Edward R.B. McCabe

Phenotypes of Patients with “Simple” Mendelian Disorders Are Complex Traits: Thresholds, Modifiers, and Systems Dynamics

One of the firmly held concepts in human molecular genetics has been that, if we can understand the details of specific genetic mutations and their effects on protein products, we will be better able to correlate genotype with phenotype. One of the promises of this concept is that such a knowledge base will move clinical genetics into a predictive mode: knowledge of the mutant alleles responsible for a disease would permit an accurate prediction of the prognosis and a better-informed selection among therapeutic strategies for any individual patient.

DNA microextraction from dried blood spots on filter paper blotters: potential applications to newborn screening

SummaryMicroextraction of DNA from dried blood specimens would ease specimen transport to centralized laboratory facilities for recombinant DNA diagnosis in the same manner as use of dried blood spots allowed the broad application of screening tests to newborn populations. A method is described which reproducibly yields 0.5μg DNA from the dried equivalent of 50μl whole blood. Though DNA yields decreased with storage of dried specimens at room temperature, good-quality DNA was still obtained. Sufficient DNA was routinely obtained for Southern blot analysis using repetitive and unique sequences. This microextraction procedure will allow immediate application of molecular genetic technology to direct newborn screening follow-up of disorders amenable to DNA diagnosis, such as sickle cell anemia, and may eventually permit primary DNA screening for specific mutations.

Current estimate of Down Syndrome population prevalence in the United States.

OBJECTIVE To calculate a reliable estimate of the population prevalence of Down syndrome in the US. STUDY DESIGN The annual number of births of infants with Down syndrome were estimated by applying published birth prevalence rates of Down syndrome by maternal age to US data from the Centers for Disease Control and Prevention for the years for which births by maternal age were available (1940-2008). Death certificate data for persons with Down syndrome were available for the years 1968-2007. We estimated the number of people with Down syndrome on January 1, 2008, using a life table approach based on proportions of deaths by age. Monte Carlo sampling was used to create 90% uncertainty intervals (UIs) for our estimates. RESULTS We estimated the January 1, 2008, population prevalence of Down syndrome as approximately 250700 (90% UI, 185900-321700) based on proportions of deaths by age from the most recent 2 years (2006-2007) of death certificate data. This estimate corresponds to a prevalence of 8.27 people with Down syndrome per 10000 population (90% UI, 6.14-10.62). CONCLUSION Our estimate of Down syndrome prevalence is roughly 25%-40% lower than estimates based solely on current birth prevalence. The results presented here can be considered a starting point for facilitating policy and services planning for persons with Down syndrome.

Multiple Acyl-CoA Dehydrogenase Deficiency (Glutaric Aciduria Type II) with Transient Hypersarcosinemia and Sarcosinuria; Possible Inherited Deficiency of an Electron Transfer Flavoprotein

Summary: When amino acids were infused at a rate of 4 g/kg/day, an infant with hypoglycemia, metabolic acidemia and chronic regurgitation showed hypersarcosinemia and excreted abnormal amounts of sarcosine, isovalerylglycine, isobutyrylglycine, α-methylbutyrylglycine, and β-hydroxyisovaleric, glutaric, α-hydroxyglutaric, methylsuccinic, and α-hydroxyisobutyric acids in urine. On all other occasions, when protein intake was lower and lipid intake higher, urine organic acids were dominated by methylsuccinic, ethylmalonic, and α-hydroxyglutaric acids, and hypersarcosinemia was absent. Autopsy showed severe fatty changes in liver, kidneys, and skeletal muscle. A previous female sibling had died with similar autopsy findings at 4 days of age. While activity of glutaryl-CoA dehydrogenase was completely deficient in liver and almost completely so in kidney, it was normal in cultured fibroblasts in the presence of flavin adenine dinucleotide (FAD) and only marginally low in its absence. Incorporation of d-(2-14C) riboflavin into flavin mononucleotides (FMN) and FAD by kidney tissue was normal.The authors conclude that this disorder is not due to generalized deficiency of glutaryl-CoA dehydrogenase or to a defect in FAD synthesis. The amino and organic acid abnormalities noted are most consistent with a defect in the flavoprotein which transfers electrons from the FAD of sarcosine and acyl-CoA dehydrogenases into the respiratory chain, although a defect in intercompartmental transfer of C4-5 acyl CoA esters across cell membranes is not excluded.The variability of the organic aciduria, which possibly reflects changes in protein and fat intake, suggests that a previous name for this disorder, i.e., glutaric aciduria type II, is inappropriate and should be replaced, perhaps by “multiple acyl-CoA dehydrogenase deficiency.”Speculation: What appears to be simultaneous deficiency of several acyl-CoA dehydrogenases may be caused by a number of different primary gene defects; the presence of hypersarcosinemia and/or sarcosinuria may delineate a subtype due to deficiency of an electron carrier flavoprotein. Further, the presence of organic aciduria may define a form of hypersarcosinemia more likely to be associated with phenotypic abnormalities than isolated deficiency of the sarcosine dehydrogenase apoenzyme.

