Linda L. McCabe

DAX1 Mutations Map to Putative Structural Domains in a Deduced Three-Dimensional Model

The DAX1 protein is an orphan nuclear hormone receptor based on sequence similarity in the putative ligand-binding domain (LBD). DAX1 mutations result in X-linked adrenal hypoplasia congenita (AHC). Our objective was to identify DAX1 mutations in a series of families, to determine the types of mutations resulting in AHC and to locate single-amino-acid changes in a DAX1 structural model. The 14 new mutations identified among our 17 families with AHC brought the total number of families with AHC to 48 and the number of reported mutations to 42; 1 family showed gonadal mosaicism. These mutations included 23 frameshift, 12 nonsense, and six missense mutations and one single-codon deletion. We mapped the seven single-amino-acid changes to a homology model constructed by use of the three-dimensional crystal structures of the thyroid-hormone receptor and retinoid X receptor alpha. All single-amino-acid changes mapped to the C-terminal half of the DAX1 protein, in the conserved hydrophobic core of the putative LBD, and none affected residues expected to interact directly with a ligand. We conclude that most genetic alterations in DAX1 are frameshift or nonsense mutations and speculate that the codon deletion and missense mutations give insight into the structure and function of DAX1.

Newborn screening: rationale for a comprehensive, fully integrated public health system

Newborn screening has existed for approximately four decades. During that period of time, newborn screening has evolved conceptually from a laboratory test for a single disorder, phenylketonuria (PKU), to a multi-part public health system involving education, screening, diagnostic follow-up, treatment/management, and system evaluation. At a time when newborn screening is recognized as a model for predictive medicine, it also faces critical challenges that will determine its future credibility and viability. In order to understand these challenges, it is helpful to review briefly the history of newborn screening.

Expanded Newborn Screening: Implications for Genomic Medicine

Newborn screening (NBS) represents the largest volume of genetic testing. The 45-year history of NBS has demonstrated its benefits, as well as the importance of an evidence base. The recent addition of tandem mass spectrometry (MS/MS) resulted in a fivefold increase in the number of tests. Experience with MS/MS also showed that laboratory tests are just one part of the NBS system. The lessons learned from NBS will provide important insights as we move into the predictive, preventive, and personalized era of genomic medicine.

Newborn screening as a model for population screening

1. IntroductionThe incorporation of newborn screening into thestates’ departments of public health represented theformal initiation of population-based predictive medi-cine with the goal of prevention of morbidityandmortalityfrom genetic disease. Starting with a meth-odologybasedonmicrobiology,thebacterialinhibitionassay[1],thefieldhasincorporatedfarmoreadvancedtechnologies such as molecular genetic analyses [2–7]and tandem mass spectrometry(MS/MS) [8–12]. Pre-dictionandpreventionarefundamentaltopublichealthandgenomicmedicine.Effectiveintegrationofscreeningacrossthelifespanforgeneticpredispositionanddiseasewill require utilization of advanced technologies andinformation systems, and incorporation of appropriatesafeguardstoprotectautonomy,privacyandconfiden-tiality.As newborn screening testing menus expand andpopulation-based genetic screening extends throughadulthood, pilot programs must be developed andevaluatedtodeterminetheirabilitytoachievethegoalsand objectives for which theyare designed [13]. Suchpilot programs will require careful design, not onlytoassessthetechnologicalbasesofthesesystems,butalsoto evaluate their impact on the individuals tested. De-terminationofimpactmustincludenotonlytraditionaleffectivenessmeasuressuchaschangesinmorbidityandmortalityfrom disease, but also the ethical, legal andsocial implications of testing such as genetic discrimi-nation[14–18].Design and implementation of genetic screeningprogramswillrequirebroad-basedexpertise.Inadditionto the traditional membership on screening programadvisorycommittees, such as medical specialists andsubspecialists, laboratorians, and public health andgovernment officials, these groups must represent allstakeholders [13]. Affected individuals and their familymembers have the best-informed experience with theprocessandconsequencesofscreening,andtheyshouldbeincorporatedasfullandequalmembers.1.1.RoleofscreeninginpublichealthThe goal of screening has been to identifydiseasebefore irreparable damage is done, to determine indi-viduals whose offspring are at risk for disease, and toevaluate new screening strategies and their potentialimpacts [19–21]. Implementation of these principleswithin a public health paradigm means that theywilltranslate to presymptomatic identification of affectedindividuals, recognition of carriers of genetic diseases,andperformanceofresearchtoimprovescreeningpro-grams.Therefore,screeningforgeneticdiseasewillfulfillthe public health need of providing an ever-evolving‘‘safety-net’’topredictandpreventdisease.While screening mayinvolve clinical evaluation ofindividuals, here we will be concerned primarilywithlaboratory-based screening programs. By their verynature, such programs will be driven byadvances inlaboratorytechnology. Until veryrecentlypopulation-based screening programs, such as neonatal screening,have relied on biochemical measurements of analytesthathaveincluded:metabolites(e.g.,phenylalanineforphenylketonuria,orgalatoseandgalactose1-phosphatefor galactosemia); hormones (e.g., thyroid stimulatinghormoneand thyroid hormone forhypothyroidism, or17-hydroxyprogesterone for 21-hydroxylase deficientcongenital adrenal hyperplasia); and proteins (e.g.,hemoglobin for hemoglobinopathies, galactose 1-phos-phateuridyltransferaseforgalactosemia,orimmunore-activetrypsinogen(IRT)forcysticfibrosis(CF))[21,22].Beginninginthe1980s, thetools ofmolecular genetics

Postgenomic medicine: Presymptomatic testing for prediction and prevention

Significant changes are occurring in genetic screening paradigms. Genetic screening is moving from traditional analytes, such as small molecules and proteins, to molecular genetic testing involving DNA and RNA. There are significant consequences to these changes, involving issues for the family unit, such as misattribution of parentage, and concerns regarding discrimination, confidentiality, and privacy. Although these latter issues have broader concerns for medicine and medical information, in the context of genetic testing, information derived from one individual can have a significant impact on others within their family. Screening is also changing from mendelian disease ascertainment to predictive testing. Issues that arise involve appropriate age at testing for adult-onset disorders, the clinical validity and clinical use of genetic testing for complex diseases, and the efficacy of interventions following genetic testing. We are also learning that the phenotypes of even simple mendelian disorders are influenced by complex genetic and environmental factors. The observations that genotypes rarely predict phenotypes absolutely have significant ramifications for counseling based on mutation analysis, for example in neonates who have not yet manifested symptoms and in older children and in adults undergoing predictive testing. Molecular genetic testing often proceeds rapidly from the research laboratory to the clinical setting. We must recognize that for single-gene disorders with high penetrance, the information derived from such testing may be relatively easy to interpret and apply. For complex diseases, however, the populations studied and their demographic characteristics are extremely important for extrapolation to counseling of individual patients. The value of population-based predictive testing is exemplified by newborn screening. It is clear that the Human Genome Project, and the information and technologies from it, will have a much broader impact on public health by presymptomatic prediction and prevention of disease.