Progression of organ manifestations upon enzyme replacement therapy in a patient with mucopolysaccharidosis type I/Hurler

BACKGROUND

Enzyme replacement therapy (ERT) has been increasingly used as an interim treatment in severe mucopolysaccharidosis type I (MPSI)/Hurler patients prior to hematopoietic stem cell transplantation (HSCT).

METHODS

We present the outcome of a patient with MPSI/Hurler after 14 months of ERT prior to HSCT.

RESULTS

Urinary glucosaminoglycan excretion decreased by 70% after one month of ERT. Liver volume decreased by 14% of baseline after 12 months of ERT. Pre-existing thoracolumbar kyphosis progressed to thoracolumbar dislocation with complete displacement of facets after 12 months of ERT. New development of mitral valve thickening was found by echocardiography and mild hearing loss progressed to severe sensorineural hearing loss after 13 months of ERT.

CONCLUSIONS

ERT over a period of 14 months did not prevent progression of organ manifestations in our patient. Patients should be monitored every 6 months for cardiac, skeletal and audiological involvement on ERT.

Intracranial calcification after cord blood neonatal transplantation for krabbe disease

Infantile-onset Krabbe disease results from a deficiency of the lysosomal enzyme galactocerebrosidase and leads to death from profound central and peripheral demyelination. Neonatal hematopoietic cell transplantation may result in near-normal cognitive development and partial rescue of gross motor development. The long-term course of the disorder for treated patients seems to involve slowly progressive neurological impairment. We describe the detailed 3-year outcomes of this experimental procedure using umbilical cord blood in a prenatally-diagnosed newborn with Krabbe disease. Substantial perivascular calcifications and atrophy of the white matter developed in the first year post-transplantation. Despite persistent neuroradiological and electrophysiological evidence of leukodystrophy, at age 3 years she has had only mildly impaired non-motor development and moderately impaired motor skills. The cause of these severe white matter changes may have been due to ongoing Krabbe disease or to effects of the chemotherapy regimen or to an interaction of these factors. Extended long-term follow-up of children neonatally transplanted for Krabbe disease is needed before the full utility and limitations of neonatal transplantation can be determined.

Mutation detection in DNA isolated from cerebrospinal fluid and urine: Clinical utility and pitfalls of multiple displacement amplification

This study describes the use of cerebral spinal fluid (CSF) and/or urine as source of DNA for mutation analysis combined with multiple displacement amplification. The findings illustrate the opportunities and pitfalls of these methods in the search for identification of the pathogenic mutations in the case that only scarce material is available such as CSF.

The paradox of the carnitine palmitoyltransferase type Ia P479L variant in Canadian Aboriginal populations

Investigation of seven patients from three families suspected of a fatty acid oxidation defect showed mean CPT-I enzyme activity of 5.9+/-4.9 percent of normal controls. The families, two Inuit, one First Nation, live in areas of Canada geographically very distant from each other. The CPT1 and CPT2 genes were fully sequenced in 5 of the patients. All were homozygous for the same P479L mutation in a highly conserved region of the CPT1 gene. Two patients from the first family were also homozygous for the CPT2 F352C polymorphism in the CPT2 gene. Genotyping the patients and their family members confirmed that all seven patients were homozygous for the P479L variant allele in the CPT1 gene, as were 27 of 32 family members. Three of the seven patients and two cousins had hypoketotic hypoglycemia attributable to CPT-Ia deficiency, but adults homozygous for the variant denied hypoglycemia. We screened 422 consecutive newborns from the region of one of the Inuit families for this variant; 294 were homozygous, 103 heterozygous, and only 25 homozygous normal; thus the frequency of this variant allele is 0.81. There was an infant death in one family and at least 10 more deaths in those infants (7 homozygous, 3 heterozygous) consecutively tested for the mutation at birth. Thus there is an astonishingly high frequency of CPT1 P479L variant and, judging from the enzyme analysis in the seven patients, also CPT-I deficiency in the areas of Canada inhabited by these families. Despite the deficiency of CPT-Ia which is the major rate-limiting enzyme for long chain fatty acid oxidation, clinical effects, with few exceptions, were slight or absent. One clue to explaining this paradox is that, judging from the fatty acid oxidation studies in whole blood and fibroblasts, the low residual activity of CPT-Ia is sufficient to allow a reasonable flux through the mitochondrial oxidation system. It is likely that the P479L variant is of ancient origin and presumably its preservation must have conveyed some advantage.

