Selective epigenetic alteration of layer I GABAergic neurons isolated from prefrontal cortex of schizophrenia patients using laser-assisted microdissection

Abstract

Among the most consistent results of studies of post-mortem brain tissue from schizophrenia patients (SZP) is the finding that in this disease, several genes expressed by GABAergic neurons are downregulated. This downregulation may be caused by hypermethylation of the relevant promoters in affected neurons. Indeed, increased numbers of GABAergic interneurons expressing DNA methyltransferase 1 (DNMT1) mRNA have been demonstrated in the prefrontal cortex (PFC) of SZP using in situ hybridization. The present study expands upon these findings using nested competitive reverse transcription-polymerase chain reaction combined with laser-assisted microdissection to quantitate the extent of DNMT1 mRNA overexpression in distinct populations of GABAergic neurons obtained from either layer I or layer V of the PFC of SZP. In a cohort of eight SZP and eight non-psychiatric subject (NPS) post-mortem BA9 tissue samples, DNMT1 mRNA was found to be selectively expressed in GABAergic interneurons and virtually absent in pyramidal neurons. DNMT1 mRNA expression was approximately threefold higher in GABAergic interneurons microdissected from layer I of SZP relative to the same neurons microdissected from NPS. GABAergic interneurons obtained from layer V of the same samples displayed no difference in DNMT1 mRNA expression between groups. In the same samples, the GABAergic neuron-specific glutamic acid-decarboxylase 67 (GAD 67 ) and reelin mRNAs were underexpressed twofold in GABAergic interneurons isolated from layer I of SZP relative to GABAergic interneurons microdissected from layer I of NPS, and unaltered in GABAergic interneurons of layer V. These findings implicate an epigenetically mediated layer I GABAergic dysfunction in the pathogenesis of schizophrenia, and suggest novel strategies for treatment of the disease.

Postmortem Brain: An Underutilized Substrate for Studying Severe Mental Illness

“…Recent work has found cell-specific changes in transcript expression in schizophrenia using laser-capture microdissection (LCM) (Ruzicka et al, 2007;Sodhi et al, 2011). These findings suggest that there are cell type-specific abnormalities of protein expression in this illness, and there are numerous hypotheses that could be tested if the LCM technique could be coupled with a reliable, high-sensitivity protein assay.…”

Neurodevelopment, GABA System Dysfunction, and Schizophrenia

“…RELN is expressed in Cajal-Retzius cells during early development and from many GABAergic cells in multiple cortical layers shortly after birth (Alcantara et al, 2006). Brain tissue from schizophrenic patients also consistently report decreased expression of the RELN gene Fatemi et al, 2000Fatemi et al, , 2001Folsom and Fatemi, 2013;Guidotti et al, 2000a;Habl et al, 2012;Impagnatiello et al, 1998;Maloku et al, 2010;Ruzicka et al, 2007), which is likely the result of altered genetic and/ or epigenetic regulation (Costa et al, 2003;Grayson et al, 2005Grayson et al, , 2006Tochigi et al, 2008;Veldic et al, 2004Veldic et al, , 2007. While the RELN deficiency observed in post-mortem tissue clearly does not impact cortical architecture to the same degree as total RELN loss during cortical development, it is likely that even a small reduction of RELN would affect synaptic integration during development and/or synaptic stability and plasticity in adulthood (Frotscher, 2010).…”

“…In addition to being a necessary component of cortical development, RELN also has a role in stabilizing neurons and synapses throughout life (Abraham and Meyer, 2003;Frotscher, 2010;Guidotti et al, 2000b). It is expressed by GABAergic interneurons and the expression of the GAD1 and RELN genes is tightly coordinated by a common epigenetic mechanism (Costa et al, 2004;Grayson et al, 2005Grayson et al, , 2006Guidotti et al, 2000a;Impagnatiello et al, 1998;Kundakovic et al, 2009;Maloku et al, 2010;Noh et al, 2005;Pesold et al, 1999;Rodriguez et al, 2002;Ruzicka et al, 2007;Tochigi et al, 2008;Veldic et al, 2004Veldic et al, , 2007. In addition, rodent models show that RELN deficiency alone can result in downstream reductions of both GAD1 (Kutiyanawalla et al, 2012;Nullmeier et al, 2011;Pascual et al, 2004;Takayama, 1994) and BDNF (Pillai and Mahadik, 2008).…”

Nicotine decreases DNA methyltransferase 1 expression and glutamic acid decarboxylase 67 promoter methylation in GABAergic interneurons

“…Recent studies suggest that the decrease of GAD 67 and other GABAergic genes expression in cortical, hippocampal, and caudate GABAergic neurons of SZP may be caused by promoter hypermethylation (18,19). Such hypermethylation is very likely caused by the overexpression of DNA methyltransferase 1 (DNMT1) in cortical BA9, BA10, and BA17 layers I and II but not in layer V and VI GABAergic interneurons of SZP (18,19).…”

“…Such hypermethylation is very likely caused by the overexpression of DNA methyltransferase 1 (DNMT1) in cortical BA9, BA10, and BA17 layers I and II but not in layer V and VI GABAergic interneurons of SZP (18,19).…”