The hereditary spastic paraplegias (HSP) are a clinically and genetically heterogeneous group of disorders characterized by progressive lower limb spasticity. Mutations in subunits of the heterotetrameric (ε-β4-μ4-σ4) adaptor protein 4 (AP-4) complex cause an autosomal recessive form of complicated HSP referred to as “AP-4 deficiency syndrome”. In addition to lower limb spasticity, this syndrome features intellectual disability, microcephaly, seizures, thin corpus callosum and upper limb spasticity. The pathogenetic mechanism, however, remains poorly understood. Here we report the characterization of a knockout (KO) mouse for the AP4E1 gene encoding the ε subunit of AP-4. We find that AP-4 ε KO mice exhibit a range of neurological phenotypes, including hindlimb clasping, decreased motor coordination and weak grip strength. In addition, AP-4 ε KO mice display a thin corpus callosum and axonal swellings in various areas of the brain and spinal cord. Immunohistochemical analyses show that the transmembrane autophagy-related protein 9A (ATG9A) is more concentrated in the trans-Golgi network (TGN) and depleted from the peripheral cytoplasm both in skin fibroblasts from patients with mutations in the μ4 subunit of AP-4 and in various neuronal types in AP-4 ε KO mice. ATG9A mislocalization is associated with increased tendency to accumulate mutant huntingtin (HTT) aggregates in the axons of AP-4 ε KO neurons. These findings indicate that the AP-4 ε KO mouse is a suitable animal model for AP-4 deficiency syndrome, and that defective mobilization of ATG9A from the TGN and impaired autophagic degradation of protein aggregates might contribute to neuroaxonal dystrophy in this disorder. Hereditary spastic paraplegia (HSP) is a genetic disorder characterized by progressive stiffness of the legs. To date, mutations in more than 70 genes have been shown to cause HSP. Four of these genes encode the subunits of adaptor protein 4 (AP-4), a heterotetrameric complex involved in intracellular protein trafficking. AP-4 deficiency belongs to a subset of complicated HSPs featuring intellectual disability, seizures, microcephaly and thinning of the corpus callosum in addition to leg stiffness. Here we describe the characteristics of mice lacking the expression of the ε subunit of AP-4. These mice exhibit motor deficits consistent with those seen in the human patients. In addition, they have an abnormally thin corpus callosum and defective axons in various parts of the brain and the spinal cord. Importantly, fibroblasts from AP-4 deficient patients as well as neurons from AP-4 deficient mice fail to deliver the autophagy-related protein 9A (ATG9A) from the Golgi complex to the cell periphery. This defect is associated with accumulation of abnormal protein aggregates in axons from AP-4-deficient neurons. We conclude that AP-4 mutant mice are a suitable animal model for studies of AP-4 deficiency in humans, and that altered localization of ATG9A and aggregate formation in neurons likely contribute to the development of the disease.

Parkinson disease, the second most common movement disorder, is a complex neurodegenerative disorder hallmarked by the accumulation of alpha-synuclein, a neural-specific small protein associated with neuronal synapses. Mutations in the glucocerebrosidase gene (GBA1), implicated in the rare, autosomal recessive lysosomal disorder Gaucher disease, are the most common known genetic risk factor for Parkinson disease. Insights into the inverse relationship between glucocerebrosidase and alpha-synuclein have led to new therapeutic approaches for the treatment of Gaucher disease and GBA1-associated Parkinson disease. Unlike the current drugs used to treat Gaucher disease, which are highly expensive and do not cross the blood-brain-barrier, new small molecules therapies, including competitive and non-competitive chaperones that enhance glucocerebrosidase levels are being developed to overcome these limitations. Some of these include iminosugars, ambroxol, other competitive glucocerebrosidase inhibitors, and non-inhibitory chaperones or activators that do not compete for the active site. These drugs, which have been shown in different disease models to increase glucocerebrosidase activity, could have potential as a therapy for Gaucher disease and GBA1- associated Parkinson disease. Some have been demonstrated to reduce α-synuclein levels in pre-clinical studies using cell-based or animal models of GBA1-associated Parkinson disease, and may also have utility for idiopathic Parkinson disease.