Two mtDNA mutations 14487T>C (M63V, ND6) and 12297T>C (tRNA Leu) in a Leigh syndrome family

Mitochondrial cytopathies are characterized by a large variability of clinical phenotypes and severity. The 14487T>C mutation in mtDNA has been recently described to be associated with Leigh syndrome. The 12297T>C mutation has been described in isolated dilated cardiomyopathy patients. Here, we report a family with multiple members who harbor both mutations, with only a few individuals who are affected with Leigh syndrome. Mitochondrial whole genome sequencing analysis in the proband's muscle specimen detected two nearly homoplasmic mutations: 14487T>C (M63V in ND6) and 12297T>C in the tRNA (Leu) (CUN) gene. These two mutations were also detected in the blood, urine sediments, hair follicles, and buccal swab samples of all matrilineal relatives tested. All individuals tested were nearly homoplasmic for the 12297T>C mutation, but had variable degrees of heteroplasmy for 14487T>C. We also screened for the frequency of these two mutations. Of 268 patients with Leigh or Leigh-like disease, one case was found to harbor the 14487T>C mutation (0.3%), and one had the 12297T>C mutation (0.3%). Neither mutation was detected in the 88 patients meeting MELAS syndrome criteria nor in the 56 patients with respiratory chain complex I or I+III deficiency. In conclusion, the 14487T>C mutation appears as the primary etiology of Leigh syndrome in this family, demonstrating the high level of heteroplasmy needed for a clinically significant phenotype with this mutation. The 12297T>C mutation was not associated with dilated cardiomyopathy for the family members who were clinically evaluated and who were shown by testing to be nearly homoplasmic for that mutation.

Newborn screening for MCAD deficiency: experience of the first three years in British Columbia, Canada

BACKGROUND

Medium Chain Acyl-CoA Dehydrogenase (MCAD) Deficiency is an autosomal recessive disorder of fatty acid oxidation, with potential fatal outcome. MCAD deficiency is diagnosed by acylcarnitine analysis on newborn screening blood spot cards by tandem mass spectrometry. Early diagnosis of MCAD and presymptomatic treatment can potentially reduce morbidity and mortality.

OBJECTIVES

To evaluate incidence, clinical outcome, biochemical and molecular phenotype of MCAD cases detected in the first three years of newborn screening in British Columbia (BC).

METHODS AND RESULTS

Medium chain length acylcarnitines, octanoylcarnitine (C8) and decanoylcarnitine (C10), were measured on newborn screening blood spot cards. Out of 121,000 live births, 17 newborns had C8 values above the screening cut-off of 0.38 umol/L. Ten newborns had elevated C8 on repeat cards and were investigated further. Both C8 and C8/C10 ratios remained abnormal in all confirmed MCAD cases. Positive predictive value of screening was 58% with no false negative results. Seven patients were homozygous for the common c.985A > G MCAD mutation and three others were compound heterozygous for the c.985A > G and a second mutation. Two novel mutations were identified (c.260T > C and c.382T > A). The estimated incidence of MCAD was approximately 1:12,000 live births. Upon frequent feeding and carnitine supplementation, none of the patients had metabolic crises or adverse outcomes.

CONCLUSION

Frequency of MCAD in BC is comparable to reports from other newborn screening programs. Persistence of elevated C8 levels and C8/C10 ratios in confirmed MCAD cases suggest that these are sensitive markers for newborn screening. Early detection and treatment have successfully prevented adverse health outcomes in patients with MCAD.

Rapidly progressive neurological deterioration in a child with Alpers syndrome exhibiting a previously unremarkable brain MRI

Alpers syndrome is a fatal disorder due to mutations in the POLG gene encoding the catalytic subunit of mitochondrial DNA polymerase gamma (Pol gamma) involved in mitochondrial DNA (mtDNA) replication. We describe a case of Alpers syndrome due to POLG mutations, with rapidly progressive course, a fatal outcome, and an essentially normal brain MRI in the early oligo-symptomatic phase. Our observation suggests that Alpers syndrome should be considered even in patients with an initially unremarkable brain MRI. The patient was found to harbor the p.Q497H, p.W748S and p.E1143G mutations in cis on one allele, and a fourth mutation, the p.G848S on the other allele. Although the individual mutations detected in the presented case have been previously reported, the specific genotype formed by the particular combination of these is novel.

