotides by the reduction of the corresponding ribonucleotides. Eukaryotic ribonucleotide reductases are alpha2beta2 tetramers; each of the larger, alpha subunits possesses binding sites for substrate and allosteric effectors, and each of the smaller, beta subunits contains a binuclear iron complex. The iron complex interacts with a specific tyrosine residue to form a tyrosyl free radical which is essential for activity. Previous work has identified two genes in the yeast Saccharomyces cerevisiae, RNR1 and RNR3, that encode alpha subunits and one gene, RNR2, that encodes a beta subunit. Here we report the identification of a second gene from this yeast, RNR4, that encodes a protein with significant similarity to the beta-subunit proteins. The phenotype of rnr4 mutants is consistent with that expected for a defect in ribonucleotide reductase; rnr4 mutants are supersensitive to the ribonucleotide reductase inhibitor hydroxyurea and display an S-phase arrest at their restrictive temperature. rnr4 mutant extracts are deficient in ribonucleotide reductase activity, and this deficiency can be remedied by the addition of exogenous Rnr4p. As is the case for the other RNR genes, RNR4 is induced by agents that damage DNA. However, Rnr4p lacks a number of sequence elements thought to be essential for iron binding, and mutation of the critical tyrosine residue does not affect Rnr4p function. These results suggest that Rnr4p is catalytically inactive but, nonetheless, does play a role in the ribonucleotide reductase complex.     

Impaired mitochondrial function protects against free radical-mediated cell death

Free radical damage can have fatal consequences. Mitochondria carry out essential cellular functions and produce high levels of reactive oxygen species (ROS). Many agents also generate ROS. Using the yeast Saccharomyces cerevisiae as a eukaryotic model, the role of functional mitochondria in surviving free radical damage was investigated. Respiratory-deficient cells lacking mitochondrial DNA (rho(0)) were up to 100-fold more resistant than isogenic rho(+) cells to killing by ROS generated by the bleomycin-phleomycin family of oxidative agents. Up to approximately 90% of the survivors of high oxidative stress lost mitochondrial function and became "petites." The selective advantage of respiratory deficiency was studied in several strains, including DNA repair-deficient rad52/rad52 and blm5/blm5 diploid strains. These mutant strains are hypersensitive to lethal effects of free radicals and accumulate more DNA damage than related wild-type strains. Losses in mitochondrial function were dose-dependent, and mutational alteration of the RAD52 or BLM5 gene did not affect the resistance of surviving cells lacking mitochondrial function. The results indicate that inactivation of mitochondrial function protects cells against lethal effects of oxygen free radicals.     

The newer trace elements

The application of the concept of a “controlled environment” led to the identification of eight trace elements with a proven or postulated biological function during this decade. Rigorous reduction of metallic contamination from air, drinking water, and diet is necessary to reduce exposure to the element under investigation below requirement levels. An essential function for a trace element is suggested when deficiency is produced, and indepently confirmed, in two or more animal species, and also when the signs of deficiency are prevented or cured by supplements. As yet, a direct role for the “new trace elements” in the human organism has not been demonstrated.     

Clinical aspects of selenium metabolism

Whereas defined selenium deficiency diseases are well characterized in animals, analogous syndromes in humans are unknown or occur only rarely under conditions of extreme selenium depletion. However, selenium deficiency has since been recognized to play important secondary roles in a variety of human diseases, and several of these can be prevented or treated by means of selenium supplementation. In clinical practice, the selenium status of patients should be monitored routinely, and corrective measures should be instituted to replete the Se body stores of patients where necessary. Interactions of Se with other elements are also of potentially great clinical significance. The importance of such interactions is exemplified by the effects of cadmium on selenium metabolism in spontaneously hypertensive rats. Deceased.     

Iodine deficiency,other trace elements,and goitrogenic factors in the etiopathogeny of iodine deficiency disorders (IDD)

Severe goiter, cretinism, and the other iodine deficiency disorders (IDD) have their main cause in the lack of availability of iodine from the soil linked to a severe limitation of food exchanges. Apart from the degrees of severity of the iodine deficiency, the frequencies and symptomatologies of cretinism and the other IDD are influenced by other goitrogenic factors and trace elements. Thiocyanate overload originating from consumption of poorly detoxified cassava is such deficiency. Very recently, a severe selenium deficiency has also been associated with IDD in the human population, whereas in animals, it has been proven to play a role in thyroid function either through a thyroidal or extrathyroidal mechanism. The former involves oxidative damages mediated by free radicals, whereas the latter implies an inhibition of the deiodinase responsible for the utilization of T4 into T3. One concludes that:

1. Goiter has a multifactorial origin
2. IDD are an important public health problem; and
3. IDD are a good model to study the effects of other trace elements whose actions in many human metabolisms have been somewhat underestimated.

     

Choline analogs as potential tools in developing selective animal models of central cholinergic hypofunction

A chronic deficiency in central cholinergic function has been implicated in a number of neuropsychiatric disease states. This deficiency most probably exists at the presynaptic nerve terminal in the brain, where acetylcholine metabolism is known to occur. To date there are no reports on animals that could simulate the neurochemical conditions which appear to cause these diseases in humans, as a result of a direct manipulation of the central cholinergic system. Several compounds related to choline have however been studied, which might be useful agents for developing such an animal model, through their specific action on the high-affinity choline transport system in the brain. This minireview presents an overview of results obtained with these potentially neurotoxic choline analogs, and provides a critical analysis of current knowledge in this area of investigation.     

