Molecular biology of anal squamous cell carcinoma: implications for future research and clinical intervention

Summary

Anal squamous cell carcinoma is a human papillomavirus-related disease, in which no substantial advances in treatment have been made in over 40 years, especially for those patients who develop disease relapse and for whom no surgical options exist. HPV can evade the immune system and its role in disease progression can be exploited in novel immunotherapy platforms. Although several studies have investigated the expression and inactivation (through loss of heterozygosity) of tumour suppressor genes in the pathways to cancer, no clinically valuable biomarkers have emerged. Regulators of apoptosis, including survivin, and agents targeting the PI3K/AKT pathway, offer opportunities for targeted therapy, although robust data are scarce. Additionally, antibody therapy targeting EGFR may prove effective, although its safety profile in combination with standard chemoradiotherapy has proven to be suboptimal. Finally, progress in the treatment of anal cancer has remained stagnant due to a lack of preclinical models, including cell lines and mouse models. In this Review, we discuss the molecular biology of anal squamous cell carcinoma, clinical trials in progress, and implications for novel therapeutic targets. Future work should focus on preclinical models to provide a resource for investigation of new molecular pathways and for testing novel targets.

Introduction

Anal squamous cell carcinoma is the most common histological type of malignant disease of the anal canal. Anatomically, the anal canal is defined as the last 3–5 cm of the gastrointestinal tract between the anorectal junction and the anal verge. The region proximal to the dentate line, which is visible macroscopically, is lined by columnar epithelia, and the region distal to the dentate line is lined by stratified squamous epithelia. At the junction between the columnar and squamous epithelia is a 6–12 mm transitional zone containing cells of varying types including unkeratinised squamous, transitional, basal, cuboidal, and columnar cells. This heterogeneous histological zone gives rise to most anal squamous cell carcinomas,1, 2 although the specific cell of origin has no apparent effect on treatment or outcome.³

The incidence of anal squamous cell carcinoma is 0·2–4·4 per 100 000 people per year with a female predominance.⁴ The incidence has risen around the world over the past three decades,5, 6, 7 particularly in men who have sex with men (35 per 100 000 per year) and those with HIV (75–135 per 100 000 per year).⁸ Nevertheless, the standard treatment for anal cancer, which consists of chemoradiotherapy with fluorouracil and mitomycin C, has remained essentially unchanged since this sphincter-preserving strategy was first proposed.⁹ The main advances since that time have been in the use of intensity-modulated radiation therapy that tailors radiation distribution to the tumour target,¹⁰ and in the use of cisplatin, which has shown similar efficacy to fluorouracil and mitomycin C in a large randomised trial.¹¹ For non-metastatic disease, 5-year survival rates range from 39·6% to 69·5% depending on overall stage.¹² 3-year progression-free survival is only 62–67% for those patients with a high disease stage (T3–4), and 68% in those with node-positive disease.¹³ Up to 25% of patients relapse, mainly within the first 3 years of completing chemoradiotherapy.¹⁴ Locoregional persistence or recurrence is the most common type of relapse for patients with anal cancer, with the only chance of cure being salvage surgery in the form of a radical abdominoperineal resection, resulting in a permanent colostomy, and with 5-year survival rates ranging from 40% to 60%. This surgery is very morbid, and results in further local relapse in 40–50% of patients.¹⁵ At present, patients with anal squamous cell carcinoma who either are not eligible for salvage surgery, have relapse after surgery, or have metastatic disease, are considered for a systemic chemotherapy regimen, but with 5-year survival rates of 15%.¹²

These approaches to treatment and their respective outcomes highlight several important aspects in the management of patients with anal cancer. First, although clinicopathological staging using the TNM model¹² provides relative consistency throughout clinical trials in predicting patients' response to treatment, it is not perfectly reliable as there is some heterogeneity in responses for individual patients in the same stage of cancer, which suggests that sensitivity to chemoradiation is somewhat heterogeneous. There is no way of predicting response to standard chemoradiotherapy, where some patients derive little benefit or are left with substantial late and morbid toxic effects.

Due to the relative rarity of anal squamous cell carcinoma and the reasonable responses to primary chemoradiotherapy, it is difficult to undertake suitable trials for the proportion of patients who have local disease relapse after surgery or who have metastatic disease. Consequently, there have been few advances in available treatment options for these patient populations. The most recent National Comprehensive Cancer Network guidelines¹⁶ for metastatic anal cancer remain broad, but acknowledge that only cisplatin-based chemotherapy is recommended as a treatment option for patients with anal cancer, as no other regimens have shown efficacy.

The DNA of human papillomavirus (HPV) is present in 88% of cancers and in most anal precursor lesions, and is recognised as the main cause of anal squamous cell carcinoma.17, 18 This nomenclature has been updated to squamous intraepithelial lesions of the anus: low-grade squamous intraepithelial lesions, analogous to anal intraepithelial neoplasia I, or high-grade squamous intraepithelial lesions, analogous to anal intraepithelial neoplasia II/III.¹⁹ Much research has been done to attempt to elucidate the molecular mechanisms by which HPV is associated with the development of anal intraepithelial neoplasia with subsequent progression to squamous cell carcinoma. This knowledge can hopefully then be exploited to offer novel therapeutic options. As knowledge on the molecular biology of anal cancer is scarce, parallels are often drawn from what is known regarding other HPV-related cancers, especially cervical cancer and head and neck squamous cell carcinoma. Most of the translational research in anal squamous cell carcinoma has been directed at identifying prognostic and predictive biomarkers in the hope they might lead to the development of tailored individualised therapy, and the ability to predict response to treatment, and hence improve patient outcomes.

Few preclinical models to test novel therapies have been reported for anal squamous cell carcinoma. These include a cell line derived from a lymph node metastasis,²⁰ two transgenic mouse models,21, 22 and a xenograft from a single patient.²³ In this Review, we discuss the results of these diverse approaches in improving our knowledge and treatment options for patients with anal squamous cell carcinoma.