Expert recommendations for the laboratory diagnosis of MPS VI

Mucopolysaccharidosis VI (MPS VI) is a lysosomal storage disease caused by a deficiency of N-acetylgalactosamine 4-sulfatase (arylsulfatase B, ASB). This enzyme is required for the degradation of dermatan sulfate. In its absence, dermatan sulfate accumulates in cells and is excreted in large quantities in urine. Specific therapeutic intervention is available; however, accurate and timely diagnosis is crucial for maximal benefit. To better understand the current practices for diagnosis and to establish diagnostic guidelines, an international MPS VI laboratory diagnostics scientific summit was held in February of 2011 in Miami, Florida. The various steps in the diagnosis of MPS VI were discussed including urinary glycosaminoglycan (uGAG) analysis, enzyme activity analysis, and molecular analysis. The following conclusions were reached. Dilute urine samples pose a significant problem for uGAG analysis and MPS VI patients can be missed by quantitative uGAG testing alone as dermatan sulfate may not always be excreted in large quantities. Enzyme activity analysis is universally acknowledged as a key component of diagnosis; however, several caveats must be considered and the appropriate use of reference enzymes is essential. Molecular analysis supports enzyme activity test results and is essential for carrier testing, subsequent genetic counseling, and prenatal testing. Overall the expert panel recommends caution in the use of uGAG screening alone to rule out or confirm the diagnosis of MPS VI and acknowledges enzyme activity analysis as a critical component of diagnosis. Measurement of another sulfatase enzyme to exclude multiple sulfatase deficiency was recommended prior to the initiation of therapy. When feasible, the use of molecular testing as part of the diagnosis is encouraged. A diagnostic algorithm for MPS VI is provided.

A synonymous mutation in TCOF1 causes Treacher Collins syndrome due to mis-splicing of a constitutive exon

Interpretation of the pathogenicity of sequence alterations in disease-associated genes is challenging. This is especially true for novel alterations that lack obvious functional consequences. We report here on a patient with Treacher Collins syndrome (TCS) found to carry a previously reported mutation, c.122C > T, which predicts p.A41V, and a novel synonymous mutation, c.3612A > C. Pedigree analysis showed that the c.122C > T mutation segregated with normal phenotypes in multiple family members while the c.3612A > C was de novo in the patient. Analysis of TCOF1 RNA in lymphocytes showed a transcript missing exon 22. These results show that TCS in the patient is due to haploinsufficiency of TCOF1 caused by the synonymous de novo c.3612A > C mutation. This study highlights the importance of clinical and pedigree evaluation in the interpretation of known and novel sequence alterations.

Genotype differences in cognitive functioning in Noonan syndrome

Noonan syndrome (NS) is an autosomal-dominant genetic disorder associated with highly variable features, including heart disease, short stature, minor facial anomalies and learning disabilities. Recent gene discoveries have laid the groundwork for exploring whether variability in the NS phenotype is related to differences at the genetic level. In this study, we examine the influence of both genotype and nongenotypic factors on cognitive functioning. Data are presented from 65 individuals with NS (ages 4-18) who were evaluated using standardized measures of intellectual functioning. The cohort included 33 individuals with PTPN11 mutations, 6 individuals with SOS1 mutations, 1 individual with a BRAF mutation and 25 participants with negative, incomplete or no genetic testing. Results indicate that genotype differences may account for some of the variation in cognitive ability in NS. Whereas cognitive impairments were common among individuals with PTPN11 mutations and those with unknown mutations, all of the individuals with SOS1 mutations exhibited verbal and nonverbal cognitive skills in the average range or higher. Participants with N308D and N308S mutations in PTPN11 also showed no (or mild) cognitive delays. Additional influences such as hearing loss, motor dexterity and parental education levels accounted for significant variability in cognitive outcomes. Severity of cardiac disease was not related to cognitive functioning. Our results suggest that some NS-causing mutations have a more marked impact on cognitive skills than others.

Unique case of trisomy 2p24.3-pter with no associated monosomy

We report a 3-year-old girl with trisomy 2p24.3-pter who presented with marked psychomotor delay and dysmorphic features. This patient represents the only known case of trisomy 2p24.3-2pter that does not involve an associated functional monosomy. In contrast to recent reports highlighting the fatal or serious complications in patients described as having partial trisomy 2p, this patient does not have significant birth defects or life threatening medical problems. Notably, this patient does not have a neural tube defect (NTD), which has been attributed to a putative locus at 2p24 in other patients with a partial trisomy 2p.

