Preimplantation genetic diagnosis for medium-chain acyl-CoA dehydrogenase (MCAD) deficiency

Inborn Errors of Metabolism

Inborn errors of metabolism are single gene disorders resulting from the defects in the biochemical pathways of the body. Although these disorders are individually rare, collectively they account for a significant portion of childhood disability and deaths. Most of the disorders are inherited as autosomal recessive whereas autosomal dominant and X-linked disorders are also present. The clinical signs and symptoms arise from the accumulation of the toxic substrate, deficiency of the product, or both. Depending on the residual activity of the deficient enzyme, the initiation of the clinical picture may vary starting from the newborn period up until adulthood. Hundreds of disorders have been described until now and there has been a considerable clinical overlap between certain inborn errors. Resulting from this fact, the definite diagnosis of inborn errors depends on enzyme assays or genetic tests. Especially during the recent years, significant achievements have been gained for the biochemical and genetic diagnosis of inborn errors. Techniques such as tandem mass spectrometry and gas chromatography for biochemical diagnosis and microarrays and next-generation sequencing for the genetic diagnosis have enabled rapid and accurate diagnosis. The achievements for the diagnosis also enabled newborn screening and prenatal diagnosis. Parallel to the development the diagnostic methods; significant progress has also been obtained for the treatment. Treatment approaches such as special diets, enzyme replacement therapy, substrate inhibition, and organ transplantation have been widely used. It is obvious that by the help of the preclinical and clinical research carried out for inborn errors, better diagnostic methods and better treatment approaches will high likely be available.

Preimplantation genetic diagnosis

Preimplantation genetic diagnosis (PGD) was introduced at the beginning of the 1990s as an alternative to prenatal diagnosis, to prevent termination of pregnancy in couples with a high risk for offspring affected by a sex-linked genetic disease. At that time, embryos obtained in vitro were tested to ascertain their sex, and only female embryos were transferred. Since then, techniques for genetic analysis at the single-cell level, involving assessment of first and second polar bodies from oocytes or blastomeres from cleavage-stage embryos, have evolved. Fluorescence in-situ hybridisation (FISH) has been introduced for the analysis of chromosomes and PCR for the analysis of genes in cases of monogenic diseases. In-vitro culture of embryos has also improved through the use of sequential media. Here, we provide an overview of indications for, and techniques used in, PGD, and discuss results obtained with the technique and outcomes of pregnancies. A brief review of new technologies is also included.

Current status of preimplantation diagnosis for single gene disorders

Preimplantation genetic diagnosis (PGD) has become an established procedure for avoiding the birth of affected children with single gene disorders. PGD is performed through polar body or blastomere biopsy, which has no deleterious effect on pre- and post-implantation development. This review describes the most recent developments and current changes in the spectrum of conditions for which PGD has been applied. The most recent applications of PGD include congenital malformations, blood group incompatibility and an increasing number of late onset disorders with genetic predisposition, all of which have not previously been diagnosed using PGD. Despite ethical concerns, PGD has also been used for preselection of unaffected and HLA matched embryos, and recently for preimplantation HLA matching without testing for the causative gene. This extends the practical value of PGD, with its utility being no longer limited to prevention of single gene disorders, by expanding it to treatment of siblings requiring stem cell transplantation.

Long-chain L-3-hydroxyacyl-coenzyme a dehydrogenase deficiency: a molecular and biochemical review

Since the first report of long-chain L-3-hydroxyacyl-coenzyme A dehydrogenase deficiency a little more than a decade ago, its phenotypic and genotypic heterogeneity in individuals homozygous for the enzyme defect has become more and more evident. Even more interesting is its association with pregnancy-specific disorders, including preeclampsia, HELLP syndrome (hemolysis, elevated liver enzymes, low platelets), hyperemesis gravidarum, acute fatty liver of pregnancy, and maternal floor infarct of the placenta. In this review we discuss the biochemical and molecular basis, clinical features, diagnosis, and management of long-chain L-3-hydroxyacyl-coenzyme A dehydrogenase deficiency.

Preimplantation genetic diagnosis

Preimplantation genetic diagnosis is essentially an alternative to prenatal diagnosis, in which genetic testing is performed on embryos before a clinical pregnancy is established. Preimplantation genetic diagnosis has been applied to patients carrying chromosomal rearrangements, such as translocations, in which it has been proven to decrease the number of spontaneous abortions and prevent the birth of children affected with chromosome imbalance. Preimplantation genetic diagnosis techniques have also been applied to increase implantation rates, reduce the incidence of spontaneous abortion and prevent trisomic offspring in women of advanced maternal age undergoing fertility treatment. A third group of patients receiving preimplantation genetic diagnosis are those at risk of transmitting a single gene disorder to their children. The number of monogenic disorders that have been diagnosed in preimplantation embryos has increased each year. Recent protocols have tended to be more complex and more reliable than previous methods, making greater use of multiplex polymerase chain reaction. As well as an expansion in the variety of disorders for which preimplantation genetic diagnosis is offered, new indications have been reported including the use of human leukocyte antigen histocompatibility typing and the application of preimplantation genetic diagnosis to late onset diseases.

Mutation analysis in mitochondrial fatty acid oxidation defects: Exemplified by acyl-CoA dehydrogenase deficiencies, with special focus on genotype-phenotype relationship

Mutation analysis of metabolic disorders, such as the fatty acid oxidation defects, offers an additional, and often superior, tool for specific diagnosis compared to traditional enzymatic assays. With the advancement of the structural part of the Human Genome Project and the creation of mutation databases, procedures for convenient and reliable genetic analyses are being developed. The most straightforward application of mutation analysis is to specific diagnoses in suspected patients, particularly in the context of family studies and for prenatal/preimplantation analysis. In addition, from these practical uses emerges the possibility to study genotype-phenotype relationships and investigate the molecular pathogenesis resulting from specific mutations or groups of mutations. In the present review we summarize current knowledge regarding genotype-phenotype relationships in three disorders of mitochondrial fatty acid oxidation: very-long chain acyl-CoA dehydrogenase (VLCAD, also ACADVL), medium-chain acyl-CoA dehydrogenase (MCAD, also ACADM), and short-chain acyl-CoA dehydrogenase (SCAD, also ACADS) deficiencies. On the basis of this knowledge we discuss current understanding of the structural implications of mutation type, as well as the modulating effect of the mitochondrial protein quality control systems, composed of molecular chaperones and intracellular proteases. We propose that the unraveling of the genetic and cellular determinants of the modulating effects of protein quality control systems may help to assess the balance between genetic and environmental factors in the clinical expression of a given mutation. The realization that the effect of the monogene, such as disease-causing mutations in the VLCAD, MCAD, and SCAD genes, may be modified by variations in other genes presages the need for profile analyses of additional genetic variations. The rapid development of mutation detection systems, such as the chip technologies, makes such profile analyses feasible. However, it remains to be seen to what extent mutation analysis will be used for diagnosis of fatty acid oxidation defects and other metabolic disorders.