Fifteen novel mutations in PKLR associated with pyruvate kinase (PK) deficiency: Structural implications of amino acid substitutions in PK

Pyruvate kinase (PK) deficiency is a rare disease but an important cause of hereditary nonspherocytic hemolytic anemia. The disease is caused by mutations in the PKLR gene and shows a marked variability in clinical expression. We report on the molecular characterization of 38 PK‐deficient patients from 35 unrelated families. Twenty‐nine different PKLR mutations were detected, of which 15 are reported here for the first time. Two novel deletions are reported: c.142_159del18 is the largest in‐frame deletion described thus far and predicts the loss of six consecutive amino acids (p.Thr48_Thr53del) in the N‐terminal domain of red blood cell PK. The other deletion removes nearly 1.5 kb of genomic DNA sequence (c.1618+37_2064del1477) and is one of a few large deletional mutants in PKLR. In addition, 13 novel point mutations were identified: one nonsense mutant, p.Arg488X, and 12 missense mutations, predicting the substitution of a single amino acid: p.Arg40Trp, p.Leu73Pro, p.Ile90Asn, p.Gly111Arg, p.Ala154Thr, p.Arg163Leu, p.Gly165Val, p.Leu272Val, p.Ile310Asn, p.Val320Leu, p.Gly358Glu, and p.Leu374Pro. We used the three‐dimensional (3D) structure of recombinant human tetrameric PK to evaluate the protein structural context of the affected residues. In addition, in selected patients red blood cell PK antigen levels were measured by enzyme‐linked immunosorbent assay (ELISA). Collectively, the results provided us with a rationale for the observed enzyme deficiency and contribute to both a better understanding of the genotype‐to‐phenotype correlation in PK deficiency as well as the enzyme's structure and function. Hum Mutat 0, 1–8, 2008. © 2008 Wiley‐Liss, Inc.     

Functional consequences and structural interpretation of mutations of human choline acetyltransferase

Choline acetyltransferase (ChAT; EC 2.3.1.6) catalyzes synthesis of acetylcholine from acetyl-CoA (AcCoA) and choline in cholinergic neurons. Mutations in CHAT cause potentially lethal congenital myasthenic syndromes associated with episodic apnea (ChAT-CMS). Here, we analyze the functional consequences of 12 missense and one nonsense mutations of CHAT in 11 patients. Nine of the mutations are novel. We examine expression of the recombinant missense mutants in Bosc 23 cells, determine their kinetic properties and thermal stability, and interpret the functional effects of 11 mutations in the context of the atomic structural model of human ChAT. Five mutations (p.Trp421Ser, p.Ser498Pro, p.Thr553Asn, p.Ala557Thr, and p.Ser572Trp) reduce enzyme expression to less than 50% of wild-type. Mutations with severe kinetic effects are located in the active-site tunnel (p.Met202Arg, p.Thr553Asn, and p.Ala557Thr) or adjacent to the substrate binding site (p.Ser572Trp), or exert their effect allosterically (p.Trp421Ser and p.Ile689Ser). Two mutations with milder kinetic effects (p.Val136Met and p.Ala235Thr) are also predicted to act allosterically. One mutation (p.Thr608Asn) below the nucleotide binding site of CoA enhances dissociation of AcCoA from the enzyme-substrate complex. Two mutations introducing a proline residue into an α-helix (p.Ser498Pro and p.Ser704Pro) impair the thermal stability of ChAT.     

Mutation update: Variants of the CYB5R3 gene in recessive congenital methemoglobinemia

NADH‐cytochrome b5 reductase 3 deficiency is an important genetic cause of recessive congenital methemoglobinemia (RCM) and occurs worldwide in autosomal recessive inheritance. In this Mutation Update, we provide a comprehensive review of all the pathogenic mutations and their molecular pathology in RCM along with the molecular basis of RCM in 21 new patients from the Indian population, including four novel variants: c.103A>C (p.Thr35Pro), c.190C>G (p.Leu64Val), c.310G>T (p.Gly104Cys), and c.352C>T (p.His118Tyr). In this update, over 78 different variants have been described for RCM globally. Molecular modeling of all the variants reported in CYB5R3 justifies association with the varying severity of the disease. The majority of the mutations associated with the severe form with a neurological disorder (RCM Type 2) were associated with the FAD‐binding domain of the protein while the rest were located in another domain of the protein (RCM Type 1).     