Autosomal recessive GTP cyclohydrolase I deficiency without hyperphenylalaninemia: evidence of a phenotypic continuum between dominant and recessive forms

We describe a unique presentation of autosomal recessive (AR) GTP cyclohydrolase I (GTPCH) deficiency, with severe CNS involvement but without hyperphenylalaninemia. A male infant presented with progressive spasticity, dystonia and oculogyric episodes. Blood phenylalanine levels were persistently normal: whereas an oral phenylalanine loading test revealed impaired phenylalanine clearance. CSF neopterin and tetrahydrobiopterin (BH(4)) were low, homovanillic acid marginally low and 5-hydroxyindoleacetic acid normal. Fibroblasts showed decreased GTPCH enzyme activity. A homozygous novel mutation of GCH1, p.V206A, was identified. On treatment (BH(4), L-Dopa/Carbidopa and 5-hydroxytryptophan), motor development improved. Mutational analysis provided neonatal diagnosis of a younger brother who, after 18 months on treatment, shows normal development. AR GTPCH I deficiency can present without hyperphenylalaninemia and with normal or subtle CSF neurotransmitter profiles. Testing for GTPCH deficiency should be considered for patients with unexplained neurological symptoms and extrapyramidal movement disorder.

Mutation-based diagnostic testing for primary hyperoxaluria type 1: survey of results

OBJECTIVES

To test for specific mutations in the alanine:glyoxylate aminotransferase (AGT) gene, in order to diagnose primary hyperoxaluria type 1 (PH1).

DESIGN AND METHODS

Samples of liver and/or DNA from 81 patients were submitted to our laboratory for diagnostic testing for PH1. Using a panel of selected mutations, DNA was examined in 64 cases, of which 36 had the diagnosis of PH1 confirmed by liver AGT assay. DNA sequencing was employed if mutation testing revealed only one mutation.

RESULTS

Identification of 100% of the mutations in the AGT-confirmed samples led to the development of a focused testing panel currently involving 4 common mutations, 7 mutations recurring at lower frequency and 5 with apparent ethnic associations.

CONCLUSIONS

This mutation panel alone would have identified the two causative mutations in 64% of the PH1 samples.

Screening for Fabry disease in patients with chronic kidney disease: limitations of plasma alpha-galactosidase assay as a screening test

BACKGROUND AND OBJECTIVES

Fabry disease is a progressive X-linked disorder of glycosphingolipid metabolism that typically presents in childhood and progresses to heart failure and renal failure in adulthood. This study sought to determine the prevalence of Fabry disease in a multiethnic male chronic kidney disease population, involving dialysis-dependent, non-dialysis-dependent, and transplant patients.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

A total of 499 patients were screened with assay of alpha-galactosidase activity using fluorometric enzyme assay on plasma prepared from fresh heparinized blood, followed by leukocyte alpha-galactosidase activity in the subset of patients with plasma alpha-galactosidase activity below the second percentile (corresponding to a value <3.0 nmol/h per ml plasma).

RESULTS

This study did not identify any new cases of Fabry disease; however, repeat testing of some of the study patients identified three limitations of the plasma enzyme assay that is commonly used as a high throughput screening method for Fabry disease: (1) False-negative results can occur; (2) these false-negative results are not prevented by use of inhibitors of alpha-galactosidase B activity; and (3) considerable intraindividual variation in plasma alpha-galactosidase levels reduces the discriminatory power of the screening test.

CONCLUSION

Clinicians need to be aware that screening using plasma will fail to detect some patients with Fabry disease.

Isolated sulfite oxidase deficiency in the newborn: lactic acidaemia and leukoencephalopathy

We report a newborn with progressive leukoencephalopathy and lactic acidaemia, diagnosed with isolated sulfite oxidase deficiency. We show that low plasma total homocysteine (PTHcy) is a valuable early indicator of sulfite oxidase dysfunction, providing a crucial first-line screen, whereas plasma cystine is not always informative in the first few days of life.

Neonatal hyperphenylalaninemia, perinatal hemochromatosis, and renal tubulopathy: a unique patient or a novel metabolic disorder?

A neonate presented with hyperphenylalaninemia (HPA), with a persistently elevated phenylalanine/tyrosine ratio. The HPA was responsive to tetrahydrobiopterin (BH4). His clinical course was dominated by liver failure, associated with perinatal hemochromatosis. He also developed renal tubulopathy. HPA has not previously been reported in association with any of these features. We investigated the etiology of his condition, and discuss the possibility that this represents a novel single-gene disorder.

