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Zorluk: ZorNeoplastic Processes

A 4545-year-old man is evaluated following a screening colonoscopy that revealed a 4-cm4\text{-cm} ulcerated mass in the ascending colon. He has no significant past medical history. His family history is notable for a father who died of colon cancer at age 5050 and a paternal aunt who was diagnosed with endometrial cancer at age 4848. Biopsy of the colonic mass confirms invasive adenocarcinoma. Immunohistochemical staining of the tumor specimen demonstrates an absence of nuclear staining for MLH1 and PMS2 proteins, while MSH2 and MSH6 proteins maintain normal nuclear expression. Further molecular analysis of the tumor reveals wild-type BRAFBRAF and high microsatellite instability (MSI-H). Based on the underlying molecular pathogenesis of this patient's neoplasm, which of the following is the most appropriate next step in management?

  1. Germline genetic sequencing of the MLH1 geneCevap
  2. B
    Assessment of the tumor for MLH1 promoter hypermethylation
  3. C
    Germline genetic sequencing of the APC gene
  4. D
    Whole-body positron emission tomography (PET) scan
  5. E
    Immediate prophylactic total abdominal colectomy

Cevap

Germline genetic sequencing of the MLH1 gene
The correct sequence of diagnostic reasoning relies on understanding the neoplastic process of colorectal adenocarcinoma. This patient's tumor exhibits high microsatellite instability (MSI-H) with an absence of MLH1 and PMS2 proteins. MLH1 and PMS2 form a functional heterodimer, and isolated primary loss of MLH1 leads to the subsequent degradation of PMS2. This specific mismatch repair (MMR) deficiency can occur sporadically (due to MLH1 promoter hypermethylation) or hereditarily (due to a germline mutation in MLH1, known as Lynch syndrome). Sporadic MLH1 promoter hypermethylation is strongly associated with the BRAF V600E mutation. Because this patient's tumor is BRAF wild-type, a sporadic etiology is highly unlikely. Combined with a strong family history of Lynch syndrome-associated malignancies (colon and endometrial cancers), the pretest probability for Lynch syndrome is extremely high. Therefore, the most appropriate next step to establish the diagnosis and guide definitive surgical management is germline genetic sequencing of the MLH1 gene.

Adım Adım Çözüm

1
Interpret the immunohistochemistry (IHC) results of the tumor.
The tumor shows loss of MLH1 and PMS2, indicating mismatch repair deficiency (dMMR).
MLH1 and PMS2 function as a heterodimer. Loss of MLH1 leads to concurrent degradation of PMS2, pointing to an issue with the MLH1 gene.
2
Differentiate between sporadic and hereditary causes of MLH1 loss using molecular markers.
The tumor is BRAF wild-type.
Sporadic MLH1 loss is typically caused by epigenetic silencing via promoter hypermethylation, which is strongly associated with the BRAF V600E mutation. A wild-type BRAF result makes a sporadic cause highly unlikely.
3
Synthesize molecular findings with the patient's clinical and family history.
The patient meets Amsterdam II criteria for Lynch syndrome (early-onset colon cancer, endometrial cancer in a first/second-degree relative) and has a molecular profile consistent with a hereditary etiology.
Clinical presentation and molecular pathology both point away from sporadic disease and directly toward Lynch syndrome (Hereditary Nonpolyposis Colorectal Cancer).
4
Determine the next best step in management.
Order germline genetic sequencing for the MLH1 gene.
To definitively diagnose Lynch syndrome and guide subsequent surgical decision-making and family screening, the specific germline mutation must be identified.

Anahtar Kavram

Molecular pathogenesis and diagnostic algorithm for differentiating sporadic mismatch repair deficient colon cancer from Lynch syndrome.
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Neoplastic Processes

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