Functional characterization of missense variants in the creatine transporter gene (SLC6A8): improved diagnostic application

Creatine transporter deficiency is an X-linked mental retardation disorder caused by mutations in the creatine transporter gene (SLC6A8). So far, 20 mutations in the SLC6A8 gene have been described. We have developed a diagnostic assay to test creatine uptake in fibroblasts. Additionally, we expanded the assay to characterize novel SLC6A8 missense variants. A total of 13 variants were introduced in the SLC6A8 cDNA by site-directed mutagenesis. All variants were transiently transfected in SLC6A8-deficient fibroblasts and tested for restoration of creatine uptake in deficient primary fibroblasts. Thus, we proved that nine variants (p.Gly87Arg, p.Phe107del, p.Tyr317X, p.Asn336del, p.Cys337Trp, p.Ile347del, p.Pro390Leu, p.Arg391Trp, and p.Pro554Leu) are pathogenic mutations and four variants (p.Lys4Arg, p.Gly26Arg, p.Met560Val, and p.Val629Ile) are nonpathogenic. The present study provides an improved diagnostic tool to classify sequence variants of unknown significance.     

Functional,structural, and genetic evaluation of 20 CDKN2A germ line mutations identified in melanoma‐prone families or patients

Germline mutations of the CDKN2A gene are found in melanoma‐prone families and individuals with multiple sporadic melanomas. The encoded protein, p16^INK4A, comprises four ankyrin‐type repeats, and the mutations, most of which are missense and occur throughout the entire coding region, can disrupt the conformation of these structural motifs as well as the association of p16^INK4a with its physiological targets, the cyclin‐dependent kinases (CDKs) CDK4 and CDK6. Assessing pathogenicity of nonsynonymous mutations is critical to evaluate melanoma risk in carriers. In the current study, we investigate 20 CDKN2A germline mutations whose effects on p16^INK4A structure and function have not been previously documented (Thr18_Ala19dup, Gly23Asp, Arg24Gln, Gly35Ala, Gly35Val, Ala57Val, Ala60Val, Ala60Arg, Leu65dup, Gly67Arg, Gly67_Asn71del, Glu69Gly, Asp74Tyr, Thr77Pro, Arg80Pro, Pro81Thr, Arg87Trp, Leu97Arg, Arg99Pro, and [Leu113Leu;Pro114Ser]). By considering genetic information, the predicted impact of each variant on the protein structure, its ability to interact with CDK4 and impede cell proliferation in experimental settings, we conclude that 18 of the 20 CDKN2A variants can be classed as loss of function mutations, whereas the results for two remain ambiguous. Discriminating between mutant and neutral variants of p16^INK4A not only adds to our understanding of the functionally critical residues in the protein but provides information that can be used for melanoma risk prediction. Hum Mutat 0, 1–11, 2009. © 2009 Wiley‐Liss, Inc.     

Identification of novel mutations in the human ornithine transcarbamylase (OTC) gene of Korean patients with OTC deficiency and transient expression of the mutant proteins in vitro

The urea cycle plays key roles to prevent the accumulation of toxic nitrogenous compound and synthesize arginine de novo. Ornithine transcarbamylase (OTC) deficiency is the most common inborn error of urea cycle, which is inherited in an X-linked manner. This study was undertaken to characterize molecular defects in Korean patients with OTC deficiency. With direct sequence analysis of OTC gene of 26 unrelated Korean patients with OTC deficiency, 23 different mutations were identified. Among these mutations, eleven were novel mutations. The novel mutations were p.Leu9X, p.Arg26Pro, p.Gly100Arg, p.Met205Thr, p.Lys221Asn, p.Asp249Gly, p.Phe281Ser, p.Val323Met, c.571delC, c.853delC, and c.796-805del. All the novel mutations in this study were tested in 100 normal alleles. In vitro expression study of some of novel missense mutations elucidated the correlation of genotype and phenotype of the OTC deficiency.     

