Newborn urine screening programme in the province of Quebec: an update of 30 years' experience

The MMACHC variant c.158T>C: Mild clinical and biochemical phenotypes and marked hydroxocobalamin response in cblC patients

Mutations in MMACHC cause cobalamin C disease (cblC, OMIM 277400), the commonest inborn error of vitamin B12 metabolism. In cblC, deficient activation of cobalamin results in methylcobalamin and adenosylcobalamin deficiency, elevating methylmalonic acid (MMA) and total plasma homocysteine (tHcy). We retrospectively reviewed the medical files of seven cblC patients: three compound heterozygotes for the MMACHC (NM_015506.3) missense variant c.158T>C p.(Leu53Pro) in trans with the common pathogenic mutation c.271dupA (p.(Arg91Lysfs*14), "compounds"), and four c.271dupA homozygotes ("homozygotes"). Compounds receiving hydroxocobalamin intramuscular injection monotherapy had age-appropriate psychomotor performance and normal ophthalmological examinations. In contrast, c.271dupA homozygotes showed marked psychomotor retardation, retinopathy and feeding problems despite penta-therapy (hydroxocobalamin, betaine, folinic acid, l-carnitine and acetylsalicylic acid). Pretreatment levels of plasma and urine MMA and tHcy were higher in c.271dupA homozygotes than in compounds. Under treatment, levels of the compounds approached or entered the reference range but not those of c.271dupA homozygotes (tHcy: compounds 9.8-32.9 μM, homozygotes 41.6-106.8 (normal (N) < 14); plasma MMA: compounds 0.14-0.81 μM, homozygotes, 10.4-61 (N < 0.4); urine MMA: compounds 1.75-48 mmol/mol creatinine, homozygotes 143-493 (N < 10)). Patient skin fibroblasts all had low cobalamin uptake, but this was milder in compound cells. Also, the distribution pattern of cobalamin species was qualitatively different between cells from compounds and from homozygotes. Compared to the classic cblC phenotype presented by c.271dupA homozygous patients, c.[158T>C];[271dupA] compounds had mild clinical and biochemical phenotypes and responded strikingly to hydroxocobalamin monotherapy.

A case of hyperlysinemia identified by urine newborn screening

Hyperlysinemia is a rare autosomal recessive deficiency of 2-aminoadipic semialdehyde synthase (AASS) affecting the initial step in lysine degradation. It is thought to be a benign biochemical abnormality, but reports on cases remain scarce. The description of additional cases, in particular, those identified without ascertainment bias, may help counseling of new cases in the future. It may also help to establish the risks associated with pharmacological inhibition of AASS, a potential therapeutic strategy that is under investigation for other inborn errors of lysine degradation. We describe the identification of a hyperlysinemia case identified in the Provincial Neonatal Urine Screening Program in Sherbrooke, Quebec. This case presented with a profile of cystinuria but with a very high increase in urinary lysine. A diagnosis of hyperlysinemia was confirmed through biochemical testing and the identification of biallelic variants in AASS. The p.R146W and p.T371I variants are novel and affect the folding of the lysine-2-oxoglutarate domain of AASS. The 11-month-old boy is currently doing well without any therapeutic interventions. The identification of this case through newborn urine screening further establishes that hyperlysinemia is a biochemical abnormality with limited clinical consequences and may not require any intervention.

Methylmalonic acid analysis using urine filter paper samples to screen for metabolic vitamin B12 deficiency in older adults

Aim: Methylmalonic acid (MMA) analysis in urine represents a noninvasive approach to screening for vitamin B₁₂ deficiency in older adults. A method allowing the analysis of MMA/creatinine in fasting urine collected on filter paper was developed/validated. Method: Dry urine specimens were eluted using a solution containing internal standards, filtrated and analyzed by ultra-performance LC-MS/MS. Results: The method allowed the chromatographic separation of MMA from succinic acid. Dried urine samples were stable for 86 days at room temperature. The MMA/creatinine ratios measured in urine collected on filter paper were highly correlated with values derived from the corresponding liquid specimens. Conclusion: This robust filter paper method might greatly improve the accessibility and cost-effectiveness of vitamin B₁₂ deficiency screening in older adults.

