Germline mismatch repair gene mutations in children with tumors: a case series from two centers

Introduction

Both constitutional mismatch repair deficiency (CMMRD) syndrome and Lynch syndrome (LS) are hereditary cancer syndromes caused by germline mutations in the mismatch repair (MMR) genes. These two conditions differ in many ways in terms of their genetic mutations, pattern of inheritance, cancer susceptibility, tumor spectrum and benign manifestations, but atypical cases share similarities (1). CMMRD syndrome-related malignant tumors usually appear in childhood. Brain tumors, particularly gliomas, are common; however, other brain tumors, such as medulloblastomas, are rare. Individuals with LS do not usually develop cancer before the age of 40 years. The hallmark tumor in LS depends partly on the underlying mutation and on gender, and childhood onset is extremely uncommon. We present this article in accordance with the AME Case Series reporting checklist (available at https://tp.amegroups.com/article/view/10.21037/tp-24-343/rc).

Case presentation

A total of five patients with MMR germline mutations and tumors were included in this review. Of the five patients, two were female and three were male, and four were diagnosed at the University of Hong Kong-Shenzhen Hospital, and one was diagnosed at Hong Kong Children’s Hospital from June 2021 to June 2023. Table 1 summarizes the main clinical characteristics of the five patients in terms of age at diagnosis, age at first tumor onset, tumor type, respective gene mutations, clinical phenotype, and family history. The CMMRD syndrome patients had a mean age of 6 years (range, 2 to 11.75 years). All the patients had café-au-lait macules (CALMs) since birth. The patients had a mean age of 4.77 years (range, 1 month to 10.75 years) when the first tumor occurred, and in 3 (75%) patients, the tumor occurred before 10 years of age. However, the diagnosis of a genetic predisposition due to CMMRD syndrome, which occurred at a mean age of 5.38 years (range, 2 months to 11.58 years), was often delayed until the tumors emerged. One LS patient without CALMs was 10 years old when the first tumor occurred, and 13 years old when LS was diagnosed. Among the four CMMRD syndrome patients, three had brain tumors, of whom two had medulloblastomas {one developed osteochondroma 1 year after the first tumor diagnosis, and one had low-grade gliomas [pleomorphic xanthoastrocytoma (PXA)-like]}, and one had desmoid fibromatosis of the wrist and also had pulmonary stenosis and first-degree atrioventricular block). The case of LS was complicated by medulloblastoma.

In terms of the gene mutations, among the CMMRD syndrome patients, two had the MSH6 mutation (one patient had the de novo mutation), and one patient had the MLH1 mutation, and one patient has not been tested. The LS patient had the MSH2 mutation. Three of the four (75%) CMMRD syndrome patients and the one LS patient had a positive family history of malignant tumors. In the past four generations of the LS family, 12 patients had malignant tumors, with a total of seven different types of tumors observed. There were three cases of colorectal carcinomas, three cases of nasopharyngeal carcinomas, three cases of brain tumors (including one case of brain tumor in an adult, one case of glioma in a young adult, and one case of medulloblastoma in a child), one case of gastric cancer, one case of liver cancer, and one case of breast cancer.

All procedures performed in this study were in accordance with the ethical standards of the institutional research committee and with the Helsinki Declaration (as revised in 2013). Written informed consent for publication of this case series and accompanying images was not obtained from the patients or the relatives after all possible attempts were made.

Discussion

Germline mutations in MMR genes may lead to a variety of hereditary cancer syndromes, including CMMRD syndrome, LS, and other disorders. There are four known clinically relevant MMR genes; that is, MSH2, MSH6, MLH1, and PMS2. CMMRD syndrome is caused by biallelic pathogenic germline mutations (2) and is inherited as an autosomal recessive disorder, while LS is an autosomal dominant disease caused by monoallelic germline mutations (3). In CMMRD syndrome families, the incidence of PMS2 mutations is greater than 60%, that of MSH6 is 20–30%, and that of MLH1 and MSH2 is 10–20% (4-6). Conversely, LS mutations are inversely distributed, with MLH1 variants accounting for 40%, MSH2 variants for 30%, MSH6 variants for 20%, and PMS2 variants for 10% (7). In the vast majority of cases, CMMRD syndrome is inherited from parents, and de novo variants of MMR genes are rare (8).