Mutations and Polymorphisms in the Human Argininosuccinate Lyase (ASL) Gene

Argininosuccinate lyase deficiency (ASLD) is caused by a defect of the urea cycle enzyme argininosuccinate lyase (ASL) encoded by the ASL gene. Patients often present early after birth with hyperammonemia but can also manifest outside the neonatal period mainly triggered by excessive protein catabolism. Clinical courses comprise asymptomatic individuals who only excrete the biochemical marker, argininosuccinic acid, in urine, and patients who succumb to their first hyperammonemic decompensation. Some patients without any hyperammonemia develop severe neurological disease. Here, we are providing an update on the molecular basis of ASLD by collecting all published (n = 67) as well as novel mutations (n = 67) of the ASL gene. We compile data on all 160 different genotypes ever identified in 223 ASLD patients, including clinical courses whenever available. Finally, we are presenting structural considerations focusing on the relevance of mutations for ASL homotetramer formation. ASLD can be considered as a panethnic disease with only single founder mutations identified in the Finnish (c.299T>C, p.Ile100Thr) and Arab (c.1060C>T, p.Gln354*) population. Most mutations are private with only few genotypes recurring in unrelated patients. The majority of mutations are missense changes including some with more frequent occurrence such as p.Arg12Gln, p.Ile100Thr, p.Val178Met, p.Arg186Trp, p.Glu189Gly, p.Gln286Arg, and p.Arg385Cys.     

Molecular characterization of multidrug- and extensively drug-resistant Mycobacterium tuberculosis strains in Jiangxi, China

In this study, a total of 77 multidrug-resistant and extensively drug-resistant (MDR and XDR, respectively) isolates of Mycobacterium tuberculosis were characterized among samples from patients living in Jiangxi province, China. The following two approaches were used: (i) genotyping all drug-resistant isolates by the 15-locus MIRU-VNTR (mycobacterial interspersed repetitive-unit-variable-number tandem-repeat) method and identifying the Beijing family genotype using the RD105 deletion targeted multiplex PCR and (ii) determining the mutation profiles associated with the resistance to the first-line antituberculous drugs rifampin (RIF) and isoniazid (INH) and the second-line drugs ofloxacin (OFX), kanamycin (KAN), amikacin (AMK), and capreomycin (CAP) with DNA sequencing. Six loci were examined: rpoB (for resistance to RIF), katG and mabA-inhA (INH), gyrA and gyrB (OFX), and rrs (KAN, AMK, and CAP). It is shown that the Beijing genotype was predominant (80.5%) among these strains and that the selected drug-resistant strains were genetically diverse, suggesting that they probably had independently acquired drug resistance. In comparison to the phenotypic data, the sensitivities for the detection of RIF, INH, OFX, and KAN/AMK/CAP resistance by DNA sequencing were 94.8, 80.5, 84.6, and 78.9%, respectively. The most prevalent mutations involved in RIF, INH, OFX, and KAN/AMK/CAP resistance were Ser531Leu in rpoB (44.2%), Ser315Thr in katG (55.8%) and C-15T in mabA-inhA (11.7%), Asp94Gly in gyrA (48.7%), and A1401G in rrs (73.7%), respectively. Five novel katG mutants (Trp191Stop, Thr271Pro, Trp328Phe, Leu546Pro, and Asp695Gly) and six new alleles (Ile569Val, Ile572Met, Phe584Ser, Val615Met, Asp626Glu, and Lys972Thr) in the rpoB gene were identified.     

