We describe here individuals with dilated cardiomyopathy who possess a truncation mutation in the PLN gene that results in an absence of detectable PLN protein — that is, PLN null. This experiment of nature is therefore analogous to previously published PLN gene ablation or “knockout” studies in mice (11
). The most important finding is that PLN ablation in humans appears to have effects that are the opposite of those observed with PLN ablation in mice. Whereas the PLN-null mouse displays chronically increased basal cardiac contractile function without developing heart failure, even in advanced age (13
), both human subjects homozygous for the PLN-L39stop mutation developed severe dilated cardiomyopathies requiring cardiac transplantation at an early age, and two heterozygous individuals from an independent family also developed delayed dilated cardiomyopathy. Although our findings indicate the virtual absence of PLN in the homozygous hearts, we cannot rule out the possibility that a highly unstable/rapidly degraded and inactive form of PLN, associated with the PLN-L39stop mutation, may initiate human dilated cardiomyopathy.
The mouse has been used extensively as a model system for human disease and for the design of appropriate therapies, but the phenotype of mouse models may differ from the phenotypes observed in human diseases with the corresponding genotype (29
). In this study, the surprising difference between human and mouse PLN-null cardiac phenotypes emphasizes the need to better understand inherent differences in cardiac physiology and Ca2+
-cycling mechanisms between mice and humans (31
). The mouse heart beats over 600 times per minute, or about 10 times faster than the human heart, indicating different mechanisms regulating the dynamic balance of cardiomyocyte Ca2+
fluxes. Whereas Ca2+
removal from the cytosol during cardiac relaxation in mice relies almost exclusively on SERCA2a (92% of total), in humans Na+
exchanger activity accounts for approximately one third of Ca2+
removal, with SERCA2a function largely responsible for the remainder (32
). The basic ventricular motor proteins are also different, with α-myosin heavy chain predominating in adult mice but β-myosin heavy chain predominating in humans (30
). An absence of PLN in mouse hearts, already operating close to their theoretical maximum, might therefore be less deleterious than in the slower-beating human hearts, in part through the regulatory effects of PLN.
In human subjects homozygous for the PLN-L39stop mutation, PLN is absent and SERCA2a should be tonically disinhibited — that is, activated — with enhancement of inotropy and lusitropy analogous to that observed in PLN-null mice (11
). Increased cardiac work over a period of years could conceivably lead to ventricular hypertrophy, as seen in some of the heterozygous PLN-L39stop individuals, which may then progress to ventricular failure as observed in the homozygous mutants. Indeed, there are numerous examples of a direct association between the development of heart failure and chronic inotropic stimulation, such as pheochromocytoma (15
) and catecholamine cardiomyopathies (16
), as well as the increased incidence of heart failure from genetic polymorphisms that augment norepinephrine release and cardiac β1
-adrenergic receptor (βAR) function (35
). For each of these conditions, prolonged, unregulated stimulation of the human heart causes or accelerates the progression of heart failure, and the same may be true of the PLN-L39stop mutant. On the other hand, chronic suppression of the β-adrenergic signaling pathway stimulatory effects by a dominant R9C-PLN mutant also results in human cardiomyopathy and heart failure (36
). Thus, a fine balance between the degree of SERCA2a inhibition and augmentation by PLN is essential for normal cardiac function.
The variable expression of the clinical phenotype, elicited by PLN-L39stop, is not unusual in inherited cardiomyopathy (17
). In this study, heterozygosity of PLN-L39stop was associated with hypertrophy in some members of both families and with overt dilated cardiomyopathy in other members. As such, environmental perturbations are likely to contribute to the mechanism by which absence of functional PLN induces cardiac dysfunction. For example, in PLN-null mice, the persistently enhanced inotropic state impairs functional recovery from ischemia (37
), and chronic β-adrenergic stimulation increases the incidence of ventricular failure after ischemia or pressure overload (38
). Likewise, there are undoubtedly as-yet-unidentified genetic influences that can modify the PLN-L39stop phenotype, as strongly suggested by the development of dilated cardiomyopathy in two heterozygous subjects from one of our kindreds but ventricular hypertrophy with normal cardiac function in heterozygous individuals from the other kindred. Future studies in additional human kindreds, and in PLN mice with different genetic backgrounds, will help to resolve this issue.
In conclusion, a genetic mutation resulting in lack of expression of PLN protein was identified in individuals from two families with inherited dilated cardiomyopathy. To our knowledge, this is the first instance of a human equivalent of the PLN-null mouse. The apparent pathological effects of absent PLN in human hearts contrast strikingly with benefits observed in many, although not all (39
), mouse heart failure models. These data emphasize a general concern that targeted therapies whose design is based exclusively on results of studies in rodent models, in which phenotypes can differ radically from those observed in the corresponding human genetic condition, may not ultimately be successful in human disease. A specific cautionary note is sounded as to whether drugs targeted to inhibition of PLN will ultimately have a therapeutic role in chronic, as compared with acute, human heart failure.