Prevalence of methylmalonic acidemia among newborns and the clinical-suspected population: a meta-analyse

IMPORTANCE

Knowing the scale of rare inborn errors is important for screening and resource allocation. Evidence on the prevalence of methylmalonic acidemia (MMA) among newborns and the clinical-suspected population from large-scale screening programs needs to be systematically synthesized.

OBJECTIVE

To estimate the worldwide prevalence of MMA for newborns and the clinical-suspected population and explore the differences in different regions, periods, and diagnostic technologies.

DATA SOURCES

MEDLINE, Embase, CRD, Cochrane Library, Scopus, CINAHL, and PROSPERO. Study Selection: All studies reporting the epidemiology characteristics of MMA were selected.

DATA EXTRACTION AND SYNTHESIS

Characteristics of study, subjects, and epidemiology were extracted, random-effect models were used for meta-analyses.

MAIN OUTCOME AND MEASURE

Pooled prevalence of MMA.

RESULTS

This study included 111 studies. The pooled prevalence of MMA worldwide was 1.14 per 100,000 newborns (1516/190,229,777 newborns, 95% CI: 0.99-1.29) and 652.11 per 100,000 clinical-suspected patients (1360/4,805,665 clinical-suspected individuals, CI: 544.14-760.07). Asia and Africa got a higher pooled prevalence of MMA. The prevalence of MMA in newborns increased through the years, while that in the clinical-suspected population decreased. Collecting blood ≥ 72 h after birth had a higher pooled prevalence of MMA than collecting during 24 h-72 h after birth. The combining-use of MS/MS and GC/MS had a higher pooled prevalence than the single-use of MS/MS or GC/MS. Prevalence of cbl C, mut, cbl B, cbl A, isolated MMA, combined MMA and homocystinuria, vitamin B12-responsive MMA was synthesized.

CONCLUSIONS AND RELEVANCE

Prevalence of MMA among newborns was extremely low, but considerably high in the clinical-suspected population, indicating the need for more efficient newborn screening strategies and closer monitoring of the high-risk population for the early signs of MMA. Asia and Africa should attach importance to the high prevalence of MMA. Further diagnostic tests were recommended for the combining-use vs single-use of MS/MS and GC/MS and for collecting blood after 72 h vs during 24-72 h after birth.

Neonatal Urine Screening Program in the Province of Quebec: Technological Upgrade from Thin Layer Chromatography to Tandem Mass Spectrometry

The Quebec Neonatal Urine Screening Program was initiated in 1971 with overall screening inception of newborns in 1973. Forty-seven years later, over 3.5 million babies have been screened for up to 25 inborn errors of metabolism divided into two groups: (1) urea cycle disorders and organic acidurias; and (2) disorders of amino acid metabolism and transport. The main goal of this preventive genetic medicine program is the detection of treatable diseases before the onset of clinical symptoms. Urine specimens from 21-day-old babies are collected and dried on filter paper by parents at home. The participation is voluntary with a high compliance rate over the years (~90%). Specimens are analyzed by thin layer chromatography (TLC). The main objective of this evaluative research project was to assess the feasibility of a technological upgrade towards mass spectrometry. A 2.85-min flow injection method was devised, normal values established, and abnormal profiles confirmed using second-tier tests. The validated assays are sensitive, specific, and suitable for populational screening, as well as for high-risk screening laboratories. Triple H syndrome, which would not be detected in newborns by blood screening at two days of age was found to be positive in the urine of an affected patient.

Amino Acid Transport Defects in Human Inherited Metabolic Disorders

Amino acid transporters play very important roles in nutrient uptake, neurotransmitter recycling, protein synthesis, gene expression, cell redox balance, cell signaling, and regulation of cell volume. With regard to transporters that are closely connected to metabolism, amino acid transporter-associated diseases are linked to metabolic disorders, particularly when they involve different organs, cell types, or cell compartments. To date, 65 different human solute carrier (SLC) families and more than 400 transporter genes have been identified, including 11 that are known to include amino acid transporters. This review intends to summarize and update all the conditions in which a strong association has been found between an amino acid transporter and an inherited metabolic disorder. Many of these inherited disorders have been identified in recent years. In this work, the physiological functions of amino acid transporters will be described by the inherited diseases that arise from transporter impairment. The pathogenesis, clinical phenotype, laboratory findings, diagnosis, genetics, and treatment of these disorders are also briefly described. Appropriate clinical and diagnostic characterization of the underlying molecular defect may give patients the opportunity to avail themselves of appropriate therapeutic options in the future.