The analysis of myotonia congenita mutations discloses functional clusters of amino acids within the CBS2 domain and the C‐terminal peptide of the ClC‐1 channel

Myotonia congenita (MC) is a skeletal‐muscle hyperexcitability disorder caused by loss‐of‐function mutations in the ClC‐1 chloride channel. Mutations are scattered over the entire sequence of the channel protein, with more than 30 mutations located in the poorly characterized cytosolic C‐terminal domain. In this study, we characterized, through patch clamp, seven ClC‐1 mutations identified in patients affected by MC of various severities and located in the C‐terminal region. The p.Val829Met, p.Thr832Ile, p.Val851Met, p.Gly859Val, and p.Leu861Pro mutations reside in the CBS2 domain, while p.Pro883Thr and p.Val947Glu are in the C‐terminal peptide. We showed that the functional properties of mutant channels correlated with the clinical phenotypes of affected individuals. In addition, we defined clusters of ClC‐1 mutations within CBS2 and C‐terminal peptide subdomains that share the same functional defect: mutations between 829 and 835 residues and in residue 883 induced an alteration of voltage dependence, mutations between 851 and 859 residues, and in residue 947 induced a reduction of chloride currents, whereas mutations on 861 residue showed no obvious change in ClC‐1 function. This study improves our understanding of the mechanisms underlying MC, sheds light on the role of the C‐terminal region in ClC‐1 function, and provides information to develop new antimyotonic drugs.     

Phenotypic and genotypic spectrum of Turkish patients with isovaleric acidemia

We aim to investigate the genetic basis of isovaleryl-CoA dehydrogenase (IVD) gene mutations and genotype–phenotype correlations in Turkish patients. Accordingly, bi-directional sequencing was performed to screen 26 patients with isovaleric acidemia (IVA). Nine novels (c.145delC, c.234 + 3G > C, c.506_507insT, p.Glu85Gln, p.Met147Val, p.Ala268Val, p.Ile287Met, p.Gly346Asp and p.Arg382Trp) and six previously reported (c.456 + 2T > C, p.Arg21His, p.Arg21Pro, p.Arg363Cys, p.Arg363His p.Glu379Lys) pathogenic mutations were identified. Pathogenicity of the novel mutations was supported using computational programs. No clear genotype–phenotype correlation could be determined. One of the cases with the novel c.234 + 3G > C mutation has portoseptal liver fibrosis, the clinical condition that was first reported for IVA. This study is the first comprehensive report from Turkey related to IVA genetics that provides information about the high number of disease-causing novel mutations.     

Vacuolating megalencephalic leukoencephalopathy with subcortical cysts: functional studies of novel variants in MLC1

Nine new unrelated patients presenting vacuolating myelinopathy with subcortical cysts were identified and analyzed for variations in the MLC1 gene. We detected 12 mutations (p.Leu37fs, p.Met80Val, p.Leu83Phe, p.Pro92Ser, p.Ser93Leu, p.Ile108fs, p.Gly130Arg, p.Cys171fs, p.Glu202Lys, p.Ser269Tyr, p.Ala275Asn, and p.Leu310_311insLeu) of which nine were novel. In one patient we did not detect mutations. Using a heterologous system, three new missense variants (p.Glu202Lys, p.Ser269Tyr, and p.Ala275Asn) and a single leucine insertion (p.Leu310insLeu)--lying in a stretch of seven leucines--were functionally assayed by determining total protein levels and mutant protein expression at the plasma membrane. No correlation was observed between mutation, clinical features, and plasma membrane expression of mutant protein.     

Allelic heterogeneity of the carbohydrate sulfotransferase-6 gene in patients with macular corneal dystrophy

