Smoking May Shorten Colorectal Cancer Survival Most in Tumors With Low LINE-1 Methylation

For decades, doctors have known that patients with colorectal cancer who smoke tend to fare worse than those who never picked up the habit. What has remained murky is why only some smokers experience this survival penalty. A new prospective cohort study published in Epigenetics Communications offers a striking clue: the danger of smoking may concentrate in a specific molecular subset of tumors, those with low levels of methylation at a repetitive DNA element called LINE-1. The findings, drawn from two of the longest-running health studies in the United States, suggest that the epigenetic state of a tumor could determine whether cigarette smoke becomes a lethal accomplice in cancer progression.

The research team, led by investigators at Brigham and Women’s Hospital and Harvard Medical School, drew on data from the Nurses’ Health Study, established in 1976 with 121,701 participants, and the Health Professionals Follow-up Study, established in 1986 with 51,529 participants. Across these cohorts, the researchers identified 4,420 people who developed colorectal cancer during follow-up. For 1,208 of these cases, archived tumor tissue was available, allowing the team to measure LINE-1 methylation levels directly in the cancer cells. After a median follow-up of 16 years among surviving patients, the study documented 776 deaths from any cause, including 343 deaths specifically attributable to colorectal cancer.

LINE-1, short for long interspersed nucleotide element-1, is a transposable element, a genetic relic that appears in hundreds of thousands of copies scattered throughout the human genome. In healthy cells, these elements are normally silenced by chemical tags called methyl groups attached to the DNA. When cancer cells lose this methylation, a phenomenon known as hypomethylation, the genome becomes destabilized. LINE-1 methylation serves as a convenient surrogate marker for this genome-wide loss of methylation, and previous work has linked LINE-1 hypomethylation in colorectal tumors to genomic instability, worse prognosis, reduced response to certain chemotherapies, and even the emerging epidemic of early-onset colorectal cancer diagnosed before age 50.

To measure methylation, the researchers extracted DNA from formalin-fixed, paraffin-embedded tissue blocks, carefully focusing on tumor areas identified by a pathologist blinded to clinical information. They then performed bisulfite treatment, polymerase chain reaction, and pyrosequencing, quantifying methylation across four CpG sites within the LINE-1 element. The resulting scores, ranging from 0 to 100 percent, were normally distributed and categorized as low (below 60 percent), intermediate (60 to below 68 percent), or high (68 percent and above). The team also characterized other molecular features of each tumor, including microsatellite instability, the CpG island methylator phenotype, and mutations in the KRAS, BRAF, and PIK3CA genes, all of which were accounted for in the statistical models.

Smoking status at diagnosis was classified as never, past, or current, based on questionnaires completed before the cancer diagnosis. The researchers then applied a sophisticated statistical framework: multivariable Cox proportional hazards regression with inverse probability weighting, a technique that used data from all 4,420 cases to correct for potential selection bias arising from the fact that tumor tissue was available for only a subset of patients. The models adjusted for a long list of potential confounders, including age, body mass index, alcohol consumption, diet quality, physical activity, aspirin use, family history, and tumor characteristics. The team also adopted an unusually stringent significance threshold of P less than 0.005, following recommendations from a panel of methodologists seeking to reduce false-positive findings.

The central result was a statistical interaction between smoking and tumor LINE-1 methylation. The association between smoking status and mortality differed depending on the tumor’s methylation level, with a P value for interaction of 0.050 for overall mortality and 0.017 for colorectal cancer-specific mortality. Among patients whose tumors showed low LINE-1 methylation, below 60 percent, current smokers at diagnosis had an 80 percent higher risk of death from any cause compared with never smokers, corresponding to a hazard ratio of 1.80 with a 95 percent confidence interval of 1.19 to 2.73. For colorectal cancer-specific mortality in the same low-methylation group, the hazard ratio was 1.55, though its confidence interval crossed one. In sharp contrast, among tumors with high LINE-1 methylation of 68 percent or more, current smoking showed no clear association with either endpoint, with hazard ratios of 1.33 for overall mortality and 0.93 for cancer-specific mortality.