Neonatal Mass Urine Screening Approach for Early Detection of Mucopolysaccharidoses by UPLC-MS/MS

Mucopolysaccharidoses (MPSs) are lysosomal storage disorders caused by deficiencies of enzymes involved in the catabolism of glycosaminoglycans (GAGs). Various treatments such as enzyme replacement therapy and/or hematopoietic stem cell transplant are available for MPSs. Early initiation of treatment improves the outcome and delays the onset of symptoms, highlighting the need for newborn screening for MPSs. The main objective of this project was to devise and validate a multiplex urine filter paper method for GAG analysis using a tandem mass spectrometry (MS/MS) approach to screen newborns for MPSs. Eluted urine samples from 21-day-old newborns were evaporated and a methanolysis reaction was performed. Samples were resuspended and analyzed using a UPLC-MS/MS system. A one-minute chromatographic method allowed the absolute quantification of heparan sulfate (HS), dermatan sulfate (DS), and creatinine. Method validation revealed high precision (< 9% relative standard deviation) and accuracy (< 7% bias) for all analytes. The reference values normalized to creatinine obtained by the analysis of five hundred 21-day-old newborn urine samples were 34.6 +/-6.2 mg/mmol of creatinine and 17.3 +/-3.9 mg/mmol of creatinine for HS and DS, respectively. We present a rapid and efficient method for populational newborn urine screening using MS/MS, which could also be applied to high-risk screening.

Systematic literature review and meta-analysis on the epidemiology of methylmalonic acidemia (MMA) with a focus on MMA caused by methylmalonyl-CoA mutase (mut) deficiency

Methylmalonic acidemia/aciduria (MMA) is a genetically heterogeneous group of inherited metabolic disorders biochemically characterized by the accumulation of methylmalonic acid. Isolated MMA is primarily caused by the deficiency of methylmalonyl-CoA mutase (MMA mut; EC 5.4.99.2). A systematic literature review and a meta-analysis were undertaken to assess and compile published epidemiological data on MMA with a focus on the MMA mut subtype (OMIM #251000). Of the 1114 identified records, 227 papers were assessed for eligibility in full text, 48 articles reported on disease epidemiology, and 39 articles were included into the quantitative synthesis. Implementation of newborn screening in various countries has allowed for the estimation of birth prevalence of MMA and its isolated form. Meta-analysis pooled point estimates of MMA (all types) detection rates were 0.79, 1.12, 1.22 and 6.04 per 100,000 newborns in Asia-Pacific, Europe, North America and the Middle East and North Africa (MENA) regions, respectively. The detection rate of isolated MMA was < 1 per 100,000 newborns in all regions with the exception of MENA where it approached 6 per 100,000 newborns. Few studies published data on the epidemiology of MMA mut, therefore no meta-analysis could have been performed on this subtype. Most of the identified papers reported birth prevalence estimates below 1 per 100,000 newborns for MMA mut. The systematic literature review clearly demonstrates that MMA and its subtypes are ultra-rare disorders.

High-Risk Screening for Fabry Disease: Analysis by Tandem Mass Spectrometry of Globotriaosylceramide (Gb3 ) in Urine Collected on Filter Paper

Fabry disease is a complex, panethnic lysosomal storage disorder. It is characterized by the accumulation of glycosphingolipids in tissues, organs, the vascular endothelium, and biological fluids. The reported incidence in different populations is quite variable, ranging from 1:1400 to 1:117,000. Its complexity lies in the marked genotypic and phenotypic heterogeneity. Despite the fact that it is an X-linked disease, more than 600 mutations affect both males and females. In fact, some females may be affected as severely as males. The purpose of this protocol is to focus on the high-risk screening of patients who might have Fabry disease using a simple, rapid, non-invasive high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) method for urinary globotriaosylceramide (Gb₃ ) analysis. Urine filter paper samples are easily collected at home by patients and sent by regular mail. This method has been successfully used for high-risk screening of patients with ophthalmologic manifestations and in an on-going study for high-risk screening of Fabry disease in patients with chronic kidney diseases. © 2017 by John Wiley & Sons, Inc.