Human glycerol kinase deficiency: An inborn error of compartmental metabolism

Twelve individuals have been described with glycerol kinase deficiency. Five of these individuals are adults who were noted incidentally to have pseudohypertriglyceridemia. Six of these individuals are children who manifest a clinical complex which includes adrenal hypoplasia/insufficiency and developmental delay. Another child has intermittent coma, a normal IQ, and no evidence of adrenal insufficiency. Genetic and biochemical hypotheses are proposed to explain this clinical variability. Glycerol kinase binds specifically and reversibly to the porin, the pore-forming protein of the outer mitochondrial membrane, which also binds hexokinase. Mutations affecting any component of this kinase-binding system will alter the properties of this system. Glycerol kinase deficiency, as an inborn error of this compartmented metabolic system, offers an investigational opportunity for studying this microenvironment.

Minipuberty of infancy and adolescent pubertal function in adrenal hypoplasia congenita

An infant and his uncle, both with adrenal hypoplasia congenita, shared the same DAX1 mutation. The adolescent uncle had hypogonadotropic hypogonadism, but the infant had a normal minipuberty of infancy. These observations suggest differences in the physiologic mechanisms regulating the hypothalamic-pituitary-gonadal axis in infancy and adolescence.

Acute leukemias in children with Down syndrome

Children with Down syndrome (DS) often present with hematopoietic abnormalities, and are at increased risk of developing leukemia. Specifically, 3-10% of newborns with DS are diagnosed with transient myeloproliferative disease, and children with DS are 500 times more likely to develop acute megakaryoblastic leukemia (AMKL) and 20 times more likely to develop acute lymphoblastic leukemia (ALL) than typical children. This review examines the characteristics of these leukemias and their development in the unique genetic background of trisomy 21. A discussion is also provided for areas of future research and potential therapeutic development.

Homotransplantation of the liver in a patient with hepatoma and hereditary tyrosinemia

A girl with hereditary tyrosinemia, diagnosed at 6 months of age, was treated with a diet restricted in phenylalanine and tyrosine. At 9 1/2 years of age she developed an acutely enlarged liver and spleen, and the diagnosis of hepatocarcinoma was made. The patient received a liver transplant and tyrosine metabolites became normal while she was receiving a regular diet. Three months later, an infected thrombosis of the portal vein caused her death. Liver transplant appears to be an effective method of enzyme replacement in tyrosinemia and should be considered for prevention of hepatoma.

Developing a national collaborative study system for rare genetic diseases

There are thousands of rare genetic diseases and many genetic and nongenetic contributors to common genetic diseases. The evidence base that is currently available about the great majority of these conditions is limited to case studies and relatively small observational study sets derived from one or several institutions. Hence, the statistical power in any one study is usually quite limited. Further, in the absence of organized registries and data collection on particular patient groups, the information available is weak and the patient resources that are available are limited. It is only through organized and coordinated clinical investigation systems that a sufficient number of patients with these diseases can be accumulated to provide the statistical power needed to inform about clinical history of treated and untreated forms, provide the resources needed for clinical trials of new tests and treatments, provide a sufficiently powered evidence base for public health decision-making and other uses. The meeting in which these issues were raised resulted in a set of proposed principles and associated recommendations as to how best to achieve the vision of creating an extensive and comprehensive collaboration of professional and lay communities to enable translational research to improve clinical care and therapies for persons with rare genetic diseases.

