CHMP2B C-truncating mutations in frontotemporal lobar degeneration are associated with an aberrant endosomal phenotype in vitro

Abstract

The charged multivesicular body protein 2B gene (CHMP2B) was recently associated with frontotemporal lobar degeneration (FTLD) linked to chromosome 3 in a Danish FTLD family (FTD-3). In this family, a mutation in the acceptor splice site of exon 6 produced two aberrant transcripts predicting two C-truncated CHMP2B proteins due to a read through of intron 5 (p.Met178ValfsX2) and a cryptic splicing event within exon 6 (p.Met178LeufsX30). Extensive mutation analysis of CHMP2B in Belgian patients (N = 146) identified one nonsense mutation in exon 5 (c.493C>T) in a familial FTLD patient, predicting a C-truncated protein p.Gln165X analogous to the Danish mutant proteins. Overexpression of Belgian p.Gln165X in human neuroblastoma SK-N-SH cells showed the formation of large, aberrant endosomal structures that were highly similar to those observed for Danish p.Met178ValfsX2. Together, these data suggest that C-truncating mutations in CHMP2B might underlie the pathogenic mechanism in FTLD by disturbing endosome function. We also describe a missense mutation in exon 5 of CHMP2B (p.Asn143Ser) in a familial patient with cortical basal degeneration. However, the pathogenic character of this mutation remains elusive.

ESCRT, autophagy, and frontotemporal dementia

“…The aberrant mRNAs were much less abundant than the wild-type mRNAs in FTD patient brains (53). A second C-terminal truncation mutation (Q165X) was found in another FTD patient (56), further supporting the notion that CHMP2B mutations are responsible for some FTD cases. However, a different C-terminal truncation mutation (R186X) has been found in non-affected members of an FTD family (57).…”

“…Ectopic overexpression of CHMP2B intron5 in undifferentiated PC12 cells led to the accumulation of vesicular structures (53). A similar phenotype was observed when other CHMP2B C-terminal truncating mutations were expressed in SK-N-SH cells (56). Studies in cultured mature cortical neurons suggested that CHMP2B intron5 caused dendritic retraction, autophagosome accumulation, and eventual neuronal cell loss (36).…”

Frontotemporal Lobar Degeneration

“…IBMPFD is caused by mutations in the VCP gene. Accumulation of TDP-43 can be appreciated on microscopic examination (van der Zee, Pirici et al 2009). Adult-onset proximal/distal muscle weakness, spine or hip pain and deformity and enlargement of long bones, as well as signs of FTLD characterize IBMPFD.…”

“…Identification of TDP-43, but not VCP protein, within ubiquitinpositive inclusions supports the hypothesis that VCP gene mutations lead to a dominant negative loss or alteration of VCP function culminating in impaired degradation of TDP-43 (Neumann, Mackenzie et al 2007). TDP-43 positive Intranuclear inclusions and dystrophic neurites are characteristic (van der Zee, Pirici et al 2009;Watts, Thomasova et al 2007) and are referred to as FTLD-TDP pathology type D . Inclusions are also present in muscle and heart and are immunoreactive for TDP-43 and beta-amyloid (Watts, Thomasova et al 2007;Kimonis, Fulchiero et al 2008).…”

“…CHMP2B protein is a member of ESCRT-III (endosomal sorting complex required for transport III) and is involved in vesicular fusion events within the endosome -lysosome compartments plays an important role in the process of degradation via autophagy. Mutations in this gene are very rare (Cannon, Baker et al 2006;van der Zee, Urwin et al 2008) and have been first described in a Danish family (Skibinski, Parkinson et al 2005). Pathogenic mutations described so far lead to a partial truncation of the C-terminal region.…”