stomach. In conclusion, this rare condition occurred in 2 Gaucher patients who had been stable on ERT. This report expands the differential diagnosis for onset of new abdominal pain in otherwise stable Gaucher patients and discusses the appropriate evaluation and management of wandering spleen. Dweck A, Abrahamov A, Hadas- Halpern I, Zimran A, Elstein D. Wandering spleen in a young girl with Gaucher disease. Isr Med Assoc J. 2001 Aug;3(8):623-4. doi:10.1016/j.ymgme.2015.12.333 176 Utilizing activity assays and population-wide allele frequencies to assess the contribution of novel mutations in NAGLU to MPS IIIB incidence Jonathan H. LeBowitz, Wyatt T. Clark, G. Karen Yu, Mika Aoyagi- Scharber, BioMarin, Novato, CA, United States Mucopolysaccharidosis type IIIB (MPS IIIB; Sanfilippo syndrome type B), caused by mutations in the gene encoding alpha-N- acetylglucosaminidase (NAGLU), has an estimated birth incidence that varies substantially in European populations from 0.08/100,000 in France to 0.78/100,000 in Greece and Portugal. Such variability in incidence reported from country to country increases the challenge of developing accurate assessments of global disease incidence. Given the large and expanding number of publically available sequencing data, it should be possible to extract incidence information from allele frequencies, provided one knows which mutations are associated with disease. Out of the 189 NAGLU missense variants found in the ExAC dataset (http://exac.broadinstitute.org, release 0.3) which comprises ~60,000 individual sequenced exomes, 24 are known to be disease associated. The potential contribution of the remaining 165 missense variants to disease incidence is unknown. This illustrates the funda- mental problem with efforts to extract incidence information from allele frequency data: given a rare disease caused by a large number of ultra-rare mutations, there will be a large number of variants of unknown significance whose contribution to disease is difficult to assess. To address this problem, we have functionally assessed the enzymatic activity of each of the 165 novel NAGLU missense mutations in the ExAC dataset. Plasmids containing cDNAs encoding each of the mutant proteins were transiently transfected into HEK293 cells and NAGLU activity was assessed using the artificial substrate 4-methyl- umbelliferyl N-acetyl-alpha-D-glucosaminide. We found a number of variants that decreased the activity of NAGLU to levels consistent with the MPS IIIB phenotype. The results of this analysis suggest that a substantial fraction of MPS IIIB incidence is accounted for by novel mutations not previously identified in patients, illustrating the utility of combining assays of enzymatic activity for variants of unknown significance with population wide allele frequency data. doi:10.1016/j.ymgme.2015.12.334 177 Accumulation of MPS GAG influences stem cell differentiation Rebecca Lehmann a,b , Ainslie Derrick Roberts a,b , Sharon Byers a,b , a SA Pathology, Adelaide, Australia, b University of Adelaide, Adelaide, Australia The mucopolysaccharidoses (MPS) are a series of inherited lysosomal disease which result from a deficiency in one of the enzymes required for the stepwise degradation of glycosaminoglycans (GAG). The MPS disorders are characterised by increased lysosomal accumulation of GAG, leading to significant cellular dysfunction. GAG are integral to many signalling pathways involved in osteogenesis and neurogenesis, and their binding capacity is determined by distinct patterns of sulphation along the length of the chain. MPS GAG have altered sulphation patterns compared to normal GAG, which may disrupt osteogenesis and neurogenesis in patients, leading to the prominent skeletal and neurological pathology that characterises the MPS disorders. In this study, we investigated the differentiative capacity of mesenchymal stem cells (MSC) isolated from MPS and normal bone. MPS I, IIIA and normal MSC were isolated from compact murine bone and expanded in culture. Normal MSC proliferate rapidly and differentiate down the osteogenic and adipogenic lineages, as evidenced by von Kossa and oil red staining respectively. MSC isolated from MPS I murine compact bone also proliferated rapidly; however, mineralisation commenced earlier than normal and cells accumulated approximately twice as much mineral as determined by calcium assay. In contrast, adipogenesis was delayed in MPS I MSC, despite reaching the same final level as normal. These results support the high bone mass phenotype observed in the MPS I mouse model. In contrast to normal MSC, MSC isolated from MPS IIIA murine compact bone failed to proliferate and cell number decreased with time. Exogenous supplementation of recombinant murine sulphamidase enzyme to the cell culture media restored the proliferative capacity of MPS IIIA MSC. To date our results suggest that the lysosomal turnover of N-sulphated HS GAG is crucial to MSC proliferation, while the accumulation of DS and lesser sulphated HS directs MSC differentiation towards the osteoblast lineage. doi:10.1016/j.ymgme.2015.12.335 178 Fabry patients after enzyme replacement therapy dose reduction and treatment switch: Renal impairment after 24 months follow-up Malte Lenders a , Sima Canaan-Kühl b , Johannes Krämer c,d , Thomas Duning a , Stefanie Reiermann a , Claudia Sommer d , Jörg Stypmann a , Daniela Blaschke b , Nurcan Üçeyler d , Hans-Werner Hense a , Stefan- Martin Brand a , Christoph Wanner d , Frank Weidemann e,d , Eva Brand a , a University Hospital Muenster, Muenster, Germany, b Charité Berlin, Berlin, Germany, c University of Ulm, Ulm, Germany, d University of Würzburg, Würzburg, Germany, e Katharinen-Hospital Unna, Unna, Germany Due to the restriction of the agalsidase-beta supply between 2009 and 2012, patients with Fabry disease (FD) were treated either with reduced doses (0.3 - 0.5 mg/kg body weight) or were switched to agalsidase-alfa. In this observational study, we assessed potential end-organ damage and clinical symptoms after 24 months of dose- reduction and/or switch to agalsidase-alfa. A total of 89 adult patients with FD who had previously received agalsidase-beta (1.0 mg/kg body weight) for N 12 months were non-randomly assigned to continue this treatment regimen (regular-dose group, n = 24), to receive a reduced dose of 0.3 - 0.5 mg/kg and a subsequent switch to 0.2 mg/kg agalsidase-alfa (dose-reduction-switch group, n = 28), or to directly switch to 0.2 mg/kg agalsidase-alfa (switch group, n = 37) and were followed-up for 24 months. Clinical events including death, myocar- dial infarction, severe arrhythmia, stroke, progression to ESRD, changes in cardiac and renal function, Fabry-related symptoms (pain, hypohidrosis, and diarrhea) and disease severity scores were assessed. Renal function was determined by creatinine and cystatin C-based estimated glomerular filtration rates (eGFR) and revealed decreasing eGFRs in the dose-reduction-switch as well as the switch group. In addition, the Mainz Severity Score Index increased significantly in these two groups (p = 0.0174 and p = 0.0002) and higher Abstracts / Molecular Genetics and Metabolism 117 (2016) S14–S124 S72