Dados do Resumo

Título

Rare somatic and germline mechanisms in patients with unexplained mismatch repair deficiency (u-dMMR)

Introdução

Deficiency in mismatch repair (dMMR) is caused by the inactivation of MMR genes (MLH1, MSH2, MSH6, and PMS2), leading to the absence of one or more of the four MMR proteins by immunohistochemistry (IHC) and microsatellite instability (MSI). Some patients present with dMMR tumors without a detectable pathogenic germline variant (PGV) in the MMR genes, somatic methylation of the MLH1 promoter, or BRAF mutation. These cases comprise a group known as unexplained dMMR (u-dMMR), also referred to as Lynch-like syndrome. Recent studies have shown that more than half of patients with u-dMMR have somatic causes of dMMR (such as somatic mutations in an MMR gene). A small portion of these patients have complex PGVs in the MMR genes that are not identified by standard germline testing (such as deep intronic alterations or gene inversions), and one-third remain with undefined causes. Identifying the causes of u-dMMR has clinical implications for the management of the patient and their families, and developing efficient strategies to characterize these cases is necessary.

Objetivo

Identify dMMR cases without germline changes in MMR; evaluate the frequency of somatic causes of dMMR through analysis of MLH1 promoter methylation, BRAF gene mutation (V600E) and somatic sequencing of MMR genes; evaluate the frequency of complex germline changes that carry u-dMMR through constitutive RNA sequencing.

Métodos

We used ACCC databases (CEP 3415/23) to analyze patients with unexplained mismatch repair deficiency (u-dMMR). DNA was extracted from tumors (fresh frozen/FFPE) and leukocytes by the A.C. Camargo Biobank. We selected patients with colorectal, endometrial, or gastric cancers showing dMMR and known IHC loss. NGS was employed to assess MLH1 promoter methylation, BRAF mutations (CRC cases), and MMR gene sequencing using a 409-gene panel. For isolated PMS2, MSH2, or MSH6 loss, only MMR sequencing was performed. RNA analysis will use a 27-gene panel.

Resultados

In our preliminary results, we identified 35 cases of patients with dMMR tumors and germline genetic testing with a negative result. The clinical characteristics correspond to 18 cases with CRC, 13 with EC and 3 with GC. The female cases recorded were 26/35 and the age at diagnosis varied between 34 and 83 years, with an average of 59 years. For male patients (9/35), ages ranged from 24-80 years, with an average of 52 years. The most common cause of dMMR was loss of the MLH1/PMS2 genes (76.4%), followed by MSH2/MSH6 (11.7%), PMS2 (5.8%), MSH6 (5.8%). MLH1 promoter methylation and BRAF mutation analyzes were performed in 8 cases, 4 of which were negative for both biomarkers and continue to be considered u-dMMR. A somatic MMR sequencing analysis was performed on 5 patients, revealing mono- or biallelic mutations in 4/5 patients and to date, one negative patient has selected to the next phase (germline RNA sequencing).

Conclusões

Multiple molecular tests are crucial for diagnosing MMR causes and identifying sporadic tumors, aiding in patient management and genetic counseling. This study aims to clarify rare somatic and germline causes of u-dMMR, improving patient follow-up.

Palavras Chave

Câncer Colorretal; síndrome hereditária ao câncer de Lynch; u-dMMR