The authors propose a biological explanation rooted in the mechanics of genome maintenance. Experimental studies have shown that disruption of DNA methyltransferase 1, the enzyme responsible for copying methylation patterns during cell division, can trigger global DNA hypomethylation, which in turn promotes chromosomal instability, including loss of heterozygosity at the TP53 tumor suppressor locus. Tumors that have already accumulated this kind of instability may be especially vulnerable to additional mutagenic insults from the carcinogens in cigarette smoke. In other words, patients whose tumors ended up LINE-1 hypomethylated may have been more susceptible all along to the damaging effects of tobacco, and continued smoking after cancer develops may accelerate an already unstable genome toward lethal progression.

An immune dimension may compound the effect. Prior research has linked LINE-1 hypomethylation to reduced infiltration of T cells into colorectal tumors, hinting at an immunosuppressive tumor microenvironment. Separately, smoking has been associated with colorectal cancer subtypes that harbor fewer T cells and macrophages, implying that tobacco suppresses the effector immune cells that would otherwise attack the tumor. Taken together, the study’s results suggest that smoking and LINE-1 hypomethylation may interact to jointly weaken the tumor-immune relationship, producing a particularly aggressive disease course in patients who carry both exposures. This convergence of genomic instability and immune evasion could explain why the smoking penalty appears only in the low-methylation subgroup.

The study has notable strengths, particularly its foundation in molecular pathological epidemiology, a discipline that integrates lifestyle data collected prospectively over decades with detailed tumor molecular profiling. Repeated biennial questionnaires captured smoking, diet, and physical activity long before diagnosis, and the inverse probability weighting approach guarded against bias from missing tissue data. Sensitivity analyses without weighting, or restricted to early-stage cancers, produced consistent results. Still, the authors acknowledge limitations: post-diagnosis smoking changes could not be analyzed because many current smokers quit after diagnosis; treatment and recurrence data were limited; and the interaction findings did not meet the strict P less than 0.005 threshold, prompting the team to emphasize biological plausibility alongside statistical evidence. The sample size also precluded stratification by additional molecular markers such as microsatellite instability status.

Looking forward, the researchers argue that emerging technologies could sharpen the picture. Locus-specific methylation assays combined with machine learning can predict genome-wide methylation more accurately than the four-CpG pyrosequencing average used here, and nanopore sequencing now allows direct, real-time reading of long DNA fragments without amplification bias, opening the door to comprehensive profiling of repetitive element methylation. If larger studies confirm the interaction, LINE-1 methylation could become a practical biomarker for identifying which patients with colorectal cancer have the most to gain from smoking cessation, adding a molecularly targeted dimension to a modifiable behavior. For now, the message is clear: in a subset of colorectal tumors marked by epigenetic destabilization, cigarette smoke appears to do its worst damage.

Subject of Research: The interaction between cigarette smoking and tumor LINE-1 DNA methylation levels in colorectal cancer survival

Article Title: Smoking and colorectal cancer survival in relation to tumor LINE-1 methylation levels: a prospective cohort study

Article References: Kishikawa, J., Ugai, T., Fujiyoshi, K., Chen, Y., Haruki, K., Liu, L., Arima, K., Akimoto, N., Hamada, T., Inamura, K., Kosumi, K., Twombly, T. S., Shi, S., Lau, M. C., Du, C., Li, P., Guo, C., Väyrynen, J. P., Väyrynen, S. A., … Wu, K. (2022). Smoking and colorectal cancer survival in relation to tumor LINE-1 methylation levels: a prospective cohort study. Epigenetics Communications, 2(1), Article 4. https://doi.org/10.1186/s43682-022-00012-y

Image Credits: AI Generated

DOI: 10.1186/s43682-022-00012-y

Keywords: colorectal cancer, smoking, LINE-1 methylation, DNA methylation, epigenetics, cancer survival, molecular pathological epidemiology, genomic instability, tumor immunology, prospective cohort study, biomarker, Nurses' Health Study