The distribution of transposable elements within and between chromosomes in a population of Drosophila melanogaster. II. Inferences on the nature of selection against elements.

Data were collected on the distribution of nine families of transposable elements among a sample of autosomes isolated from a natural population of Drosophila melanogaster, by means of in situ hybridization of biotinylated probes to polytene chromosomes. There is no general tendency for elements to accumulate at the tips of chromosomes. Elements tend to be present in excess of random expectation in the euchromatin proximal to the centromeres of the major autosomes, and on chromosome four. There is considerable heterogeneity between different families in the extent of this excess. The overall abundance of element families is inversely related to the extent to which they accumulate proximally. The level of proximal accumulation for the major autosomes is similar to that on the fourth chromosome, but less than that for the X chromosome. There is an overall deficiency of elements in the mid-section of the X compared with the mid-sections of the major autosomes, with considerable heterogeneity between families. The magnitude of this deficiency is positively related to the extent to which elements accumulate proximally. No such deficiency is seen if the proximal regions of the X and autosomes are compared. There is a small and non-significant excess of elements in third chromosomes carrying inversions. There is some between-year heterogeneity in element abundance. The implications of these findings are discussed, and it is concluded that they generally support the hypothesis that transposable element abundance is regulated primarily by the deleterious fitness consequences of meiotic ectopic exchange between elements. If this is the case, such exchange must be very infrequent in the proximal euchromatin, and the elements detected in population surveys of this kind must be inserted into sites where they have negligible mutational effects on fitness.     

New Insight into the Biochemical Pathology of Liver in Choline Deficiency

Abstract

A diet deficient in choline can cause liver cancer in rats. The previous work since 1932 emphasized the fat-removing ability of choline from the liver. There are other dietary factors, including methionine, which, like choline, can remove fat from the liver. These factors were termed as lipotropes. Since then, choline deficiency and lipotrope deficiency are used synonoumously. Recent work since 1980 has clearly demonstrated that choline deficiency (CD) and lipotrope deficiency (LD) are not the same. Generation of free radicals, DNA alterations, liver cell death, and liver cancer that occur due to CD are not generated by LD. Generation of free radicals due to CD diet and some of the agents that counteract free radical action also prevent CD effects except for lipid accumulation in the liver. Despite the recent observations on the role of phospholipase A, (PLA) as the protector of the membranes, it has been found that by preventing the rise of PLA, in the liver, cell death can be prevented. These new findings give choline a distinct role in liver cell death and cancer rather than the role of lipotrope. A new hypothesis linking dietary choline deficiency and liver cancer has been discussed.     

Lithium and Other Elements in Scalp Hair of Residents of Tokyo Prefecture as Investigational Predictors of Suicide Risk

The high suicide rates in Japan and several reports of inverse associations of suicide rates with the levels of lithium (Li) in drinking water prompted determinations of Li along with other elements in samples of scalp hair of 100 male and 100 female residents of Tokyo Prefecture. In more than half of the samples of both genders, Li levels were below the instrumental detection limit or below or the lower limit of the laboratory reference ranges. Among other elements, the concentrations namely of cobalt were also frequently below the laboratory reference range, suggesting that low circulating levels of vitamin B(12) were common in this study population. As vitamin B(12) deficiency is associated with depression and other psychiatric conditions, and there is evidence of interactions between Li and vitamin B(12), Li deficiency as well as suboptimal vitamin B(12) status must be considered as potential suicide risk factors. In view of its established positive effects on mood and brain function, an adequate supply of selenium (Se) is important as well. Although the analytical results suggested that the Se status of the subjects was generally adequate, as seafood was a major dietary source of Se, much of it was actually sequestered by mercury and only a fraction was bio-available. In addition, the hair samples were found to contain not insignificant levels of As, Cd, Ni, and Pb, arising from the adventitious presence of these elements in foods and the environment. As these elements also interact with Se in vivo and are known to adversely affect mood and behavior, in investigational studies, subjects at risk need to be evaluated also with respect to these elements.     

The role of exosomal transport of viral agents in persistent HIV pathogenesis

Human immunodeficiency virus (HIV) infection, despite great advances in antiretroviral therapy, remains a lifelong affliction. Though current treatment regimens can effectively suppress viral load to undetectable levels and preserve healthy immune function, they cannot fully alleviate all symptoms caused by the presence of the virus, such as HIV-associated neurocognitive disorders. Exosomes are small vesicles that transport cellular proteins, RNA, and small molecules between cells as a mechanism of intercellular communication. Recent research has shown that HIV proteins and RNA can be packaged into exosomes and transported between cells, to pathogenic effect. This review summarizes the current knowledge on the diverse mechanisms involved in the sorting of viral elements into exosomes and the damage those exosomal agents can inflict. In addition, potential therapeutic options to counteract exosome-mediated HIV pathogenesis are reviewed and considered.     

Mutagenicity, carcinogenicity and teratogenicity of zinc

Zinc is a common element in the human environment and constitutes an important trace element intervening in many biological processes. Toxicity of zinc is low; zinc deficiency represents, however, a hazard for human health. Zinc is not mutagenic and has little, if any, clastogenic properties. Zinc can induce tumours but only following local application, and does not represent a carcinogenic risk to man. It is still uncertain whether zinc can cause malignant transformation but zinc is needed for cellular proliferation of existing tumours and tumour growth is retarded by zinc deficiency. Zinc is not teratogenic; it can, in fact, avert teratogenicity of other agents. Conversely, zinc deficiency may be harmful to the developing organism.