TBC1D20 mediates autophagy as a key regulator of autophagosome maturation

In humans, loss of TBC1D20 (TBC1 domain family, member 20) protein function causes Warburg Micro syndrome 4 (WARBM4), an autosomal recessive disorder characterized by congenital eye, brain, and genital abnormalities. TBC1D20-deficient mice exhibit ocular abnormalities and male infertility. TBC1D20 is a ubiquitously expressed member of the family of GTPase-activating proteins (GAPs) that increase the intrinsically slow GTP-hydrolysis rate of small RAB-GTPases when bound to GTP. Biochemical studies have established TBC1D20 as a GAP for RAB1B and RAB2A. However, the cellular role of TBC1D20 still remains elusive, and there is little information about how the functional loss of TBC1D20 causes clinical manifestations in WARBM4-affected children. Here we evaluate the role of TBC1D20 in cells carrying a null mutant allele, as well as TBC1D20-deficient mice, which display eye and testicular abnormalities. We demonstrate that TBC1D20, via its RAB1B GAP function, is a key regulator of autophagosome maturation, a process required for maintenance of autophagic flux and degradation of autophagic cargo. Our results provide evidence that TBC1D20-mediated autophagosome maturation maintains lens transparency by mediating the removal of damaged proteins and organelles from lens fiber cells. Additionally, our results show that in the testes TBC1D20-mediated maturation of autophagosomes is required for autophagic flux, but is also required for the formation of acrosomes. Furthermore TBC1D20-deficient mice, while not mimicking severe developmental brain abnormalities identified in WARBM4 affected children, display disrupted neuronal autophagic flux resulting in adult-onset motor dysfunction. In summary, we show that TBC1D20 has an essential role in the maturation of autophagosomes and a defect in TBC1D20 function results in eye, testicular, and neuronal abnormalities in mice implicating disrupted autophagy as a mechanism that contributes to WARBM4 pathogenesis.     

Polymicrogyria in fetal alcohol syndrome

BACKGROUND: Intrauterine exposure to alcohol may result in a distinct pattern of craniofacial abnormalities and central nervous system dysfunction, designated fetal alcohol syndrome (FAS). The spectrum of malformations of the brain associated with maternal alcohol abuse during pregnancy is much broader than the relatively uniform clinical phenotype of FAS. Among these malformations the most striking abnormalities involve the impairment of neuronal cell migration. However, polymicrogyria (PMG) has so far been reported only once in a human autopsy study of a child with FAS. CASE: A 16‐year‐old girl with confirmed maternal alcohol consumption during pregnancy and full phenotype of FAS presented after two generalized epileptic seizures for neurologic assessment. Cranial magnetic resonance imaging revealed bilateral PMG in the superior frontal gyrus with asymmetric distribution. History, clinical features, and genetic investigations provided no evidence for any of the known genetic or acquired causes of PMG. Therefore, we propose that prenatal alcohol exposure is the cause of PMG in this patient rather than a mere coincidence. CONCLUSION: Our observation represents only the second patient of PMG in FAS and confirms the phenotypic variability of cerebral malformations associated with maternal alcohol abuse during pregnancy. In patients with clinical features of FAS and neurologic deficits or seizures neuroimaging is recommended. Furthermore, FAS should be considered as a differential diagnosis for PMG. Birth Defects Research (Part A), 2010. © 2009 Wiley‐Liss, Inc.