Molecular characterization of 355 mucopolysaccharidosis patients reveals 104 novel mutations.

Mucopolysaccharidosis (MPS) disorders are heterogeneous and caused by deficient lysosomal degradation of glycosaminoglycans, resulting in distinct but sometimes overlapping phenotypes. Molecular analysis was performed for a total of 355 MPS patients with MPSI (n = 15), MPSII (n = 218), MPSIIIA (n = 86), MPSIIIB (n = 20), MPSIVA (n = 6) or MPSVI (n = 10). This analysis revealed 104 previously unreported mutations: seven in IDUA (MPSI), 61 in IDS (MPSII), 19 in SGSH (MPSIIIA), 11 in NAGLU (MPSIIIB), two in GALNS (MPSIVA) and four in ARSB (MPSVI). The intergenic comparison of the mutation data for these disorders has revealed interesting differences. Whereas IDUA, IDS, NAGLU and ARSB demonstrate similar levels of mutation heterogeneity (0.6-0.675 different mutations per total alleles), SGSH and GALNS have lower levels of mutation heterogeneity (0.282 and 0.455, respectively), due to more recurrent mutations. The type of mutation also varies significantly by gene. SGSH, GALNS and ARSB mutations are usually missense (76.5 %, 81.8 % and 85 %), while IDUA has many more nonsense mutations (56 %) than the other genes (≤20%). The mutation spectrum is most diverse for IDS, including intergenic inversions and multi-exon deletions. By testing 102 mothers of MPSII patients, we determined that 22.5 % of IDS mutations are de novo. We report the allele frequency of common mutations for each gene in our patient cohort and the exonic distribution of coding sequence alterations in the IDS, SGSH and NAGLU genes, which reveals several potential "hot-spots". This further molecular characterization of these MPS disorders is expected to assist in the diagnosis and counseling of future patients.

Severe Hunter syndrome (mucopolysaccharidosis II) phenotype secondary to large deletion in the X chromosome encompassing IDS, FMR1, and AFF2 (FMR2).

A 2-year-old boy with an initial diagnosis of Hunter syndrome (mucopolysaccharidosis II) had a more severe phenotype than expected, which warranted further evaluation. The patient had severe infantile global neurodevelopmental delays, macrocephaly with a prominent forehead, coarse facial features with clear corneas, chronic congestion with snoring, wide-spaced teeth, short thick neck, hepatomegaly, an inguinal hernia repaired, early clawhand deformities, and severe generalized hypotonia. X chromosome microarray revealed a large deletion encompassing the genes IDS, FMR1, and AFF2 (FMR2) confirming the diagnoses of both Hunter and fragile X syndromes. This case is also a reminder to clinicians that for optimum patient care, further diagnostic testing is warranted if there is concern that a patient's phenotype is more severe or complex than would be expected for the initial neurogenetic diagnosis.

Utilization of blood spot testing for metabolic-genetic disorders in Honduras: is it time for newborn screening?

Honduran infant mortality (20/1000) has fallen below the Latin American newborn screening target rate (<30/1000). The authors report 2 Honduran maple syrup urine disease cases and a newborn screening pilot study. The first infant, diagnosed by plasma/urine testing in the U.S., prompted this study. Although marked clinical/radiological improvement occurred after treatment, moderate neurodevelopmental delays persist at 5 years. This 1-month, prospective study used blood spot specimens from hospitalized term Honduran neonates shipped overnight to South Carolina for routine newborn screening with electronic result submission to Honduras for follow-up. Of 88 consecutive neonates (mean age: 4.2 days, standard deviation: 4.2 days) tested, 24 (0.6%) of 3837 completed tests were positive. Another infant with maple syrup urine disease, diagnosed after study completion by blood spot testing, later died. The study findings indicate that collaborative blood spot testing aids in the diagnosis of Honduran metabolic-genetic disease. Newborn screening is now needed to diagnose and treat these diseases before morbidity/mortality develops.

Diagnosis, treatment, and long-term outcomes of late-onset (type III) multiple acyl-CoA dehydrogenase deficiency.

We report 4 children with late-onset (type III) multiple acyl-CoA dehydrogenase deficiency, also known as glutaric aciduria type II, which is an autosomal recessive disorder of fatty acid and amino acid metabolism. The underlying deficiency is in the electron transfer flavoprotein or electron flavoprotein dehydrogenase. Clinical presentations include fatal acute neonatal metabolic encephalopathies with/without organ system anomalies (types I and II) and late-onset acute metabolic crises, myopathy, or neurodevelopmental delays (type III). Two patients were identified in childhood following a metabolic crisis and/or neurodevelopmental delay, and 2 were identified by newborn metabolic screening. Our cases will illustrate the difficulty in making a biochemical diagnosis of late-onset (type III) multiple acyl-CoA dehydrogenase deficiency from plasma acylcarnitines and urine organic acids in both symptomatic and asymptomatic children. However, they emphasize the need for timely diagnosis to urgently implement prophylactic treatment for life-threatening metabolic crises with low protein/fat diets supplemented with riboflavin and carnitine.

