Genetic Factors in Septic Shock

The study of the genetic basis of disease is a rapidly emerging field of medicine that has the potential to revolutionize both disease diagnostics and therapeutics. The first step in that process requires defining the contribution of specific genes to specific human diseases. The role of specific genes in predisposition to various chronic human metabolic, degenerative, oncologic, and autoimmune diseases has been accepted for decades. Well-known examples include cystic fibrosis, Parkinson disease, and Wilms tumor among many others. Alzheimer disease,¹ diabetes mellitus,² and systemic lupus erythematosus³ represent examples of other diseases that have less well-known associations with specific gene variations or genetic polymorphisms. Recently, a markedly increased risk of breast cancer has been demonstrated in women with an aberrant tumor-suppressor gene (BRCA1 and BRCA2)⁴ while human immunodeficiency virus (HIV) resistance has been linked to a variant of a gene coding for a surface membrane receptor used by the HIV virus (CCR5).⁵

Understanding the terminology is important for proper interpretation of the exciting data being developed in the rapidly emerging field of disease genetics. Different forms of a gene that are sustained in a stable manner at the same chromosomal site within a population are termed alleles. Such alleles typically code for different phenotypic characteristics (eg, blue vs brown eyes or less vs more tumor necrosis factor [TNF] production in response to inflammatory stimulation). In a single individual, only 2 alleles are possible (1 from each parent). Within a population, more than 2 alleles may exist at different frequencies. A genetic polymorphism is a related but broader term indicating a stable gene mutation that is sustained within a population. These mutations generate the different alleles that result in the corresponding phenotypic variants.

Most human diseases linked to specific genetic polymorphisms are chronic in nature and often involve malignancies, autoimmune disease, or inborn errors of metabolism. In contrast, relatively few data have been available on the link between specific genetic polymorphisms (unrelated to general immune function) and development and outcome of acute disease processes, particularly infection. Among the few known genetic polymorphisms that appear to affect the risk and progress of infection, the TNF2 allele stands out. The TNF2 allele represents a stable mutation of the part of the TNF-α gene that regulates TNF-α protein expression (ie, the TNF gene promoter) by altering the level of TNF-α gene transcription. The presence of this TNF2 promoter polymorphism (TNF2 allele) increases the generation of TNF-α protein in vitro⁶ and in vivo.⁷ Clinically, the presence of this allele appears to be responsible for increased susceptibility to mucocutaneous leishmaniasis⁸ and the cerebral form of malaria.⁹ In addition, this allele also appears to be associated with increased risk of death in meningococcal purpura fulminans.¹⁰

The TNF-α cytokine has a central role in the generation of septic shock, a devastating syndrome of fulminant cardiovascular collapse that can affect patients with severe infections. Mortality can range from 30% to 80%, and the disease accounts for more than 100,000 deaths annually in the United States.¹¹ Spontaneous septic shock is generated typically through a sequence beginning with the establishment of a nidus of infection (eg, pneumonia, pyelonephritis, or cellulitis). This infection is followed by the release of antigenically active structural components (eg, endotoxin from gram-negative bacteria) or exotoxins (eg, toxic shock toxin-1 from toxic shock–producing Staphylococcus aureus) from the infecting organism. Subsequently, generation of pro-inflammatory cytokines such as TNF-α and interleukin 1 β (IL-1β) by macrophages and other cells occurs in response to the released bacterial products.¹² These endogenous products, particularly TNF-α, result in the physiologic and metabolic manifestations of septic shock including severe hypotension, myocardial dysfunction,¹³ organ hypoperfusion, and lactic acidosis. Thus, any genetic predisposition to increased TNF-α production under inflammatory stress may have profound implications for the risk of development of septic shock and the probability of survival.