Stuttering is a common neurodevelopmental disorder that has been associated with mutations in genes involved in intracellular trafficking. However, the cellular mechanisms leading to stuttering remain unknown. Engineering a mutation in N-acetylglucosamine-1-phosphate transferase subunits α and β (GNPTAB) found in humans who stutter into the mouse Gnptab gene resulted in deficits in the flow of ultrasonic vocalizations similar to speech deficits of humans who stutter. Here we show that other human stuttering mutations introduced into this mouse gene, Gnptab Ser321Gly and Ala455Ser, produce the same vocalization deficit in 8-day-old pup isolation calls and do not affect other nonvocal behaviors. Immunohistochemistry showed a marked decrease in staining of astrocytes, particularly in the corpus callosum of the Gnptab Ser321Gly homozygote mice compared to wild-type littermates, while the staining of cerebellar Purkinje cells, oligodendrocytes, microglial cells, and dopaminergic neurons was not significantly different. Diffusion tensor imaging also detected deficits in the corpus callosum of the Gnptab Ser321Gly mice. Using a range of cell type-specific Cre-drivers and a Gnptab conditional knockout line, we found that only astrocyte-specific Gnptab-deficient mice displayed a similar vocalization deficit. These data suggest that vocalization defects in mice carrying human stuttering mutations in Gnptab derive from abnormalities in astrocytes, particularly in the corpus callosum, and provide support for hypotheses that focus on deficits in interhemispheric communication in stuttering. Significance Stuttering is a common neurodevelopmental disorder. However, the neurological causes of this disorder are poorly understood. The disorder is highly genetic, and recent discoveries have found several genes involved in this disorder, but how these lead to the unique clinical features of stuttering has been unknown. We have shown that mice carrying human stuttering mutations display vocalization deficits that recreate the salient features of human stuttering. In these mice, we have used several different complementary techniques to identify a specific deficit in astrocytes, a type of glial cell prominent in white matter tracts, particularly in the corpus callosum of these mice. These findings suggest astrocytes as a site of the neuropathology, leading to a deficit in interhemispheric connectivity in stuttering.

Objective: Anti–cyclic citrullinated peptide (anti‐CCP) antibodies are rheumatoid arthritis (RA)–specific serologic markers. RA susceptibility has been associated with HLA–DRB1 shared epitope (SE) alleles and single‐nucleotide polymorphism (SNP) haplotypes in the peptidyl arginine deiminase 4 gene (PADI4). This study was undertaken to determine whether anti‐CCP levels are associated with PADI4 haplotypes and/or SE alleles in Korean patients with RA.Methods: Three nonsynonymous SNPs in PADI4 (padi4_89, padi4_90, and padi4_92) and SE alleles were genotyped, and serum anti‐CCP levels were measured, in 311 patients with nonerosive or erosive RA. The relationships between anti‐CCP levels and PADI4 haplotypes and/or SE alleles were analyzed statistically.Results: Anti‐CCP levels were significantly higher in patients carrying the PADI4 RA risk haplotype than in patients who did not have the risk haplotype, among anti‐CCP–positive patients with RA with a disease duration of ≤34 months (P = 0.041), but not among patients with a longer disease duration or among those who had erosive RA versus nonerosive RA. In contrast, the levels were significantly higher in SE carriers than in noncarriers among patients with RA with a disease duration of ≥141 months (P = 0.0037) and among those who had erosive RA (P = 0.000098), but not among patients who had a shorter disease duration or those who had nonerosive RA.Conclusion: The PADI4 RA risk haplotype is associated with increased anti‐CCP levels in RA patients with disease of short duration, and PADI4 may play a role in early RA. In contrast, SE alleles are associated with increased anti‐CCP levels in RA patients with very longstanding disease and in patients with erosive RA, suggesting that SE alleles play a role in very late RA.

Objective: Recent genome‐wide association scans and replication studies of European populations have disclosed several single‐nucleotide polymorphisms (SNPs) associated with rheumatoid arthritis (RA) susceptibility. The aim of this study was to evaluate the RA‐associated loci by genotyping previously reported SNPs and additional tag SNPs in a Korean population.Methods: A total of 1,316 unrelated RA patients and 1,006 controls were genotyped for 12 SNPs identified in genome‐wide scans and for 12 additional tag SNPs in IL2RB, OLIG3–TNFAIP3, PTPN22, and TRAF1–C5, and the findings were statistically compared.Results: None of the SNPs tested was associated with RA susceptibility, except rs7021206 in TRAF1 intron 3 (P = 0.0032) and, among the SNPs previously reported, rs6457617 in HLA (P = 4.6 × 10⁻³⁵). The association of rs7021206 was positive in patients who were seropositive for rheumatoid factor (P = 0.0051) or for anti–cyclic citrullinated peptide autoantibodies (P = 0.0062). However, Korean patients were negative for the association of rs3761847 in the TRAF1–C5 intergenic region previously reported in Caucasians. Linkage disequilibrium between rs3761847 and rs7021206 was not as high in Koreans (r² = 0.37) as in Caucasians (r² = 0.67), which explains the lack of association of rs3761847 in Koreans. Accordingly, RA susceptibility was localized to an extended haplotype marked by rs7021206 rather than rs3761847, and SNPs highly correlated with rs7021206 (r² ≥ 0.81) extended from rs1953126 in the PHF19–TRAF1 intergenic region to rs2900180 in the TRAF1–C5 intergenic region, spanning 66 kb.Conclusion: Our results demonstrate that within and around TRAF1, excluding PHF19 and C5, SNPs highly correlated with rs7021206, but not those correlated with rs3761847, are associated with RA in both Asians and Caucasians and are possibly correlated with causative variations.