TSAの亜型—Mucin rich TSA

Colorectal Mucin-rich Variant of Traditional Serrated Adenoma

要旨●大腸TSAの亜型として，近年，粘液豊富な杯細胞を特徴とするMR-TSAが提唱された．MR-TSAは，好酸性の吸収上皮細胞と比較して杯細胞が全体の50%以上を占めるものと定義される．臨床病理学的には，MR-TSAは通常型TSA（C-TSA）同様に男性優位で，左側大腸に好発する．粘液形質ではC-TSAは大腸型を示す一方で，MR-TSAは高頻度に胃腸混合型を示す．β-catenin核内発現は一部で認められ，MLH1発現は保持されている．遺伝子変異解析では，MR-TSAは高頻度にBRAF変異を認め，逆にKRAS変異は少ない．これらの所見は，MR-TSAがBRAF変異陽性のmicrosatellite stableを示す大腸癌の前駆病変となる可能性を示唆している．

　Several morphological variants of TSA（traditional serrated adenoma）of the colorectum have recently been identified ; notably, mucin-rich TSA（MR-TSA）was introduced as a distinct morphological variant that differs from C-TSAs（conventional TSAs）. MR-TSA is defined as a lesion that meets the criteria for TSAs and additionally shows ≥50% goblet/mucin-rich cells. Clinicopathologically, both MR-TSAs and C-TSAs show male predominance and a predilection for the distal colon. On the basis of immunohistochemical evaluation, MR-TSAs show a higher rate of immunopositivity to MUC5AC and show lesser nuclear β-catenin expression but retain MLH1 nuclear staining compared with C-TSAs. Genetically, BRAF mutations are more common ; however, KRAS mutations are less commonly associated with MR-TSAs than with C-TSAs, suggesting that MR-TSAs could be an important precursor of BRAF-mutated, microsatellite-stable subtypes of colorectal carcinoma.