In this issue of THE JOURNAL, Mira and colleagues¹⁴ report their findings regarding the role of the TNF2 allele in septic shock. The authors demonstrate that the TNF2 allele (with an altered TNF-α promoter) occurs with increased frequency in patients with septic shock compared with a randomly selected control group of blood donors. In addition, the investigators show that mortality due to septic shock is increased in patients with this allele even when other factors such as age, acute physiologic derangements, and degree of organ failure are taken into account. In essence, this work shows that a single base pair alteration in the promoter region of the TNF-α gene, contained in the cluster of HLA class III genes on 1 of the 2 copies of chromosome 6, results in an increased risk for and mortality due to septic shock. Most previous studies in this area have demonstrated the involvement of this allele with predisposition to or worsened outcome from infections involving a specific infectious organism at a specific site.^{8 - 10} These data, in contrast, suggest an association of the allele with susceptibility and response to an etiologically heterogeneous condition such as septic shock, involving 1 or more organisms at various anatomic sites of infection.

Although intriguing, there may be as many questions raised by this study as are resolved. Interesting questions include the role of other TNF-α promoter polymorphisms that occurred at too low of a frequency to have independent statistical impact on outcome in this study and the effect of being homozygous vs heterozygous for the TNF2 allele in disease susceptibility and outcome. In addition, the issue of whether the TNF2 promoter polymorphism is specifically associated with septic shock susceptibility and mortality remains an open question. Given that circulating levels of TNF-α are increased in a wide variety of critical illnesses (eg, trauma,¹⁵ burn injury,¹⁶ pancreatitis,¹⁷ and congestive heart failure¹⁸ ), it remains possible that the frequency of the TNF2 allele is increased in all critically ill patients rather than just in those with septic shock. The possibility that the TNF2 allele may cause a more general predisposition to critical illness cannot be ruled out without comparison with a control group of critically ill patients without sepsis. Unfortunately, this important control group was not included in the study by Mira et al.

Perhaps the most important unresolved question is why the TNF2 allele has persisted at such a high population frequency despite conferring what appears to be a marked survival disadvantage. As with other ostensibly detrimental gene mutations that persist in the human population (eg, the sickle cell trait), an unrecognized selective advantage to carriers of the TNF2 allele may exist. For example, the presence of the TNF2 allele may increase the resistance of the host to local infection (by increasing local production of TNF-α at the infection site) even while increasing the risk of septic shock if local immune defenses fail. Further research is clearly needed to answer these questions and to define which patient populations are placed at risk or are protected by the TNF2 allele.

Despite the unresolved questions, this study by Mira et al carries substantial implications for the investigation of sepsis syndrome and septic shock. Although immediate pretreatment identification of this and similar genetic polymorphisms is currently impossible, the study suggests that it may be necessary to control for genetic factors in the post hoc analysis of a clinical trial when assessing the efficacy of experimental antisepsis compounds. If control and treatment populations in such studies exhibit discordant frequencies of the distribution of the TNF2 allele and similar polymorphisms, the results may be biased either in favor of or against the experimental treatment modality. Similarly, this study should provide some impetus for additional efforts to develop anti-TNF strategies for sepsis given the clear demonstration that mortality in septic shock is increased in those who carry a gene that results in increased production of TNF-α under inflammatory stress. In addition, these data support further research into the molecular aspects of sepsis including transcriptional regulation, gene expression, and therapeutic approaches using antisense (ie, artificially synthesized complementary RNA strands) technology and other techniques to modulate regulation of inflammatory gene expression or neutralize specified messenger RNA sequences that code for inflammatory cytokines.

Clinically, these data suggest the possibility that additional genetic polymorphisms exist that could have significant roles in the evolution and final outcome of infection, sepsis syndrome, and septic shock. These polymorphisms may include additional TNF-α promoter alleles and the poorly defined IL-1 and IL-10 alleles noted by Mira et al.¹⁴ Future delineation of genetic polymorphisms associated with outcome in infection, sepsis syndrome, and septic shock may be useful as a prognostic tool initially. However, it is possible to envision a day when a patient's immune response genotype may be predetermined and embedded into a patient's records to allow for rapid initiation of appropriate, patient-specific multimodal therapy for sepsis syndrome and septic shock. In the long term, it is even possible that technology will evolve sufficiently to support rapid bedside assessment of the status of crucial sepsis response genes. Such a future development would allow even more rapid deployment of therapy custom designed for a specific patient. Hopefully, the result will be an improvement of the inordinately high mortality rate of sepsis syndrome and septic shock.