Persistent growth failure in Prader-Willi syndrome associated with short-chain acyl-CoA dehydrogenase gene variant.

The authors report the rare association of Prader-Willi syndrome and short-chain acyl-CoA dehydrogenase gene variant. Prader-Willi syndrome, associated with paternal chromosome 15q11-q13 silencing, is characterized by neonatal/infantile hypotonia, growth failure, and neurodevelopmental delays in the first 1 to 2 years of life, typically followed by hyperphagia and obesity. Short-chain acyl-CoA dehydrogenase gene variant, with 625 G-to-A and 511 C-to-T changes, impairs C4-C6 fatty acid metabolism and variably causes neonatal/infantile hypotonia with developmental delays. The authors' patient continues to exhibit the classic severe growth failure of early infancy Prader-Willi syndrome at 40 months. Extensive laboratory investigations indicate that the short-chain acyl-CoA dehydrogenase gene variant is likely preventing or delaying the normal expression of the Prader-Willi syndrome phenotype.

X-linked creatine transporter (SLC6A8) mutations in about 1% of males with mental retardation of unknown etiology.

Mutations in the creatine transporter gene, SLC6A8 (MIM 30036), located in Xq28, have been found in families with X-linked mental retardation (XLMR) as well as in males with idiopathic mental retardation (MR). In order to estimate the frequency of such mutations in the MR population, a screening of 478 males with MR of unknown cause was undertaken. All 13 exons of SLC6A8 were sequenced using genomic DNA. Six novel potentially pathogenic mutations were identified that were not encountered in at least 588 male control chromosomes: two deletions (p.Asn336del, p.Ile347del) and a splice site alteration (c.1016+2C>T) are considered pathogenic based on the nature of the variant. A mutation (p.Arg391Trp) should be considered pathogenic owing to its localization in a highly conserved region. Two other missense variants (p.Lys4Arg, p.Gly26Arg) are not conserved but were not observed in over 300 male control chromosomes. Their pathogenicity is uncertain. A missense variant (p.Val182Met), was classified as a polymorphism based on a normal creatine/creatinine (Cr:Crn) ratio and cerebral creatine signal in proton magnetic resonance spectroscopy (H-MRS) in the patient. Furthermore, we found 14 novel intronic and neutral variants that were not encountered in at least 280 male control chromosomes and should be considered as unclassified variants. Our findings of a minimum of four pathogenic mutations and two potentially pathogenic mutations indicate that about 1% of males with MR of unknown etiology might have a SLC6A8 mutation. Thus, DNA sequence analysis and/or a Cr:Crn urine screen is warranted in any male with MR of unknown cause.

X-linked spermine synthase gene (SMS) defect: the first polyamine deficiency syndrome.

Polyamines (putrescine, spermidine, spermine) are ubiquitous, simple molecules that interact with a variety of other molecules in the cell, including nucleic acids, phospholipids and proteins. Various studies indicate that polyamines are essential for normal cell growth and differentiation. Furthermore, these molecules, especially spermine, have been shown to modulate ion channel activities of certain cells. Nonetheless, little is known about the specific cellular functions of these compounds, and extensive laboratory investigations have failed to identify a heritable condition in humans in which polyamine synthesis is perturbed. We report the first polyamine deficiency syndrome caused by a defect in spermine synthase (SMS). The defect results from a splice mutation, and is associated with the Snyder-Robinson syndrome (SRS, OMIM_309583), an X-linked mental retardation disorder. The affected males have mild-to-moderate mental retardation (MR), hypotonia, cerebellar circuitry dysfunction, facial asymmetry, thin habitus, osteoporosis, kyphoscoliosis, decreased activity of SMS, correspondingly low levels of intracellular spermine in lymphocytes and fibroblasts, and elevated spermidine/spermine ratios. The clinical features observed in SRS are consistent with cerebellar dysfunction and a defective functioning of red nucleus neurons, which, at least in rats, contain high levels of spermine. Additionally, the presence of MR reflects a role for spermine in cognitive function, possibly by spermine's ability to function as an 'intrinsic gateway' molecule for inward rectifier K(+) channels.

X-linked mental retardation with seizures and carrier manifestations is caused by a mutation in the creatine-transporter gene (SLC6A8) located in Xq28.

A family with X-linked mental retardation characterized by severe mental retardation, speech and behavioral abnormalities, and seizures in affected male patients has been found to have a G1141C transversion in the creatine-transporter gene SLC6A8. This mutation results in a glycine being replaced by an arginine (G381R) and alternative splicing, since the G-->C transversion occurs at the -1 position of the 5' splice junction of intron 7. Two female relatives who are heterozygous for the SLC6A8 mutation also exhibit mild mental retardation with behavior and learning problems. Male patients with the mutation have highly elevated creatine in their urine and have decreased creatine uptake in fibroblasts, which reflects the deficiency in creatine transport. The ability to measure elevated creatine in urine makes it possible to diagnose SLC6A8 deficiency in male patients with mental retardation of unknown etiology.