inheritance patterns. Currently, no treatment exists. This study represents the first longitudinal examination of NM patients, providing a more accurate estimate of the rate of change and a more precise documentation of the clinical phenotype. Participants were evaluated at family conferences in 2009 and 2016. Physical examinations, pulmonary function testing (PFT) and motor function assessment (MFM20) were performed along with collection of medical history. 58 patients were enrolled with 17 participating at both time points. Ages ranged from 1–37 years (73% under age 18). Genotypes included 24% ACTA1, 29% NEB, 3% TPM2 and 43% unknown. The majority of participants were ambulant (67%) and required respiratory support (60%). Motor function testing using MFM20 showed a clinically meaningful decrease of total scores in 54% between time points, while 31% remained stable. Participants demonstrated greatest difficulty in domain 1, representing standing and transfers. Of those requiring respiratory support, 52% required invasive ventilation. PFT revealed significant respiratory involvement with the majority of patients below the clinical cutoffs for MIPS (90%), peak cough flow (53%) and FVC (74%). Longitudinal analysis of PFT’s (n = 4) showed stable peak cough flow and FVC with increasing MIPS. Many of participants did not have a severe phenotype based on the requirement of technologies (wheelchair, trach/vent, G/GJ tube) as 41% did not require any support, while only 15% required all three. NM represents a wide clinical spectrum, with the majority of patients on the milder end compared to other congenital myopathies. Motor function appears to decrease over time, while respiratory status remains relatively stable. MFM20 and PFT’s look promising for use as outcome measures in future clinical trials. http://dx.doi.org/10.1016/j.nmd.2017.06.320 P.281 A novel copy number variation detection array for the diagnostics of neuromuscular disorders L. Sagath 1 , V. Lehtokari 1 , C. Wallgren-Pettersson 1 , K. Pelin 2 , K. Kiiski 1 1 The Folkhälsan Institute of Genetics, Helsinki, Finland; 2 University of Helsinki, Helsinki, Finland Nemaline myopathy (NM) is caused by mutations in at least eleven different genes, but most commonly by recessive mutations in the nebulin gene (NEB), a 183 exon gene essential for correct sarcomere structure and function. NEB harbors a 32 kb triplicate region (TRI), in which eight exons are usually typically repeated three times (ex 82–89, 90–97, 98–105), and is prone for copy number variation (CNV).We have designed and validated a new custom-made 4x180k comparative genomic hybridization array (aCGH) design. It includes the same tiled high density coverage of the ten previously known NM genes as our previous 8x60k NM-CGH-array, but also covers 176 additional genes that have been related to neuromuscular diseases, such as titin (TTN). As to date, we have analyzed samples from 259 NM families with the aCGH method, and identified 15 different disease-causing aberrations in NEB in 31 of these families. The majority of these pathogenic variations were detected only in one to three families each. CNVs affecting the TRI region were detected in 16% of the NM families, and in 5% of the families the CNV was interpreted to be pathogenic. Pathogenic CNVs have been found in altogether 12% of all affected families analyzed. The new 4x180k array allows the reliable detection of CNVs also in regions of repetitive DNA segments such as the NEB TRI, and the similar, not yet fully characterized, region of TTN. Our novel 4x180k array- CGH design allows CNV detection of a broader spectrum of neuromuscular disorders and brings thus a new alternative to CNV validation and mutation detection in patients with neuromuscular disorders. The method is available for mutation analysis in our laboratory. http://dx.doi.org/10.1016/j.nmd.2017.06.321 P.282 KHLH40 mutations causing severe neonatal nemaline myopathy T. Willis, R. Kulshrestha, C. Sewry Robert Jones and Agnes Hunt hospital, Oswestry, UK Mutations the KHLH40 gene are a frequent cause of severe autosomal recessive neonatal nemaline cases. The KHLH40 gene (Kelch-like family member 40 gene) mutations have been found to be a relatively common cause of the severe nemaline myopathy, in particular in the Japanese population, accounting for 28% of cases. The striated-muscle-specific protein has been shown to be absent in these cases of KHLH40-associated nemaline myopathy skeletal muscle. It plays an important part in muscle development and function and therefore should be suspected in severe cases of nemaline myopathy with contractures and pre-natal symptoms. We describe a family who presented with three children affected with severe neonatal presentations, the two previous siblings clinically had nemaline myopathy but with no genetic diagnosis and died within the neonatal period. 7 years later the third child affected clinically with nemaline myopathy presented and we have subsequently confirmed genetically that all three cases were caused by mutations in the KHLH40 gene. The family have 2 unaffected children and the third baby with nemaline myopathy is now 7 months old supported with non-invasive ventilation during sleep and gastrostomy fed. Clinical features will be described in detail. The children were all found to be compound heterozygotes, with a mutation in the KLHL40 gene; c.434_435delGCinsAA p.(Cys145Ter), this variant results in a premature termination of the KLHL40 protein and therefore pathogenic, the second KHLH40 c.557T>A p.(Leu186His) sequence variant has not been previously described. It affects a highly conserved amino acid and in silico analysis predicts that this variant will affect the function of the KHLH40 protein. The cases are described in detail along with the muscle biopsies in two of the cases. http://dx.doi.org/10.1016/j.nmd.2017.06.322 P.283 The regulation of leiomodins by Kelch proteins – deciphering the mechanism of pathogenicity in nemaline myopathy M. Yuen 1 , H. Best 2 , D. Ahmadi Rastegar 2 , B. Cenik 3 , E. Olson 3 , N. Clarke 2 , S. Cooper 2 1 VU Medical Center, Amsterdam, Netherlands; 2 The Children’s Hospital at Westmead, Sydney, Australia; 3 The University of Texas Southwestern Medical Center, Dallas, USA Recessive loss-of-function mutations in LMOD3 were recently associated with severe congenital nemaline myopathy. LMOD3 encodes leiomodin 3 (LMOD3), a member of the tropomodulin protein family which is likely involved in regulating the formation and structure of actin filaments in muscle. Mutations in KLHL40, a member of the Kelch protein family, also cause nemaline myopathy. Kelch proteins function as adaptor proteins regulating ubiquitination and protein degradation of their binding partners. KLHL40 was shown to regulate the stability of LMOD3 and nebulin, thus shedding light on how mutations in Kelch proteins may result in muscle weakness. It remains unclear how KBTBD13 and KLHL41, two myopathy associated Kelch proteins, cause disease and whether leiomodin 2 (LMOD2), a second striated muscle leiomodin, is regulated by Kelch proteins similarly to LMOD3. We studied the tissue-specific expression of striated muscle leiomodins and used a tissue culture model to closely characterise the regulation of leiomodins by KLHL40 and 41. We found that LMOD2 is expressed in human cardiac muscle, whereas LMOD3 is present in both skeletal and cardiac muscle. Similar to LMOD3, LMOD2 levels are increased when KLHL40 is co-expressed in COS-7 cells. Additionally, we showed that KLHL41 also stabilises LMOD2 and LMOD3 in vitro. Using LMOD3 deletion constructs we demonstrated that the N-terminus of LMOD3 is primarily responsible for conferring KLHL40 and 41 interactions (amino acid 1–100). In conclusion, our data indicates that cardiac and skeletal muscle leiomodins, LMOD2 and LMOD3, are regulated by both KLHL40 and 41. We thus provide a potential link for KLHL41 to the sarcomere and a S182 Abstracts 2017/Neuromuscular Disorders 27 (2017) S96–S249