Home-Based Screening for Biliary Atresia Using Infant Stool Color Cards in Canada: Quebec Feasibility Study

OBJECTIVES

Biliary atresia (BA) is a leading cause of liver failure and liver transplantation in pediatrics. BA manifests by 3 weeks of life with jaundice and pale stools. Delayed diagnosis and surgical intervention with Kasai portoenterostomy after 3 months of age is significantly associated with poor prognosis for native liver survival. A national Taiwan infant stool color card (SCC) screening program has entirely eliminated late Kasai portoenterostomy >90 days of age and improved native liver survival. A recent large-scale prospective cohort study in British Columbia, Canada, indicated that distribution of SCC on the maternity ward was feasible, led to high utilization rate, and was cost-effective. The aim of the present study was to assess the generalizability of this screening strategy in another Canadian jurisdiction with a different sociodemographic profile.

METHODS

An SCC was distributed to families of newborns discharged at St Mary's Hospital Center, Montreal, Quebec. Families were instructed to monitor their infant's stool color for 21 days and then complete and mail the SCC to the study center. Phone surveys to families who did not return cards were used to estimate total card utilization rate.

RESULTS

Two thousand two hundred forty-six infants were eligible for inclusion; 99.9% were enrolled. Mail SCC return rate was 63.3%. No cases of BA were identified. All of the 118 families who completed the phone survey reported that they had utilized the SCC. Conservative and optimistic estimates for total card utilization rates were 82% and 100%, respectively.

CONCLUSIONS

The high enrollment and utilization rates in this screening study strongly support the feasibility of implementing a Canadian SCC screening program to improve outcomes of children with BA.

Mutations in the sarcosine dehydrogenase gene in patients with sarcosinemia

Sarcosinemia is an autosomal recessive metabolic trait manifested by relatively high concentrations of sarcosine in blood and urine. Sarcosine is a key intermediate in 1-carbon metabolism and under normal circumstances is converted to glycine by the enzyme sarcosine dehydrogenase. We encountered six families from two different descents (French and Arab), each with at least one individual with elevated levels of sarcosine in blood and urine. Using the "candidate gene approach" we sequenced the gene encoding sarcosine dehydrogenase (SARDH), which plays an important role in the conversion of sarcosine to glycine, and found four different mutations (P287L, V71F, R723X, R514X) in three patients. In an additional patient, we found a uniparental disomy in the region of SARDH gene. In two other patients, we did not find any mutations in this gene. We have shown for the first time that mutations in the SARDH gene are associated with sarcosinemia. In addition, our results indicate that other genes are most probably involved in the pathogenesis of this condition.

Research use of leftover newborn bloodspots: attitudes of Canadian geneticists regarding storage and informed consent requirements

PURPOSE

Leftover newborn spots can provide a powerful research tool as a population-wide DNA bank. Some provinces/states store them for more than 20 years; however, parents are usually not informed of the retention of leftover newborn spots. To examine the opinions of Canadian geneticists regarding permission for leftover newborn spots storage for research purposes and the associated risks, a web-based survey was distributed to all members of the Canadian College of Medical Geneticists with a valid e-mail address (n = 209) and completed by 78 respondents (37%).

RESULTS

The majority of respondents (73%) favored opt-out notification for retention of samples that would be held for longer than 2 years. For research on multifactorial conditions using leftover newborn spots originally banked without parental permission, geneticists favored different types of permission depending on the level of identifiable information attached to samples. Thirty-eight percent were concerned that information pamphlets that state that leftover newborn spots will be stored and may be "a source of DNA for research" would lead to a decreased participation in newborn screening. Twenty-eight percent believed that group stigma or family anxiety was likely to result from using nonidentified leftover newborn spots to study multifactorial conditions.

CONCLUSION

The concerns of this knowledgeable cohort supports the critical importance of public engagement about both the potential risks and societal benefits associated with the use of leftover newborn spots in research as policy for leftover newborn spots is developed.

Newborn screening in Canada - Are we out of step?

OBJECTIVES

To review the status of universal newborn screening programs in Canada.

METHODS

A brief questionnaire (seven questions) was circulated to one key individual in each province (n=10) and territory (n=3). These individuals were usually physicians or clinical biochemists closely involved in the diagnosis and treatment of genetic metabolic diseases.