6-pyruvoyl-tetrahydropterin synthase deficiency with mild hyperphenylalaninemia

Severe 6-pyruvoyl-tetrahydrobiopterin synthase deficiency is a tetrahydrobiopterin deficiency disorder that presents in infancy with developmental delay, seizures, and abnormal movements associated with hyperphenylalaninemia usually detectable by neonatal phenylketonuria screening programs. We describe an 8-year-old girl with delay, seizures, and dystonia with mild hyperphenylalaninemia detected in late childhood. The diagnosis of 6-pyruvoyl-tetrahydrobiopterin synthase deficiency was made by analysis of pterins in urine, pterins and neurotransmitters in cerebrospinal fluid, and enzyme assay. The patient improved clinically taking oral tetrahydrobiopterin, levodopa/carbidopa, and 5-hydroxytryptophan. This treatable condition may not always be detected by routine population screening for hyperphenylalaninemia.

Carnitine transporter defect due to a novel mutation in the SLC22A5 gene presenting with peripheral neuropathy

The carnitine transporter defect (McKusick 212140) is an autosomal recessive disorder caused by mutations in the SLC22A5 gene, which encodes the high-affinity carnitine transporter OCTN2 (Wang et al 2001). Diagnosis is suspected when plasma carnitine levels are extremely low and secondary causes of carnitine loss are excluded. The disease can present with recurrent Reye-like episodes of hypoketotic hypoglycaemia or with cardiomyopathy associated with myopathy (Stanley et al 1991). Here we report novel clinical findings in a 3-year-old with primary carnitine deficiency.

PAHdb 2003: what a locus-specific knowledgebase can do

PAHdb, a legacy of and resource in genetics, is a relational locus-specific database (http://www.pahdb.mcgill.ca). It records and annotates both pathogenic alleles (n = 439, putative disease-causing) and benign alleles (n = 41, putative untranslated polymorphisms) at the human phenylalanine hydroxylase locus (symbol PAH). Human alleles named by nucleotide number (systematic names) and their trivial names receive unique identifier numbers. The annotated gDNA sequence for PAH is typical for mammalian genes. An annotated gDNA sequence is numbered so that cDNA and gDNA sites are interconvertable. A site map for PAHdb leads to a large array of secondary data (attributes): source of the allele (submitter, publication, or population); polymorphic haplotype background; and effect of the allele as predicted by molecular modeling on the phenylalanine hydroxylase enzyme (EC 1.14.16.1) or by in vitro expression analysis. The majority (63%) of the putative pathogenic PAH alleles are point mutations causing missense in translation of which few have a primary effect on PAH enzyme kinetics. Most apparently have a secondary effect on its function through misfolding, aggregation, and intracellular degradation of the protein. Some point mutations create new splice sites. A subset of primary PAH mutations that are tetrahydrobiopterin-responsive is highlighted on a Curators' Page. A clinical module describes the corresponding human clinical disorders (hyperphenylalaninemia [HPA] and phenylketonuria [PKU]), their inheritance, and their treatment. PAHdb contains data on the mouse gene (Pah) and on four orthologous mutant mouse models and their use (for example, in research on oral treatment of PKU with the enzyme phenylalanine ammonia lyase [EC 4.3.1.5]).

How PAH gene mutations cause hyper-phenylalaninemia and why mechanism matters: insights from in vitro expression

Mutations in the human PAH gene, which encodes phenylalanine hydroxylase are associated with varying degrees of hyperphenylalaninemia (HPA). The more severe of these manifest as a classic metabolic disease--phenylketonuria (PKU). In vitro expression analysis of PAH mutations has three major applications: 1) to confirm that a disease-associated mutation is genuinely pathogenic, 2) to assess the severity of a mutation's impact, and 3) to examine how a mutation exerts its deleterious effects on the PAH enzyme, that is, to elucidate the molecular mechanisms involved. Data on expression analysis of 81 PAH mutations in multiple in vitro systems is summarized in tabular form online at www.pahdb.mcgill.ca. A review of these findings points in particular to a prevalent general mechanism that appears to play a major role in the pathogenicity of many PAH mutations. Amino acid substitutions promote misfolding of the PAH protein monomer and/or oppose the correct assembly of monomers into the native tetrameric enzyme. The resulting structural aberrations trigger cellular defenses, provoking accelerated degradation of the abnormal protein. The intracellular steady-state levels of the mutant PAH enzyme are therefore reduced, leading to an overall decrease in phenylalanine hydroxylation within cells and thus to hyperphenylalaninemia. There is considerable scope for modulation of the enzymic and metabolic phenotypes by modification of the cellular handling--folding, assembly, and degradation--of the mutant PAH protein. This has major implications, both for our understanding of genotype-phenotype correlations and for the development of novel therapeutic approaches.

