Mutations in known disease genes account for the majority of autosomal recessive retinal dystrophies.

Retinal dystrophies (RDs) are hereditary blinding eye conditions that are highly variable in their clinical presentation. The remarkable genetic heterogeneity that characterizes RD was a major challenge in establishing the molecular diagnosis in these patients until the recent advent of next-generation sequencing. It remains unclear, however, what percentage of autosomal recessive RD remain undiagnosed when all established RD genes are sequenced. We enrolled 75 families in which RD segregates in an apparently autosomal recessive manner. We show that the yield of a multigene panel that contains known RD genes is 67.5%. The higher yield (82.3%) when whole exome sequencing was implemented instead was often due to hits in genes that were not included in the original design of the panel. We also show the value of homozygosity mapping even during the era of exome sequencing in uncovering cryptic mutations. In total, we describe 45 unique likely deleterious variants (of which 18 are novel including one deep intronic and one genomic deletion mutation). Our study suggests that the genetic heterogeneity of autosomal recessive RD is approaching saturation and that any new RD genes will probably account for only a minor role in the mutation burden.

Homozygous nonsense mutation in SCHIP1/IQCJ-SCHIP1 causes a neurodevelopmental brain malformation syndrome.

We report a consanguineous Arab family with 3 affected siblings who display a disorder of global developmental delay, learning difficulties, facial dysmorphism, hearing impairments, and cataract. The clinical phenotype was associated with characteristic brain magnetic resonance imaging (MRI) features of axonal guidance defects involving anterior commissure agenesis as well as scattered areas of polymicrogyria-cobblestone complex. Whole genome sequencing revealed a novel nonsense mutation (159609921C>T) that segregated in the family consistent in an autosomal recessive pattern. This mutation located in the C-terminal region shared by the Schwanomin-Interacting Protein1 (SCHIP1) isoforms including the IQCJ-SCHIP1. The in vitro expression of SCHIP1 and IQCJ-SCHIP1 truncated mutant isoforms (NM_001197109.1; p.R209* and NM_001197114.1; p.R501*, respectively) were markedly reduced as compared to their full-length versions suggesting protein stability/folding impairment. The pathogenic nature of this mutation is supported by a previously reported mouse knockout of Schip1 isoforms, which phenocopied the human axon guidance abnormality. This is the first report of a SCHIP1/IQCJ-SCHIP1 point mutation in humans associated with a neurological-developmental phenotype.

Cysteamine revisited: repair of arginine to cysteine mutations.

Cysteamine is a small aminothiol endogenously derived from coenzyme A degradation. For some decades, synthetic cysteamine has been employed for the treatment of cystinosis, and new uses of the drug continue to emerge. In this review, we discuss the role of cysteamine in cellular and extracellular homeostasis and focus on the potential use of aminothiols to reconstitute the function of proteins harboring arginine (Arg) to cysteine (Cys) mutations, via repair of the Cys residue into a moiety that introduces an amino group, as seen in basic amino acid residues Lys and Arg. Cysteamine has been utilized in vitro and ex vivo in four different genetic disorders, and thus provides "proof of principle" that aminothiols can modify Cys residues. Other aminothiols such as mercaptoethylguanidine (MEG) with closer structural resemblance to the guanidinium moiety of Arg are under examination for their predicted enhanced capacity to reconstitute loss of function. Although the use of aminothiols holds clinical potential, more studies are required to refine specificity and treatment design. The efficacy of aminothiols to target proteins may vary substantially depending on their specific extracellular and intracellular locations. Redox potential, pH, and specific aminothiol abundance in each physiological compartment are expected to influence the reactivity and turnover of cysteamine and analogous drugs. Upcoming research will require the use of suitable cell and animal models featuring Arg to Cys mutations. Since, in general, Arg to Cys changes comprise about 8% of missense mutations, repair of this specific mutation may provide promising avenues for many genetic diseases.

Clinical genomics expands the morbid genome of intellectual disability and offers a high diagnostic yield.

