Desmoplakin Mutant Murine Model

Desmoplakin (DSP), localized to chromosome 6p24, was initially identified as the mutant gene causing autosomal recessive Carvajal syndrome [20,21] and later shown to cause autosomal dominant ARVD8 [14]. Desmoplakin is a major component of desmosomes, complex intercellular junctions assembled through cooperative interactions between multiple proteins [22,23]. The majority of patients with DSP have the classic form of ARVC/D, although a substantial number of affected individuals have associated LV disease [42].

Using the Cre-LoxP system [43], a cardiac-restricted exon 2 deletion in DSP was created in mice by Garcia-Gras et al. [44]. These animals were engineered by crossing mice in which the second ex-on of the murine DSP gene is flanked by loxP sequence (floxed DP mice; 129/SvJ strain) with mice expressing Cre recombinase under the control of the a-MHC promoter (a-MHC-Cre mice; FVB/N strain). Homozygous (DP-/-) mutant mice had a high rate of embryonic lethality, consistent with that previously reported by Gallicano et al. [45,46] in germline DP-null mice. These homozygous mutant mouse embryos exhibited growth arrest at embryonic stage E10-E12, appeared pale, had no circulating red blood cells in organs, and were growth retarded. Histopathologic evaluation revealed poorly formed hearts with no chamber specification and unorganized cardiac myocytes. In addition, red blood cells were localized to the pericar-dial sac instead of within the cardiac chambers. Furthermore, an excess number of cells resembling adipocytes, dispersed between myocytes and localized to adjacent areas, were also detected. In comparison, cardiac phenotype was normal in DP+/+ and DP+/- embryos, with and without the a-MHC-Cre transgene. Those DP-/- mice surviving the embryonic period (approximately 5% of the litter) died typically within the first 2 weeks from birth. On the other hand, DP+/- mice were born with normal development but had age-dependent penetrance of heart involvement, including a 20% incidence of SCD by 6 months of life. Gross pathologic analysis of both DP+/- and DP-/- animals demonstrated grossly enlarged cardiac chambers and increased heart weight with increased heart weight-to-body ratio, being highest in the homozygous mutants and lowest in the WT animals. Both RV and LV were enlarged equally and this enlargement occurred at approximately the same age. The gross anatomic findings were further supported by echocardiographic measurements, which revealed thin ventricular walls, increased LV end-diastolic and end-systolic dimensions, and depressed systolic function with reduced ejection fraction. Furthermore, baseline resting electrocar-diographic evaluation identified spontaneous ventricular ectopy, including premature ventricular contractions, ventricular couplets and short runs of VT in heterozygous mutants but no ventricular arrythmias in WT mice. Histologic examination revealed poorly organized myocytes with large areas of patchy fibrosis; in the DP-/- animals, fibrosis was seen in up to 30%-40% of the myocardium. Excess accumulation of fat droplets was notable in both DP-/- and DP+/- mutant mice using Oil Red O staining, and was seen predominantly at the site of fibrosis.

In addition to these pathologic abnormalities, the authors showed that JUP, a member of the armadillo repeat protein family that plays a role in regulation of gene expression, interacts and competes with P-catenin, the effector of the canonical Wnt signaling [47], having a negative effect on this pathway. They were able to show that plakoglobin was translocated to the nucleus in cardiac-restricted DP-deficient mice and that expression levels of gene targets of the canonical Wnt/^-catenin pathway (c-myc and cyclin D1) were reduced (Fig. 7.1). Expression of adipogenic genes were increased, as was TUNEL-positive cells, but in the absence of DNA laddering consistent with low levels of apop-tosis.

Another animal model of mutant desmoplakin was recently described by our group [48]. This model, a transgenic mouse with cardiac-restricted overexpression of a C-terminal DSP mutant (R2834H), demonstrated histological evidence of increased cardiomyocyte apoptosis, cardiac fibro-sis, and lipid accumulation (Fig. 7.2). Echocardio-graphy and cardiac magnetic resonance imaging revealed ventricular enlargement and cardiac dysfunction of both ventricles, which was confirmed on necropsy (Fig. 7.2). RV wall thickness was also reduced. The mutant mice also displayed interruption of the DSP-desmin interaction at intercalated disks and marked ultrastructural abnormalities of the intercalated disks. The intercalated disks were irregularly shaped with markedly widened gaps

Fig. 7.1 • Cardiomyocyte architecture. This illustration shows the various proteins that contribute to the function of the cardiomyocyte. In the extracellular matrix, a2-laminin interacts with the intrasarcolemmal dystrophin-associated protein complex via partnering with a-dystroglycan.The a-dystroglycan interfaces with the p-dystroglycan and sarcoglycan complex, which binds dystrophin and binds with the sarcomere via the actin cytoskeleton. Interactions with the nucleus also occur. At the periphery, the desmosomal proteins are seen (left) and include cadherin, desmocollin, desmoglein, plako-globin,plakophilin,and desmoplakin,the latter interacting with the Wnt pathway.The interactions between these desmosomal proteins is shown in the expanded view at the bottom left of the figure

Fig. 7.1 • Cardiomyocyte architecture. This illustration shows the various proteins that contribute to the function of the cardiomyocyte. In the extracellular matrix, a2-laminin interacts with the intrasarcolemmal dystrophin-associated protein complex via partnering with a-dystroglycan.The a-dystroglycan interfaces with the p-dystroglycan and sarcoglycan complex, which binds dystrophin and binds with the sarcomere via the actin cytoskeleton. Interactions with the nucleus also occur. At the periphery, the desmosomal proteins are seen (left) and include cadherin, desmocollin, desmoglein, plako-globin,plakophilin,and desmoplakin,the latter interacting with the Wnt pathway.The interactions between these desmosomal proteins is shown in the expanded view at the bottom left of the figure between adjacent anchoring sarcomeres, affecting both the adherens junctions and desmosomes (Fig. 7.2). In addition, changes in other desmosomal and junctional components were notable, including increased expression and redistribution of JUP, PKP2, and P-catenin, as well as changes in gap junction components including redistribution of connexin 43.

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