Homomeric and heteromeric interactions between wild-type and mutant phenylalanine hydroxylase subunits: evaluation of two-hybrid approaches for functional analysis of mutations causing hyperphenylalaninemia

Phenylketonuria (PKU) is caused by mutations in the phenylalanine hydroxylase gene (PAH), while mutations in genes encoding the two enzymes (dihydropteridine reductase, DHPR, and pterin-4-alpha-carbinolamine dehydratase, PCD) required for recycling of its cofactor, tetrahydrobiopterin (BH(4)), cause other rarer disease forms of hyperphenylalaninemia. We have applied a yeast two-hybrid method, in which protein--protein interactions are measured by four reporter gene constructs, to the analysis of six PKU-associated PAH missense mutations (F39L, K42I, L48S, I65T, A104D, and R157N). By studying homomeric interactions between mutant PAH subunits, we show that this system is capable of detecting quite subtle aberrations in PAH oligomerization caused by missense mutations and that the observed results generally correlate with the severity of the mutation as determined by other expression systems. The mutant PAH subunits are also shown in this system to be able to interact with wild-type PAH subunits, pointing to an explanation for apparent dominant negative effects previously observed in obligate heterozygotes for PKU mutations. Based on our findings, the applications and limitations of two-hybrid approaches in understanding mechanisms by which PAH missense mutations exert their pathogenic effects are discussed. We have also used this technique to demonstrate homomeric interactions between wild-type DHPR subunits and between wild-type PCD subunits. These data provide a basis for functional studies on HPA-associated mutations affecting these enzymes.

Degradation of mutant proteins, underlying "loss of function" phenotypes, plays a major role in genetic disease

Many Mendelian monogenic disorders are caused by loss of the function of a single protein. This can result from rapid degradation of the mutant protein by cellular proteases, which reduces the steady-state concentration of the protein within the cell. The susceptibility of a protein to such proteolytic breakdown depends upon its kinetics of monomer folding and oligomer assembly and upon the intrinsic (thermodynamic) stability of its functional native-state conformation. Other cellular proteins, notably molecular chaperones, promote correct protein folding and assembly and thus provide some protection against degradation. An accumulation of recent evidence indicates that premature or accelerated degradation of mutant proteins, provoked by aberrations in their conformation, occurs in various subcellular compartments and represents a significant and prevalent pathogenic mechanism underlying genetic diseases. Inter-individual variability in proteolytic and folding systems can in part explain why "simple monogenic diseases" often display inconsistent genotype-phenotype correlations which show these disorders to be in reality quite complex. Protein folding and degradation may also be modulated artificially using exogenous small molecules. The identification or design of compounds which can interact specifically with particular target proteins, and which in so doing can exert beneficial effects on protein folding, assembly and/or stability, is beginning to open up a new and remarkably promising avenue for the treatment of diverse genetic disorders.

In vitro expression analysis of mutations in phenylalanine hydroxylase: linking genotype to phenotype and structure to function

Mutations in the human phenylalanine hydroxylase gene (PAH) altering the expressed cDNA nucleotide sequence (GenBank U49897) can impair activity of the corresponding enzyme product (hepatic phenylalanine hydroxylase, PAH) and cause hyperphenylalaninemia (HPA), a metabolic phenotype for which the major disease form is phenylketonuria (PKU; OMIM 261600). In vitro expression analysis of inherited human mutations in eukaryotic, prokaryotic, and cell-free systems is informative about the mechanisms of mutation effects on enzymatic activity and their predicted effect on the metabolic phenotype. Corresponding analysis of site-directed mutations in rat Pah cDNA has assigned critical functional roles to individual amino acid residues within the best understood species of phenylalanine hydroxylase. Data on in vitro expression of 35 inherited human mutations and 22 created rat mutations are reviewed here. The core data are accessible at the PAH Mutation Analysis Consortium Web site (http://www.mcgill.ca/pahdb).