Direct-to-consumer genetic testing: access and marketing.

The American College of Medical Genetics (ACMG), in their “Statement on Direct-to-Consumer Genetic Testing”1 (in this issue of Genetics in Medicine), argues for the involvement of appropriately qualified health care professionals in the ordering and interpretation of genetic tests, and the counseling of individuals and families regarding the meaning and significance of the test results. They point out the potential harms that may result if such health professionals are not involved, including misused tests, misinterpreted results, and misguided follow-up. We feel that this statement from the ACMG will not only educate the public and professionals about this issue, but will also lead to further discourse. In this commentary, we will provide a brief background for these discussions. One argument in favor of direct-to-consumer testing relies on respect for patient autonomy. Such reliance, however, ignores the need for information if autonomy and decisionmaking are to be meaningful. We acknowledge and cherish the autonomy of individuals to make decisions regarding their own health care. Geneticists show respect for and uphold autonomy through nondirective counseling. We agree with the ACMG, however, that decisions regarding whether and how an individual wishes to use a genetic test, and the information derived from it, should be informed by discussions with a knowledgeable health professional. Others argue that direct access to genetic testing reduces health care costs by eliminating the need for consultation with trained professionals. However, this argument fails to account for the costs likely to result from the uninformed and unnecessary uses of genetic tests and from the adverse consequences of inappropriate responses to test results, whether positive or negative, valid or invalid. Such consequences can include underuse and overuse of health care resources. Concerns regarding direct access to genetic testing are closely tied to concerns about direct-to-consumer advertising and marketing of genetic testing. Arguments have been made for direct-to-consumer marketing of genetic tests based on experience with direct-to-consumer advertisement of prescription drugs.2 Benefits cited by advocates include increased compliance and facilitation of patient-physician communication (based on the required “talk with your doctor” phrase). In the context of genetic tests, benefits of direct-to-consumer advertising may include an increased awareness of the importance of family history, the relation between risk and family history, the role of genetics in disease, and the value of genetic counseling. When advertisements for genetic tests are presented on the Internet or in the media by a commercial entity, however, significant clinical information may be missing. For example, there is frequently no information provided regarding the clinical validity and utility of the test. Consumers are not advised, for example, whether a test will provide the answers they are seeking regarding a particular disorder. An individual consumer is unlikely to know the positive predictive value (probability that a positive test result indicates the person will develop the disorder) or the negative predictive value (probability that a person will not develop a disorder if the test does not find a mutation) of the test. The consumer also is unlikely to know the performance characteristics of that test in the specific context in which they are requesting the test result; e.g., test performance may be very different in the presence or absence of a positive family history for the disease, or in the presence or absence of a known mutation. A consumer also may not be able to determine whether a particular genetic test is appropriate for them in the absence of consultation with a trained health professional.2 For example, a woman with a family history of breast cancer who is concerned about her risk for breast cancer (consultand) would do better to have a sample from an affected relative who meets the criteria for familial breast cancer (proband) tested for mutations in BRCA1 and BRCA2 than to have her own sample tested. If the proband did not have a BRCA1 or BRCA2 mutation, then the consultand would be at no different risk for breast cancer before or after she was tested. An appropriately trained health professional can help the consultand identify the best proband(s) for initial testing in order for the consultand to receive the best and most useful information from their genetic testing.2,3 Without this kind of health professional involvement and counseling, direct-to-consumer advertising may reinforce an erroneous deterministic interrelationship between genotype and phenotype.4 An appropriately trained health professional also would be able to advise the consultand regarding the actual information that may be available from the test, as well as its potential utility. The difficulties encountered in the interpretation of genetic test results are well-recognized.3 The public generally expects to receive a definitive, yes/no answer from a medical test. Results from genetic tests often inform only the estimation of the probability of developing a disease or the predictive risk of From the Departments of Human Genetics and Pediatrics, David Geffen School of Medicine at UCLA; UCLA Center for Society, the Individual and Genetics; and Mattel Children’s Hospital at UCLA, Los Angeles, California.