RESULTS

Universal newborn screening is under provincial jurisdiction. The number of diseases screened for varies and ranges from three to 28. Nine provinces/territories have a central computerized system for tracking initially positive cases. Only five provinces/territories have adequate personnel and resources for follow-up and treatment. Treatment costs are only partially covered in most jurisdictions. Only five provinces/territories have formal advisory committees with official mandates. Expensive, restricted access treatment products for adults with inherited metabolic diseases are only fully available in six provinces/territories. There is very limited access to these products in an additional four provinces/territories. To date, specific informed consent for newborn screening is not required in any province or territory.

CONCLUSIONS

Canada is far behind the rest of the developed (and some 'emerging') countries of the world in the field of universal newborn screening. New strategies for advocating expanded screening, follow-up and (long-term) payment of treatment costs on behalf of the potentially affected infants and their families must be devised, and such initiatives should include participation from the new Public Health Agency of Canada.

Loss-of-function mutations in the glutamate transporter SLC1A1 cause human dicarboxylic aminoaciduria

Solute carrier family 1, member 1 (SLC1A1; also known as EAAT3 and EAAC1) is the major epithelial transporter of glutamate and aspartate in the kidneys and intestines of rodents. Within the brain, SLC1A1 serves as the predominant neuronal glutamate transporter and buffers the synaptic release of the excitatory neurotransmitter glutamate within the interneuronal synaptic cleft. Recent studies have also revealed that polymorphisms in SLC1A1 are associated with obsessive-compulsive disorder (OCD) in early-onset patient cohorts. Here we report that SLC1A1 mutations leading to substitution of arginine to tryptophan at position 445 (R445W) and deletion of isoleucine at position 395 (I395del) cause human dicarboxylic aminoaciduria, an autosomal recessive disorder of urinary glutamate and aspartate transport that can be associated with mental retardation. These mutations of conserved residues impeded or abrogated glutamate and cysteine transport by SLC1A1 and led to near-absent surface expression in a canine kidney cell line. These findings provide evidence that SLC1A1 is the major renal transporter of glutamate and aspartate in humans and implicate SLC1A1 in the pathogenesis of some neurological disorders.

Treatment of cobalamin C (cblC) deficiency during pregnancy

OBJECTIVE

To report the successful pregnancy of a woman with methylmalonic acidemia and hyperhomocysteinemia, cblC type [cobalamin C (cblC) deficiency] (MIM 277400).

METHOD

Retrospective chart review.

RESULTS

A 24-year-old woman presented at 14 weeks gestation with nausea, self-restricted protein diet, and weight loss. She had a past history of asymptomatic methylmalonic acidemia but had been lost to follow-up since the age of 15 years. Biochemical evaluation revealed combined methylmalonic acidemia and hyperhomocysteinemia. Complementation analysis confirmed cblC deficiency. One copy of the most common mutations in the MMACHC gene, c.271dupA, was identified. The women was treated from 15 weeks of gestation with a low protein diet (64 g/day) (1.1 g /kg of weight/day), L-carnitine (1 g per os 3 times daily to 3 g per os 3 times daily in the third trimester), aspirin (salicylic acid) 80 mg per day, folic acid 5 mg per day, and hydroxocobalamin 1 mg intramuscular every week to two times per week in the third trimester. The pregnancy was uneventful and the delivery at term. The newborn was healthy at delivery and at follow-up.

CONCLUSION

We report on the successful outcome of pregnancy in a treated woman with cblC disease. The pregnancy was uneventful for both fetus and mother with the delivery of a term healthy boy. There is a need for an international registry on the management and outcomes of pregnancy in women with inborn errors of metabolism.

Experience with the treatment of argininosuccinic aciduria during pregnancy

We present the details of the management and the outcome of a pregnancy of a woman affected with argininosuccinic aciduria. Management with a closely monitored, protein-restricted diet, supplemented with L-arginine, resulted in the birth of a healthy infant boy and an uneventful perinatal course for the mother.