Allelic heterogeneity of the carbohydrate sulfotransferase-6 gene in patients with macular corneal dystrophy.Macular corneal dystrophy (MCD) is an autosomal recessive disorder characterized by grayish white opacities in the cornea. It is caused by mutations in the carbohydrate sulfotransferase-6 (CHST6) gene, which codes for the enzyme corneal N-acetylglucosamine-6-sulfotransferase. This enzyme catalyzes the sulfation of keratan sulfate, an important component of corneal proteoglycans. We screened 31 patients from 26 families with MCD for mutations in the coding region of the CHST6 gene. Twenty-six different mutations were identified, of which 14 mutations are novel. The novel mutations are one nonsense mutation found in one patient (Trp2Ter), one frameshift (insertion plus deletion) mutation in two patients (His335fs), and 12 missense mutations (Leu3Met, Ser54Phe, Val56Arg, Ala73Thr, Ser98Leu, Cys165Trp, Ser167Phe, Phe178Cys, Leu193Pro, Pro204Arg, Arg272Ser, and Arg334Cys) in 11 patients. These data demonstrate a high degree of allelic heterogeneity of the CHST6 gene in patient populations with MCD from Southern India, where this disease may have a relatively higher prevalence than in outbred communities.     

Molecular and phenotypic characteristics of seven novel mutations causing branched‐chain organic acidurias

Specific mitochondrial enzymatic deficiencies in the catabolism of branched‐chain amino acids cause methylmalonic aciduria (MMA), propionic acidemia (PA) and maple syrup urine disease (MSUD). Disease‐causing mutations were identified in nine unrelated branched‐chain organic acidurias (BCOA) patients. We detected eight previously described mutations: p.Asn219Tyr, p.Arg369His p.Val553Glyfs*17 in MUT, p.Thr198Serfs*6 in MMAA, p.Ile144_Leu181del in PCCB, p.Gly288Valfs*11, p.Tyr438Asn in BCKDHA and p.Ala137Val in BCKDHB gene. Interestingly, we identified seven novel genetic variants: p.Leu549Pro, p.Glu564*, p.Leu641Pro in MUT, p.Tyr206Cys in PCCB, p.His194Arg, p.Val298Met in BCKDHA and p.Glu286_Met290del in BCKDHB gene. In silico and/or eukaryotic expression studies confirmed pathogenic effect of all novel genetic variants. Aberrant enzymes p.Leu549Pro MUT, p.Leu641Pro MUT and p.Tyr206Cys PCCB did not show residual activity in activity assays. In addition, activity of MUT enzymes was not rescued in the presence of vitamin B12 precursor in vitro which was in accordance with non‐responsiveness or partial responsiveness of patients to vitamin B12 therapy. Our study brings the first molecular genetic data and detailed phenotypic characteristics for MMA, PA and MSUD patients for Serbia and the whole South‐Eastern European region. Therefore, our study contributes to the better understanding of molecular landscape of BCOA in Europe and to general knowledge on genotype–phenotype correlation for these rare diseases.     

Inhibitory influence of amino acids on secretagogues induced exocrine secretion in isolated perfused rat pancreas

Influence of amino acids upon pancreatic exocrine secretion has been investigated in the isolated perfused pancreas of rats. Arg produced significant and dose-related inhibition of pancreatic juice flow, protein output and amylase output evoked by CCK-PZ (1.25 pM). The secretory response evoked by CCK-PZ was inhibited by other amino acids (Ala, Asp, Asn, Gly, Ile, Leu, Lys, Met, Phe, Pro, Thr, Trp, Val, in each 20 mM). A similar inhibitory pattern was observed using 10 mixed amino acids of 2 mM each (Pro, Phe, Thr, Met, Lys, Asp, Leu, Trp, Val, Gly). Gly at a concentration of 20 mM produced significant inhibition of exocrine secretion evoked by ACh (50 nM) or GRP (36 pM). The inhibitory response induced by amino acids could not be repeated by using exogenous insulin (1 microM) and glucagon (280 nM). The inhibitory response was also not changed by increased extracellular Ca (5 or 10 mM). However, Gly (20 mM) produced inhibition of exocrine secretion evoked by Ca reintroduction into a pancreas which was pretreated with A 23187. It was suggested that the inhibitory effects of some amino acids on exocrine secretion are mainly caused by suppression of Ca influx in a stimulus-secretion coupling process.     