Alterations in protein aggregation and degradation due to mild and severe missense mutations (A104D, R157N) in the human phenylalanine hydroxylase gene (PAH)

Phenylalanine hydroxylase (PAH) catalyzes the conversion of phenylalanine to tyrosine; its activity is the major determinant of phenylalanine disposal. Mutations in the corresponding human gene (PAH), which encodes the human hepatic PAH enzyme, result in hyperphenylalaninemia; the resulting phenotypes can range in severity from mild forms of hyperphenylalaninemia with benign outcome to the severe form, phenylketonuria with impaired cognitive development. This paper describes the detailed characterization of two inherited recessive missense mutations in PAH, c.311C-->A (A104D) and [c.470G-->A;c.471A-->C] (R157N), which are associated, respectively, in the homozygous or functionally hemizygous states, with mild and severe metabolic phenotypes. We used three different in vitro PAH expression systems (in Escherichia coli, cell-free rabbit reticulocyte lysates, and human embryonal kidney cells), as well as a unique assay for phenylalanine oxidation in vivo. In each system, we observed alterations of PAH function and physical properties, compared with wild-type enzyme, and differences in relative severity of effects between these two mutations. Pulse-chase experiments showed increased PAH degradation, probably related to observed aberrations in protein folding and altered oligomerization, as a basic mechanism underlying effects of these missense mutations.

Characterization of phenylketonuria missense substitutions, distant from the phenylalanine hydroxylase active site, illustrates a paradigm for mechanism and potential modulation of phenotype

Missense mutations account for 48% of all reported human disease-causing alleles. Since few are predicted to ablate directly an enzyme's catalytic site or other functionally important amino acid residues, how do most missense mutations cause loss of function and lead to disease? The classic monogenic phenotype hyperphenylalaninemia (HPA), manifesting notably as phenylketonuria (PKU), where missense mutations in the PAH gene compose 60% of the alleles impairing phenylalanine hydroxylase (PAH) function, allows us to examine this question. Here we characterize four PKU-associated PAH mutations (F39L, K42I, L48S, I65T), each changing an amino acid distant from the enzyme active site. Using three complementary in vitro protein expression systems, and 3D-structural localization, we demonstrate a common mechanism. PAH protein folding is affected, causing altered oligomerization and accelerated proteolytic degradation, leading to reduced cellular levels of this cytosolic protein. Enzyme specific activity and kinetic properties are not adversely affected, implying that the only way these mutations reduce enzyme activity within cells in vivo is by producing structural changes which provoke the cell to destroy the aberrant protein. The F39L, L48S, and I65T PAH mutations were selected because each is associated with a spectrum of in vivo HPA among patients. Our in vitro data suggest that interindividual differences in cellular handling of the mutant, but active, PAH proteins will contribute to the observed variability of phenotypic severity. PKU thus supports a newly emerging paradigm both for mechanism whereby missense mutations cause genetic disease and for potential modulation of a disease phenotype.

PAHdb: a locus-specific knowledgebase

PAHdb is an online relational locus-specific "mutation database" (http://www.mcgill.ca/pahdb) for the human phenylalanine hydroxylase gene (symbol PAH) and its associated phenotypes (protein, metabolic, clinical). When combined with associated information (population distribution of allele, haplotype association, etc.) PAHdb functions as a knowledgebase. From the outset, and in the absence of raw data (e.g., sequence gels), PAHdb has instead been an annotated repository of information about mutations maintained by a team of curators. It is also disease-oriented, being focused on a variant phenotype (hyperphenylalaninemia (HPA) and its most important form of disease, phenylketonuria (PKU)) resulting from primary dysfunction of the PAH enzyme (EC 1.14.16.1); it is "patient friendly" in that it contains information for those personally involved with HPA/PKU (MIM# 261600). PAHdb also serves its community through direct interaction.

Monogenic traits are not simple: lessons from phenylketonuria

The classification of genetic disease into chromosomal, monogenic and multifactorial categories is an oversimplification. Phenylketonuria (PKU) is a classic 'monogenic' autosomal recessive disease in which mutation at the human PAH locus was deemed sufficient to explain the impaired function of the enzyme phenylalanine hydroxylase (enzymic phenotype), the attendant hyperphenylalaninemia (metabolic phenotype) and the resultant mental retardation (cognitive phenotype). In the era of molecular genetics, expectations for a consistently close correlation between the mutant genotype and variant phenotype have been somewhat disappointed, and PKU is used here to illustrate how and why this might be the case. So-called monogenic traits do, indeed, conform to long-accepted ideas about the expression of 'major' loci and their importance in determining parameters of phenotype, but the associated features are as complex, in their own ways, as those in so-called complex traits.