Bringing the cell nucleus in the focus of NAFLD
Authors: Kurt Zatloukal
Journal: Hepatology (2017)

Potential conflict of interest: Nothing to report. See Article On Page 1710 Cell differentiation and function are determined not only by the information encoded in the DNA bases but also epigenetic modifications and chromatin architecture. In this context, increasing emphasis is placed on the three‐dimensional architecture of chromatin, which modulates the accessibility of DNA by the transcription machinery and, on a larger scale, is reflected in the distribution of heterochromatin and euchromatin within the nucleus. Alteration of the nuclear chromatin distribution has been a well‐known disease feature used by histopathologists for decades in cancer diagnostics and is frequently observed in patients with metabolic syndrome. There is increasing knowledge about proteins that are involved in shaping nuclear morphology and chromatin architecture. One group of these proteins comprises the nuclear lamins, which are constituents of the nuclear envelope and members of the intermediate filament family of proteins forming a filamentous meshwork beneath the inner nuclear membrane, the so‐called nuclear lamina (Fig. 1).1 The nuclear lamina is composed of two B‐type lamins (B1 and B2) as well as lamins A and C, the latter generated by alternative splicing of lamin A RNA. Lamin A binds to chromatin and other proteins of the nuclear envelope, such as B‐type lamins and lamina‐associated protein 2 (LAP2), and may thereby modulate nuclear and chromatin architectures.2 The human disease relevance of lamins has been established by the observation that mutations in the lamin A gene (LMNA) are associated with premature aging, cardiac failure, neuropathies, lipodystrophy, and the metabolic syndrome.3 The latter observations and the fact that nuclear alterations are observed in patients with metabolic syndrome and nonalcoholic fatty liver disease (NAFLD) led Brady et al. to investigate whether genetic variations in genes encoding nuclear lamins, lamin‐associated proteins, or other nuclear envelope transmembrane proteins are linked with an increased risk of NAFLD.5Figure 1: The nuclear envelope with its main components; inner nuclear membrane, outer nuclear membrane, nuclear pore complexes, and nuclear lamina composed of lamins B1 and B2, lamin A, and lamin C. The nuclear lamina is connected with the nuclear pore complexes as well as with the chromatin and the cytoskeleton through a variety of different nuclear envelope transmembrane proteins. Nuclear envelope transmembrane proteins comprise a large group of different proteins including LAP2, which binds to lamins A and C, and the LINC complex, which links the nuclear lamina to the cytoplasmic cytoskeleton. Abbreviations: INM, inner nuclear membrane; Lam, lamin; LINC, linker of nucleoskeleton and cytoskeleton; NETs, nuclear envelope transmembrane proteins; NPC, nuclear pore complex; ONM, outer nuclear membrane.For this aim, they have performed an exon‐directed sequencing study of 10 nuclear lamina‐related genes in a cohort of 37 twin and sibling pairs with and without NAFLD (21 patients with NAFLD and 53 subjects without NAFLD). They found 12 coding sequence variations in four genes: in ZMPSTE24, which encodes a lamin A processing enzyme; in SREBF1 and SRBF2 encoding the sterol regulatory element binding transcription factors 1 and 2, which bind to lamin A; and in TMPO, which encodes LAP2 (for an overview on the increasing number of nuclear envelope–associated proteins, see Worman and Schirmer6). These variants were associated with a significantly increased risk of NAFLD (odds ratio, 17.0' P < 0.001), and 90% of patients with NAFLD (19 of 21) had a variant in at least one lamina‐associated gene, whereas only 30% of subjects without NAFLD (19 of 53) had variants. Brady and coworkers5 then focused on the further investigation and functional characterization of the variants found in TMPO. Of particular interest in this context was the insertion found in TMPO, which resulted in a truncated LAP2 (amino acids 1‐99). The truncated LAP2 1‐99 lost its ability to form homopolymers, did not bind lamin A, and had impaired nuclear localization. These changes in the molecular properties of LAP2 1‐99 were mirrored by a disturbance of the endogenous lamin architecture and accumulation of truncated LAP2 1‐99 in the cytoplasm. Cells expressing the truncated LAP2 1‐99 showed increased lipid accumulation after exposure to oleic acid compared to cells expressing wild‐type LAP2 (1‐694). To investigate whether this effect on lipid metabolism was related to altered binding properties of LAP2 1‐99 to other proteins, immunoprecipitation of lysates from cells transfected with LAP2 1‐99 followed by mass spectrometric analyses of coprecipitated proteins were performed. The data revealed several potential new binding partners of the truncated LAP2 1‐99, of which p62/SQSTM1 (p62) was one of the top‐ranked proteins. This change in binding properties to p62 links LAP2 variants to a protein (namely, p62) that is well known as a constant component of Mallory‐Denk bodies, which are cytoplasmic protein aggregates occurring in hepatocytes in patients with nonalcoholic steatohepatitis and alcoholic steatohepatitis.7 Furthermore, p62 has multiple functions in cell signaling including Nrf2‐mediated defense responses and lipid metabolism.8 The results reported by Brady and coworkers5 open a further chapter in the search for alleles conferring susceptibility to NAFLD and provide a molecular explanation for the heritability of susceptibility to NAFLD as evidenced by a variety of twin and family studies.10 The observation that genetic variants of proteins of the nuclear lamina are associated with NAFLD is intriguing and poses the question as to how proteins with rather general cellular functions, such as contributing to nuclear morphology or chromatin architecture, are related to specific function and disease. One possible explanation is that truncated LAP2 1‐99 loses its physiological function and at the same time acquires novel functions that exert specific (e.g., metabolic) effects which eventually confer increased risks of NAFLD. An increasing number of mechanisms have been discovered showing how ubiquitously expressed proteins or proteins with multiple functions may exert specific roles. For example, tissue‐specific effects can be explained by tissue‐specific expression of splice variants, by binding partners that are either only expressed in certain tissues or mediators of specific effects, or by altering the chromatin architecture and thereby impacting on expression of certain genes.2 In analogy, changes in amino acid composition or truncating variants in nuclear lamina–associated proteins, such as those reported for LAP2 by Brady and coworkers,5 may either directly or indirectly (e.g., through alterations of binding partners) exert specific effects related to NAFLD. Author names in bold designate shared co‐first authorship.