Choline Acetyltransferase Mutations Causing Congenital Myasthenic Syndrome: Molecular Findings and Genotype–Phenotype Correlations

Choline acetyltransferase catalyzes the synthesis of acetylcholine at cholinergic nerves. Mutations in human CHAT cause a congenital myasthenic syndrome due to impaired synthesis of ACh; this severe variant of the disease is frequently associated with unexpected episodes of potentially fatal apnea. The severity of this condition varies remarkably, and the molecular factors determining this variability are poorly understood. Furthermore, genotype–phenotype correlations have been difficult to establish in patients with biallelic mutations. We analyzed the protein expression of phosphorylated ChAT of seven CHAT mutations, p.Val136Met, p.Arg207His, p.Arg186Trp, p.Val194Leu, p.Pro211Ala, p.Arg566Cys, and p.Ser694Cys, in HEK‐293 cells to phosphorylated ChAT, determined their enzyme kinetics and thermal stability, and examined their structural changes. Three mutations, p.Arg207His, p.Arg186Trp, and p.Arg566Cys, are novel, and p.Val136Met and p.Arg207His are homozygous in three families and associated with severe disease. The characterization of mutants showed a decrease in the overall catalytic efficiency of ChAT; in particular, those located near the active‐site tunnel produced the most seriously disruptive phenotypic effects. On the other hand, p.Val136Met, which is located far from both active and substrate‐binding sites, produced the most drastic reduction of ChAT expression. Overall, CHAT mutations producing low enzyme expression and severe kinetic effects are associated with the most severe phenotypes.     

Molecular characterization of 82 patients with pyruvate dehydrogenase complex deficiency. Structural implications of novel amino acid substitutions in E1 protein

Background

Pyruvate dehydrogenase complex (PDHc) deficiencies are an important cause of primary lactic acidosis. Most cases result from mutations in the X-linked gene for the pyruvate dehydrogenase E1α subunit (PDHA1) while a few cases result from mutations in genes for E1β (PDHB), E2 (DLAT), E3 (DLD) and E3BP (PDHX) subunits or PDH-phosphatase (PDP1).

Aim

To report molecular characterization of 82 PDHc-deficient patients and analyze structural effects of novel missense mutations in PDHA1.

Methods

PDHA1 variations were investigated first, by exon sequencing using a long range PCR product, gene dosage assay and cDNA analysis. Mutation scanning in PDHX, PDHB, DLAT and DLD cDNAs was further performed in unsolved cases. Novel missense mutations in PDHA1 were located on the tridimensional model of human E1 protein to predict their possible functional consequences.

Results

PDHA1 mutations were found in 30 girls and 35 boys. Three large rearrangements, including two contiguous gene deletion syndrome were identified. Novel missense, frameshift and splicing mutations were also delineated and a nonsense mutation in a mosaic male. Mutations p.Glu75Ala, p.Arg88Ser, p.Arg119Trp, p.Gly144Asp, p.Pro217Arg, p.Arg235Gly, p.Tyr243Cys, p.Tyr243Ser, p.Arg245Gly, p.Pro250Leu, p.Gly278Arg, p.Met282Val, p.Gly298Glu in PDHA1 were predicted to impair active site channel conformation or subunit interactions. Six out of the seven patients with PDHB mutations displayed the recurrent p.Met101Val mutation; 9 patients harbored PDHX mutations and one patient DLD mutations.

Conclusion

We provide an efficient stepwise strategy for mutation screening in PDHc genes and expand the growing list of PDHA1 mutations analyzed at the structural level.     

Human lymphokine-activated killer (LAK) cells--II. Studies of various L-amino acid methyl esters on LAK generation at high cell density