Fabry disease urinary globotriaosylceramide/creatinine biomarker evaluation by liquid chromatography-tandem mass spectrometry in healthy infants from birth to 6 months

Fabry disease is an X-linked lysosomal storage disorder caused by deficiency of alpha-galactosidase A, resulting in accumulation of the principal substrate, globotriaosylceramide (Gb(3)), in various physiological fluids and tissues in affected patients. The recognition that accumulation of Gb(3) begins in utero, combined with the fact that the diagnosis of the disease is often delayed until after the development of irreversible tissue damage, has generated pressure to develop techniques for the early, pre-symptomatic diagnosis of the disease. Measurements of urinary Gb(3) have been shown to be useful for the diagnosis of Fabry disease in adults. The objective of this work was to measure the Gb(3)/creatinine biomarker in urine of healthy infants from birth to 6 months, including the establishment of reference ranges for urinary Gb(3) excretion at various postnatal ages, in male and female infants. We employed liquid chromatography-tandem mass spectrometry (LC-MS/MS) to determine Gb(3)/creatinine ratios in urine specimens dried on filter paper and mailed to the laboratory by participating parents. A total of 728 urine specimens were obtained at intervals from birth to 6 months of age from 68 healthy infants (35 male and 33 female). Parental participation was good, with 90% of the expected specimens received by the laboratory. The results of the analyses were grouped by the age of the infants into four periods. We have determined that both postnatal age and sex have an effect on urinary Gb(3) excretion levels which vary considerably in newborns. We conclude that screening for Fabry disease by measurement of urinary Gb(3) excretion is unlikely to be reliable before 30 days of age.

Iminoglycinuria and hyperglycinuria are discrete human phenotypes resulting from complex mutations in proline and glycine transporters

Iminoglycinuria (IG) is an autosomal recessive abnormality of renal transport of glycine and the imino acids proline and hydroxyproline, but the specific genetic defect(s) have not been determined. Similarly, although the related disorder hyperglycinuria (HG) without iminoaciduria has been attributed to heterozygosity of a putative defective glycine, proline, and hydroxyproline transporter, confirming the underlying genetic defect(s) has been difficult. Here we applied a candidate gene sequencing approach in 7 families first identified through newborn IG screening programs. Both inheritance and functional studies identified the gene encoding the proton amino acid transporter SLC36A2 (PAT2) as the major gene responsible for IG in these families, and its inheritance was consistent with a classical semidominant pattern in which 2 inherited nonfunctional alleles conferred the IG phenotype, while 1 nonfunctional allele was sufficient to confer the HG phenotype. Mutations in SLC36A2 that retained residual transport activity resulted in the IG phenotype when combined with mutations in the gene encoding the imino acid transporter SLC6A20 (IMINO). Additional mutations were identified in the genes encoding the putative glycine transporter SLC6A18 (XT2) and the neutral amino acid transporter SLC6A19 (B0AT1) in families with either IG or HG, suggesting that mutations in the genes encoding these transporters may also contribute to these phenotypes. In summary, although recognized as apparently simple Mendelian disorders, IG and HG exhibit complex molecular explanations depending on a major gene and accompanying modifier genes.

Iminoglycinuria and hyperglycinuria are discrete human phenotypes resulting from complex mutations in proline and glycine transporters

Iminoglycinuria (IG) is an autosomal recessive abnormality of renal transport of glycine and the imino acids proline and hydroxyproline, but the specific genetic defect(s) have not been determined. Similarly, although the related disorder hyperglycinuria (HG) without iminoaciduria has been attributed to heterozygosity of a putative defective glycine, proline, and hydroxyproline transporter, confirming the underlying genetic defect(s) has been difficult. Here we applied a candidate gene sequencing approach in 7 families first identified through newborn IG screening programs. Both inheritance and functional studies identified the gene encoding the proton amino acid transporter SLC36A2 (PAT2) as the major gene responsible for IG in these families, and its inheritance was consistent with a classical semidominant pattern in which 2 inherited nonfunctional alleles conferred the IG phenotype, while 1 nonfunctional allele was sufficient to confer the HG phenotype. Mutations in SLC36A2 that retained residual transport activity resulted in the IG phenotype when combined with mutations in the gene encoding the imino acid transporter SLC6A20 (IMINO). Additional mutations were identified in the genes encoding the putative glycine transporter SLC6A18 (XT2) and the neutral amino acid transporter SLC6A19 (B0AT1) in families with either IG or HG, suggesting that mutations in the genes encoding these transporters may also contribute to these phenotypes. In summary, although recognized as apparently simple Mendelian disorders, IG and HG exhibit complex molecular explanations depending on a major gene and accompanying modifier genes.

