Diagnostic Value of MAML2 Rearrangements in Mucoepidermoid Carcinoma

Abstract

Mucoepidermoid carcinoma (MEC) is often seen in salivary glands and can harbor MAML2 translocations (MAML2+). The translocation status has diagnostic utility as an objective confirmation of the MEC diagnosis, for example, when distinction from the more aggressive adenosquamous carcinoma (ASC) is not straightforward. To assess the diagnostic relevance of MAML2, we examined our 5-year experience in prospective testing of 8106 solid tumors using RNA-seq panel testing in combinations with a two-round Delphi-based scenario survey. The prevalence of MAML2+ across all tumors was 0.28% (n = 23/8106) and the majority of MAML2+ cases were found in head and neck tumors (78.3%), where the overall prevalence was 5.9% (n = 18/307). The sensitivity of MAML2 for MEC was 60% and most cases (80%) were submitted for diagnostic confirmation; in 24% of cases, the MAML2 results changed the working diagnosis. An independent survey of 15 experts showed relative importance indexes of 0.8 and 0.65 for "confirmatory MAML2 testing" in suspected MEC and ASC, respectively. Real-world evidence confirmed that the added value of MAML2 is a composite of an imperfect confirmation test for MEC and a highly specific exclusion tool for the diagnosis of ASC. Real-world evidence can help move a rare molecular-genetic biomarker from an emerging tool to the clinic.

Keywords: CRTC; adenosquamous; biomarker; fusion gene; molecular testing; mucoepidermoid; next-generation sequencing.

Figure 1

Survival Analysis and Morphology of Mucoepidermoid Carcinoma (MEC) and Adenosquamous Carcinoma (ASC). (a) Data were accessed using the SEER database comprised of 18 registries from 2000–2017 from the National Cancer Institute. Only patients with mucoepidermoid carcinoma (MEC, ICD-O-8430) and adenosquamous carcinoma (ASC, ICD-O-8560) were included in this analysis. Reports of observed survival were pulled as case listings and analyzed using the Kaplan–Meier method. (b) Mucoepidermoid carcinoma (MEC; MAML2+). (c) Adenosquamous carcinoma (ASC; MAML2–) with keratinizing and non-keratinizing squamous and glandular components. Abbreviations: ASC, adenosquamous carcinoma; MEC, mucoepidermoid carcinoma.

Figure 2

Molecular–genetic diagnostic test results integration. The generation of a diagnosis from a tissue sample relies on the interpretation carried out by the surgical pathologist. To follow the concept of integrating molecular diagnostics, the time of molecular testing (mTAT) needs to be considered. To perform molecularly informed decision-making to support the correct diagnosis, the molecular result must be available before surgical pathology sign out. Integration of molecular test results into the surgical pathology diagnosis results in several scenarios. (a) To obtain a diagnosis, the tissue specimen is submitted to the surgical pathologist and will be analyzed in a certain turn-around time (TAT). (b) Time points of surgical pathology and molecular order and surgical pathology and molecular sign-out for patients. Red squares represent the working diagnosis MEC, blue squares represent the working diagnosis ASC, and grey squares indicate another working diagnosis. White circles represent the time point of the molecular testing request, red circles show a detected MAML2 rearrangement, and blue circles indicate a negative (no MAML2 rearrangement) result. White horizontal triangles indicate the time point of surgical pathology order and are normalized to 0, and white vertical triangles mark a changed surgical diagnosis after molecular testing. (c) In a confirmatory intent, molecular testing is usually ordered right before or shortly after the case is diagnosed (signed out). (d) When confirmation of a diagnosis requires molecular diagnostics, the case is held until the molecular result is available to the surgical pathologist. Light grey squares indicate the final diagnosis, white circles mark molecular test ordering, and black circles show molecular testing results. (e,f) Four-field tables with cases represented according to their order intent (confirmatory vs. diagnostic) and whether their final diagnosis was changed due to molecular testing. Abbreviations: MDx, molecular diagnostics; mTAT, molecular turn-around time; TAT, turn-around time.

Figure 3

State transition probability. (a) Alluvial diagram to represent changes from working diagnosis before molecular testing to final diagnosis. The states MEC, MAML2+, MAML2−, ASC, and others were assigned as variables to parallel vertical axes. The values represent the total number of cases in each group before and after molecular diagnoses. The height of each block represents the size of the cluster, and the height of each stream field represents the size of the components contained in both blocks connected by the stream field. (b) Pre- and post-molecular fusion analysis diagnoses represent states in a memoryless (Markov) stochastic process where state transitions (arrows) are determined exclusively by the relationship between the pre-molecular diagnoses and the post-molecular fusion analysis diagnoses. The empirical estimates of state transition probabilities were computed by simply conditioning and normalizing the data. Given the size of our data set, relative to that of the state space, these empirical estimates were not expected to converge to the true values of the underlying state transition probabilities, but rather represent our observations via pure summary statistics. Numbers indicate the probability of transitions from the indicated states to another, with the 3 states of working diagnosis MEC, ASC, and others and 4 states at final diagnosis (MAML2+ MEC, MAML2− MEC, ASC, and other). The transition from ASC to MAML2+ MEC did not exist and is therefore marked with a red arrow. Abbreviations: ASC, adenosquamous carcinoma; MEC, mucoepidermoid carcinoma.

Figure 4

Summary of data acquisition. After data clean-up, the analytical cohort was divided into 2 cohorts (review by primary site and review by working diagnosis).

Figure 5

Relationship between analytical, primary site, and test order analysis cohorts. Abbreviations: MEC, mucoepidermoid carcinoma; ASC, adenosquamous carcinoma.