Nineteen L-amino acid methyl esters were studied for their cytotoxic activity on human monocytes, NK activity, and LAK activation by IL-2 at high cell density (5 x 10(6) cells/ml). Phenylalanine, Met, Trp, Cys, Tyr, Asp and Glu methyl esters depleted monocytes from PBMC, caused inhibition of NK activity, and allowed LAK activation at high cell density. Alanine, Val and Pro methyl esters were marginal. Glycine, Ser, Thr, Lys, His and Arg were not active. Leucine, Ile and cystine methyl esters depleted monocytes and also NK activity; LAK activation was suppressed. The D series of the active L-amino acid (Met, Tyr and Trp) methyl esters were not active. The position of the methyl ester is important as shown by 5-Glu methyl ester which was not active as Glu(OMe)2. Phenylalanine T-butyl ester was not as active as the methyl or the ethyl ester. This indicates that the breakage of the ester bond is the rate-limiting step for the actions of the Phe alkyl esters. Seven L-amino acid amides (Ile, Leu, Phe, Val, Glu, Asp and Tyr) were studied and only Ile, Leu and Phe were found to be active. Isoleucine and Leu amides depleted monocytes with little inhibitory effect on NK activity and thus allowed LAK activation. In summary, depletion of monocytes by the amino acid methyl esters and the amides allowed LAK activation at high cell density.     

Prevalence and associated phenotypes of PLXNA1 variants in normosmic and anosmic idiopathic hypogonadotropic hypogonadism

Idiopathic hypogonadotropic hypogonadism (IHH) can be divided into two major forms, normosmic IHH and Kallmann syndrome (KS). Genetic mutations are responsible for the majority of IHH. PLXNA1 has recently been implicated in the GnRH neuron migration and the etiology of KS. We aimed to investigate the prevalence and associated phenotypes of PLXNA1 variants in a large cohort of IHH patients. We screened the whole exome data of 215 IHH patients in a single center for causative PLXNA1 variants. Our studies showed eight novel (p.Arg836His, p.Lys1451Arg, p.Val287Met, p.Val536Ile, p.Ser1850Arg, p.Ile1701Val, p.Arg319Trp, and p.Pro485Leu) and two previously described (p.Arg528Trp and p.Gly720Glu) heterozygous PLXNA1 variants in nine affected individuals from seven unrelated families. Only three of nine patients were anosmic (KS) while the remaining patients showed normal olfactory function (nIHH). Seven of nine patients (77.7%) harbored additional one or two variants in other nIHH/KS-associated genes, including PROKR2, IGSF10, HS6ST1, SEMA3E, CCDC141, FGFR1, NRP1, POLR3A, and SRA1. Our findings indicate that PLXNA1 variants cause not only anosmic but also normosmic IHH with a relatively high prevalence (3.9%). Heterozygous missense PLXNA1 variants appear to be involved together with other IHH gene variants in bringing about the IHH disease phenotype.     

Investigation of citrullinemia type I variants by in vitro expression studies

Mild citrullinemia is an allelic variant of classical citrullinemia type I also caused by deficiency of the urea cycle enzyme argininosuccinate synthetase (ASS). Affected patients comprise a biochemical but no clinical phenotype. However, there is no reliable parameter allowing conclusions regarding the course of the disorder or its type of manifestation. The aim of this study was to test the importance of varying levels of ASS residual activities for the severity at diagnosis. Bacterial in vitro expression studies allowed the enzymatic analysis of purified wild-type and the mutant ASS proteins p.Ala118Thr (c.352G>A), p.Trp179Arg (c.535T>C), p.Val263Met (c.787G>A), p.Arg265Cys (c.793C>T), p.Met302Val (c.904A>G), p.Gly324Ser (c.970G>A), p.Gly362Val (c.1085G>T), and p.Gly390Arg (c.1168G>A). In the chosen system, classical mutations do not show any significant enzymatic activity, whereas mutations associated with a mild course yield significant ASS activity levels. The mutation p.Ala118Thr (c.352G>A) impresses by a high residual activity (62%) but a severe reduction of affinity toward the substrates citrulline and aspartate. This mutation was identified in a hitherto healthy female adult with no history of known citrullinemia who had died during the postpartum period from hyperammonemic coma. The results of this study suggest that even a high level of residual ASS activity is not a reliable prognostic marker for an uneventful clinical course. Determination of ASS residual activities, therefore, cannot help in anticipating the risk of metabolic derangement. This study should guide clinicians as well as patients with mild citrullinemia toward a lifelong awareness of the disorder.