Apical transporters for neutral amino acids: physiology and pathophysiology

Absorption of amino acids in kidney and intestine involves a variety of transporters for different groups of amino acids. This is illustrated by inherited disorders of amino acid absorption, such as Hartnup disorder, cystinuria, iminoglycinuria, dicarboxylic aminoaciduria, and lysinuric protein intolerance, affecting separate groups of amino acids. Recent advances in the molecular identification of apical neutral amino acid transporters has shed a light on the molecular basis of Hartnup disorder and iminoglycinuria.

Persistence of the Common Hartnup Disease D173N Allele in Populations of European Origin

Hartnup disorder is an aminoaciduria that results from mutations in the recently described gene SLC6A19 on chromosome 5p15.33. The disease is inherited in a simple recessive manner and ten different mutations have been described to date. One mutation, the D173N allele, is present in 42% of Hartnup chromosomes from apparently unrelated families from both Australia and North America. We report an investigation of the origins of the D173N allele using a unique combination of variants including SNPs, microsatellites, and a VNTR across 211 Kb spanning the SLC6A19 locus. All individuals who carry the mutant allele share an identical core haplotype suggesting a single common ancestor, indicating that the elevated frequency of the D173N allele is not a result of recurrent mutation. Analyses of these data indicate that the allele is more than 1000 years old. We compare the reasons for survival of this allele with other major alleles in some other common autosomal recessive diseases occurring in European Caucasians. We postulate that survival of this allele may be a consequence of failure of the allele to completely inactivate the transport of neutral amino acids.

Newborn screening in North America

Newborn screening in North America dates to the early work of Bob Guthrie in the USA. Screening programmes in both the USA and Canada began in the early 1960s, with documented programmes in both countries as early as 1962. Throughout the 1960s and 1970s, many of the screening tests that later became part of routine screening around the world were developed in US and Canadian laboratories, including tests for phenylketonuria, other inborn errors of metabolism, congenital hypothyroidism, congenital adrenal hyperplasia, and haemoglobinopathies. An automated punching machine developed in the USA facilitated screening expansion by significantly reducing sample preparation time and effort. US and Canadian programmes were leaders in applying computerized data management to newborn screening in the 1980s. In the 1990s, DNA and tandem mass spectrometry testing protocols were developed in the USA and applied to newborn screening. US programmes have continually expanded over time, while most Canadian programmes have not. With impetus from private laboratories and professional and consumer groups, many US programmes now screen for more than 50 conditions and there is increased expansion activity in Canada. NBS research in the USA is focused on improving system efficiency and translating other genetic testing to NBS, particularly where new technologies and treatment therapies exist. Although national newborn screening policies do not exist in either Canada or the USA, there are intense efforts to provide uniform access to screening nationwide in both countries. New partnerships between health professionals, consumers and politicians are benefiting the overall screening systems in both countries.

Quebec neonatal mass urinary screening programme: from micromolecules to macromolecules

The Quebec Mass Urinary Screening Programme, initiated in 1971, has resulted in the screening of more than 2,500,000 newborns in the province of Quebec for 25 inherited Mendelian disorders divided into two groups. The first group concerns urea cycle disorders (citrullinaemia, hyperargininaemia, argininosuccinic aciduria), ketotic hyperglycinaemia, and organic acidurias (methylmalonic aciduria, glutaric aciduria type I, etc.); the second group relates to disorders of amino acid metabolism (cystathioninuria, prolidase deficiency, etc.) and transport (Fanconi syndrome, cystinurias, Hartnup syndrome, etc.). The main goal of the Programme is to detect and prevent these genetic diseases, some detectable only in urine, before the onset of clinical symptoms. A multiplex thin-layer chromatography methodology was developed, in which metabolites in urine are resolved and visualized by the sequential application of four different reagents to detect aminoacidopathies and organic acidurias. The technique is simple, reproducible, inexpensive and rapid, allowing the analysis of 500 samples daily by a single technician. The voluntary compliance of the parents is excellent, averaging 90% per year. Over the years, we have established a dynamic process, developing techniques or new reagents to detect as many treatable disorders as possible, now evaluating macromolecules associated with lysosomal storage disorders, mainly globotriaosylceramide (Gb3) for Fabry disease. We present here the methodology, infrastructure in place, results and recent statistics of the well-established Quebec Mass Urinary Screening Programme. We also report a study by tandem mass spectrometric analysis of urinary Gb3 in Fabry disease for the follow-up and monitoring of Fabry patients, as well as for its possible application to mass and high-risk screening programmes.