Intellectual disability (ID) is a measurable phenotypic consequence of genetic and environmental factors. In this study, we prospectively assessed the diagnostic yield of genomic tools (molecular karyotyping, multi-gene panel and exome sequencing) in a cohort of 337 ID subjects as a first-tier test and compared it with a standard clinical evaluation performed in parallel. Standard clinical evaluation suggested a diagnosis in 16% of cases (54/337) but only 70% of these (38/54) were subsequently confirmed. On the other hand, the genomic approach revealed a likely diagnosis in 58% (n=196). These included copy number variants in 14% (n=54, 15% are novel), and point mutations revealed by multi-gene panel and exome sequencing in the remaining 43% (1% were found to have Fragile-X). The identified point mutations were mostly recessive (n=117, 81%), consistent with the high consanguinity of the study cohort, but also X-linked (n=8, 6%) and de novo dominant (n=19, 13%). When applied directly on all cases with negative molecular karyotyping, the diagnostic yield of exome sequencing was 60% (77/129). Exome sequencing also identified likely pathogenic variants in three novel candidate genes (DENND5A, NEMF and DNHD1) each of which harbored independent homozygous mutations in patients with overlapping phenotypes. In addition, exome sequencing revealed de novo and recessive variants in 32 genes (MAMDC2, TUBAL3, CPNE6, KLHL24, USP2, PIP5K1A, UBE4A, TP53TG5, ATOH1, C16ORF90, SLC39A14, TRERF1, RGL1, CDH11, SYDE2, HIRA, FEZF2, PROCA1, PIANP, PLK2, QRFPR, AP3B2, NUDT2, UFC1, BTN3A2, TADA1, ARFGEF3, FAM160B1, ZMYM5, SLC45A1, ARHGAP33 and CAPS2), which we highlight as potential candidates on the basis of several lines of evidence, and one of these genes (SLC39A14) was biallelically inactivated in a potentially treatable form of hypermanganesemia and neurodegeneration. Finally, likely causal variants in previously published candidate genes were identified (ASTN1, HELZ, THOC6, WDR45B, ADRA2B and CLIP1), thus supporting their involvement in ID pathogenesis. Our results expand the morbid genome of ID and support the adoption of genomics as a first-tier test for individuals with ID.

The Frequencies of Different Inborn Errors of Metabolism in Adult Metabolic Centres: Report from the SSIEM Adult Metabolic Physicians Group.

BACKGROUND: There are few centres which specialise in the care of adults with inborn errors of metabolism (IEM). To anticipate facilities and staffing needed at these centres, it is of interest to know the distribution of the different disorders. METHODS: A survey was distributed through the list-serve of the SSIEM Adult Metabolic Physicians group asking clinicians for number of patients with confirmed diagnoses, types of diagnoses and age at diagnosis. RESULTS: Twenty-four adult centres responded to our survey with information on 6,692 patients. Of those 6,692 patients, 510 were excluded for diagnoses not within the IEM spectrum (e.g. bone dysplasias, hemochromatosis) or for age less than 16 years, leaving 6,182 patients for final analysis. The most common diseases followed by the adult centres were phenylketonuria (20.6%), mitochondrial disorders (14%) and lysosomal storage disorders (Fabry disease (8.8%), Gaucher disease (4.2%)). Amongst the disorders that can present with acute metabolic decompensation, the urea cycle disorders, specifically ornithine transcarbamylase deficiency, were most common (2.2%), followed by glycogen storage disease type I (1.5%) and maple syrup urine disease (1.1%). Patients were frequently diagnosed as adults, particularly those with mitochondrial disease and lysosomal storage disorders. CONCLUSIONS: A wide spectrum of IEM are followed at adult centres. Specific knowledge of these disorders is needed to provide optimal care including up-to-date knowledge of treatments and ability to manage acute decompensation.

Whole exome sequencing unravels disease-causing genes in consanguineous families in Qatar.

Whole exome sequencing (WES) has greatly facilitated the identification of causal mutations for diverse human genetic disorders. We applied WES as a molecular diagnostic tool to identify disease-causing genes in consanguineous families in Qatar. Seventeen consanguineous families with diverse disorders were recruited. Initial mutation screening of known genes related to the clinical diagnoses did not reveal the causative mutations. Using WES approach, we identified the definitive disease-causing mutations in four families: (i) a novel nonsense homozygous (c.1034C>G) in PHKG2 causing glycogen storage disease type 9C (GSD9C) in a male with initial diagnosis of GSD3; (ii) a novel homozygous 1-bp deletion (c.915del) in NSUN2 in a male proband with Noonan-like syndrome; (iii) a homozygous SNV (c.1598C>G) in exon 11 of IDUA causing Hurler syndrome in a female proband with unknown clinical diagnosis; (iv) a de novo known splicing mutation (c.1645+1G>A) in PHEX in a female proband with initial diagnosis of autosomal recessive hypophosphatemic rickets. Applying WES as a diagnostic tool led to the unambiguous identification of disease-causing mutations in phenotypically complex disorders or correction of the initial clinical diagnosis in ˜25% of our cases.

