Constitutional mismatch repair deficiency in childhood colorectal cancer harboring a de novo variant in the MSH6 gene: a case report

Strategies for diagnosis and management of CMMRD in low-resource countries: report of a Tunisian family

Constitutional Mismatch Repair Deficiency (CMMRD) is a rare childhood cancer predisposition syndrome, caused by biallelic pathogenic germline variants in the mismatch repair genes. Diagnosis and management of this syndrome is challenging, especially in low-resource settings. This study describes a patient diagnosed with colorectal cancer and grade 3 astrocytoma at the age of 11 and 12 respectively. Immunohistochemistry analysis showed a loss of MSH2 and MSH6 protein expression in CRC tissues of the patient. We identified by Targeted Exome Sequencing a homozygous pathogenic germline variant in exon 9 of the MSH6 gene (c.3991 C > T; p.Ala1268Glyfs*6). Genetic investigation of the family showed that the father was heterozygous for the identified pathogenic variant while the brother was wild type for this variant. Our study highlights the importance of a correct and timely diagnosis of CMMRD which can have implications for treatment. It also underlines the imperative need to enhance awareness, diagnostic standards, and surveillance that are crucial for patients and their families.

Constitutional Mismatch Repair Deficiency Syndrome as a Cause of Numerous Malignancies in a Teenage Patient-A Case Report

Constitutional mismatch repair deficiency syndrome is a genetic disorder resulting from a biallelic mutation in one of the following genes: MLH1, MSH2, MSH6, or PMS2. Individuals with constitutional mismatch repair deficiency are highly predisposed to develop both hematological and solid cancers in childhood, particularly lymphoma, brain tumors, and gastrointestinal neoplasms. We report a case of a boy diagnosed with B-cell acute lymphoblastic leukemia at the age of 3. In 2013, at the age of 6, head magnetic resonance imaging revealed hamartoma and astrocytoma lesions in the central nervous system. Two years after treatment completion, a diagnosis of precursor T-cell lymphoblastic lymphoma, accompanied by the vena cava syndrome, was established and treated accordingly. During treatment, a genetic test using Sanger sequencing was performed-a biallelic mutation in the MSH6 gene was detected. The study revealed that the mutation 17-bp c.2277-2293del. was inherited from the patient's mother. The second mutation, 5-bp c.1135_1139delAGAGA, developed inpatient de novo. At the age of 14, the diagnosis of isolated bone marrow relapse of acute lymphoblastic leukemia B-cell type was established. Due to the almost exceeded total dose of anthracyclines, the patient's treatment included blinatumomab, and subsequently, he was qualified for allogeneic hematopoietic cell transplantation. The patient remains in complete remission for 11 months after allogeneic hematopoietic stem cell transplantation under the care of the transplant center.

A de novo pathogenic variant in the MSH6 gene in a 52 years-old woman

Lynch syndrome (LS) is a condition which predisposes individuals primarily to early-onset colorectal and endometrial cancer. LS is characterized by a germline pathogenic variant in one of the MMR (MisMatch Repair) gene, inducing a phenotype of microsatellite instability in the tumor, which may be associated with a loss of expression of MMR proteins detected by standard immunohistochemistry on tumor tissue. Most of the time, LS is inherited from a parent in whom the condition may not be known due to incomplete penetrance, but de novo pathogenic variant is a rare occurrence. Here, we describe the case of a 52-year-old woman with no family history of LS, referred to the genetics department for colorectal cancer at the age of 50. Genetic analysis revealed a de novo germline pathogenic variant in the MSH6 gene. To date, this case is only the second report of a de novo pathogenic variant in the MSH6 gene in Lynch syndrome. De novo mutations have been extensively studied over the past years, but little is known about their origin and mechanism of occurrence in MMR genes. However, knowledge of mutation status allows better cancer risk management for the patient and an appropriate genetic testing and counseling for her family.