Regulation of the human long chain acyl-CoA dehydrogenase gene by nuclear hormone receptor transcription factors

Mitochondrial fatty acid oxidation provides most of the energy required for myocardial function after birth. Long chain acyl-CoA dehydrogenase (LCAD) catalyzes the first step in the beta-oxidation spiral. Our objective was to define regulatory elements of the human LCAD gene required for high levels of expression in mature heart and to locate elements suppressing gene expression in the fetus. We characterized the human LCAD gene structure and used in vitro transfection into cardiomyocytes and hepatoma cells of LCAD genomic fragments fused to a reporter gene to examine the effects of putative regulatory elements on transcription. Binding of transcription factors to nuclear hormone receptor consensus DNA binding domains was studied by gel shift experiments. The 200 bp of the human LCAD gene immediately upstream of the transcription initiation site are sufficient to act as a minimal promoter for the gene and provide some tissue-specific positive regulatory elements. The region from -1800 bp to -250 bp contains elements which markedly suppress transcription, including nuclear hormone receptor response elements. The dominant interaction is with the repressor factor, chicken ovalbumin upstream promoter transcription factor. We conclude that the developmental and tissue-specific regulation of the human LCAD gene is mediated, in part, by these nuclear hormone receptor transcription factors.

Monosomy 9p24-->pter and trisomy 5q31-->qter: case report and review of two cases

Partial deletion of the short arm of chromosome 9 (p24-->pter) and partial duplication of the long arm of chromosome 5 (q32-->qter) were observed in an abnormal boy who died at age 8 weeks of a complex cyanotic cardiac defect. He also had minor anomalies, sagittal craniosynostosis, triphalangeal thumbs, hypospadias, and a bifid scrotum. Two other infants with similar cytogenetic abnormalities were described previously. These patients had severe congenital heart defect, genitourinary anomalies, broad nasal bridge, low hairline, apparently low-set ears, short neck, and triphalangeal thumbs, in common with our patient. We suggest that combined monosomy 9p23,24-->pter and trisomy 5q31,32-->qter may constitute a clinically recognizable syndrome.

Expression and characterization of human mutant (glutamic acid304) medium-chain acyl-coenzyme A dehydrogenase in mammalian cells

Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is a frequent and sometimes fatal inherited metabolic disorder of fatty acid beta-oxidation. A eukaryotic expression system was used to study naturally occurring mutations in MCAD. The 1263 nucleotide coding region of human MCAD cDNA was inserted downstream of the SV40 early promoter for high-level expression in Chinese hamster ovary cells. Both normal MCAD cDNA and a mutant MCAD cDNA containing the common, disease-causing A to G transition at position 985 (A985G), which alters a lysine to a glutamic acid (K304E), were inserted into expression vectors. Transient transfection of Chinese hamster ovary cells was performed with the expression constructs. The steady state level of expressed normal MCAD protein antigen was substantially higher (5-fold) than the expressed mutant protein. The MCAD enzymatic activity in protein extracts from cells containing the expressed normal MCAD cDNA was also much higher (6-fold) than the activity in cells expressing the mutant MCAD. Therefore, these data confirm that the common K304E mutation causes MCAD deficiency primarily by decreased protein stability rather than reduction of catalytic activity and, in fact, demonstrate that the K304E mutant protein has a similar sp act against octanoyl CoA substrate as the normal protein.

Detection and assessment by positron emission tomography of a genetically determined defect in myocardial fatty acid utilization (long-chain acyl-CoA dehydrogenase deficiency)

Genetic defects in fatty acid oxidation are important, inherited causes of cardiomyopathy, skeletal myopathies, and childhood sudden death. The clinical manifestations and their severity vary widely among affected subjects and different age groups. Although measurement of serum and urinary fatty acid intermediary metabolites and enzymatic assays establish the diagnosis of a defect in fatty acid oxidation, they do not predict the specific clinical manifestations nor their severity in a given subject. To determine whether impaired myocardial fatty acid utilization, indicative of cardiac phenotypic expression of a specific genetic abnormality in fatty acid oxidation, can be detected, cardiac positron emission tomography with the metabolic tracers carbon-11-labeled palmitate and acetate was performed in 6 patients with long-chain acyl-CoA dehydrogenase (ACD) deficiency and in 9 control subjects. The myocardial extraction of both tracers was similar in patients and controls. The rate of clearance of palmitate from myocardium was significantly prolonged in patients compared with that in control subjects (0.022 +/- 0.012 vs 0.061 +/- 0.033 min-1; p < 0.025), indicative of a decreased rate of oxidation of long-chain fatty acids. Furthermore, the extent of diminution of clearance of palmitate, quantified in terms of the rate of clearance for palmitate divided by that for acetate (to correct for individual differences in overall mitochondrial oxidative metabolic flux), correlated with the clinical severity of the long-chain ACD deficiency. Accordingly, noninvasive evaluation with positron emission tomography may not only facilitate diagnosis, but also enable assessment of the pathogenetic impact and effects of therapeutic interventions in the hearts of subjects with specific, inherited defects in fatty acid oxidative metabolism.

Microcephalic osteodysplastic primordial dwarfism, type II. Report of a case with characteristic skeletal features

Osteodysplastic primordial dwarfism, type II manifests typical skeletal features which have not previously been described in the radiographic literature. We present an infant with characteristic findings and discuss related conditions, especially the Seckel syndrome.