Autosomal recessive inheritance of GLUT1 deficiency syndrome.

GLUT1 deficiency syndrome (GLUT1DS) is understood as a monogenetic disease caused by heterozygous SLC2A1 gene mutations with autosomaldominant and sporadic transmission. We report on a six-year-old girl from an inbred Arab family with moderate global developmental delay, epilepsy, ataxia, hypotonia, and hypoglycorrhachia (CSF glucose 36 mg/dL; CSF lactate 1.09 mmol/L; CSF/blood glucose ratio 0.44). Molecular analysis of the SLC2A1 gene identified a novel homozygous c1402C>T (p. Arg468Trp) mutation in exon 10 in the index patient and her asymptomatic younger sister. The mutation was absent in 120 control alleles of healthy individuals as well as in 400 alleles of other GLUT1DS patients. Arg468 represents a highly conserved, functionally important amino acid residue in the GLUT1 carboxy-terminus essential for substrate recognition and transport. Both unaffected parents were heterozygous for the mutation. A younger brother and two family members were healthy and carried the GLUT1 wild type. A ketogenic diet effectively controlled seizures in the index patient. We conclude that GLUT1DS can be transmitted as an autosomal recessive disease and provide new insights into genetic counselling for this treatable disorder.

Implementation of extended neonatal screening and a metabolic unit in the State of Qatar: developing and optimizing strategies in cooperation with the Neonatal Screening Center in Heidelberg.

Qatar is a country in the Gulf area and member of the Gulf Cooperation Council states. The country is populated by original Qatari tribes that amount to about 200,000 people and about 600,000 expatriates mainly from Arabic and Asian countries. Inbreeding over centuries and high rates of consanguinity in the Qatari population and in some groups of expatriates, in addition to large family sizes and rapid population growth, have contributed to a high frequency of autosomal recessive disorders. In December 2003 Hamad Medical Corporation in Doha and the University Children's Hospital of Heidelberg, Germany, started an extended state-wide neonatal screening programme for metabolic and endocrine disorders, with the laboratory situated in Heidelberg, Germany. All aspects of the screening process had to be adapted to the unique situation of the laboratory being 6000 km from the birthplace of the neonates. Within 32 months, samples of 25,214 neonates were screened. In 28 cases an endocrine or metabolic diagnosis was identified (incidence 1:901, in Germany 1:1728). In particular, a variety of monogenic metabolic diseases were prevalent, with 19 patients detected giving an incidence of metabolic diseases of 1:1327 (Germany 1:2517). Each euro spent on the screening programme saved more than 25 euros in health and social costs. The programme revealed a high incidence of treatable inborn metabolic diseases in the population of Qatar. A reliable screening for classical homocystinuria showing a unique incidence of >1:3000 and for sickle cell disease has now been added.

Atypical phenotype in a boy with a maple syrup urine disease.

Maple syrup urine disease (MSUD) is a metabolic disorder due to a block in the decarboxylation step in the catabolic pathways of the branched-chain amino acids (BCAAs). We describe an atypical presentation in an infant male. The patient presented with psychomotor retardation, profound hypotonia and elevated plasma levels of BCAAs, but no elevation of alloisoleucine. Cranial magnetic resonance imaging showed prominent diffuse CSF spaces, delayed myelin maturation and symmetrical signal abnormality within the globi pallidi, midbrain, dorsal pons and medulla. The cerebellar white matter was specifically spared. A mitochondrial disorder was suggested. After correction of feeding problems with G-tube feeds, his high BCAAs persisted and, on fourth analysis, alloisoleucine was seen. Subsequent fibroblast enzyme and mutation analysis confirmed MSUD due to E(1)-alpha subunit deficiency. After starting dietary treatment, there was no significant improvement in his hypotonia or his psychomotor development. However, the high signal within the globi pallidi had resolved. MSUD may have diverse clinical presentations, and should be considered in children who present with chronic psychomotor delay but no acute encephalopathic episodes. BCAA levels may not be very high, alloisoleucine may not always be detected in MSUD even with severe enzyme deficiency, and imaging may be misleading if seen in the chronic phase only.