Our Case 1 had CMMRD syndrome caused by the de novo pathogenic mutation of the MSH6 gene, which is uncommon. In the past few years, extensive studies have been conducted on de novo mutations, but little is known about the origin and pathogenesis of MMR de novo mutations in CMMRD syndrome (8-10). CMMRD syndrome is a complex hereditary cancer syndrome due to the possibility of mosaic mutations in the MMR genes (8). Further research should be conducted to determine whether the mother of this patient has an MMR pathogenic mutation due to tissue mosaicism mutation in the MSH6 gene.

Different mutated genes are often associated with different tumors. The age at which the first tumor occurs also varies. In CMMRD syndrome patients, on average, the first tumor in those with the PMS2 mutation develops at 10 years, which can be compared to 7.5 years for those with the MSH6 mutation, and 8 years for those with the MLH1/MSH2 mutation (4,11). Patients with PMS2 mutations have a higher incidence of brain tumors, colorectal cancer, and endometrial cancer than those with the MSH6, MLH1, and MSH2 mutations, while the incidence of hematological malignancies is lower than that of the MSH6, MLH1, and MSH2 mutations (6). For individuals with LS, colorectal cancer and endometrial cancer are common, especially in patients with the MSH2 and MLH1 mutations (12).

CMMRD syndrome is a childhood cancer susceptibility syndrome, and the onset of the first tumor is generally in the first decade of life (6). The average age at onset is 7.5 years (range, 0.4–39 years) (11), but it also has a less-penetrant phenotype, and patients with this phenotype may not develop cancers until the age of 40 years (13). The most common tumors in CMMRD syndrome patients are brain tumors in childhood, and glioma is the most common brain tumor associated with CMMRD syndrome. Other brain tumors, such as medulloblastoma, have only been reported occasionally (11), and their clinical features, histopathological findings, and molecular genetic profile have yet to be defined.

LS-related tumors, such as rectal cancer, can occur in young adults and adult patients. LS is a cancer predisposition syndrome of adulthood, and the first cancer usually emerges between the ages of 40–50 years (14). There is a wide spectrum of LS tumors, and the common types are carcinomas in various locations, such as the colorectum, uterus, small bowel, ureter, renal pelvis, biliary tract, stomach, and urinary bladder. In children with LS, hematological malignancies, lymphomas, brain tumors, and gastrointestinal tumors are common (15,16). Primary brain tumors are a rare feature of LS, with an estimated lifetime risk of 1% to 6% (17,18). It is even rarer for LS patients to be diagnosed with medulloblastoma in childhood or young adulthood (Table 2). In our study, Case 3 was diagnosed with medulloblastoma at the age of 10 years, and is one of the very few cases reported so far. Case 4 was diagnosed with PXA, and Case 5 was diagnosed with desmoid fibromatosis in the right wrist, both of which are extremely rare cases in CMMRD syndrome. More studies are needed to confirm the association between these tumors and MMR gene mutations.

Patients with CMMRD syndrome may develop more than one tumor during their lifetime (2). It was recently demonstrated that the cumulative cancer incidence by age 18 years was 90% (19). Case 1 presented with an osteochondroma 1 year after the diagnosis of medulloblastoma. Although non-malignant, the occurrence of a second tumor in a relatively short period should also be taken seriously by clinicians, which emphasizes the importance of tumor surveillance in CMMRD syndrome patients. Tabori et al. recommends the following CMMRD surveillance protocols: blood count every 6 months from birth, abdominal ultrasound every 6 months from age of 1 year, clinical examination and brain magnetic resonance imaging (MRI) every 6 months from diagnosis, whole body MRI and ileocolonoscopy annually from age of 6 years, esophagogastroduodenoscopy (EGD) annually from age of 8 years, gynecological examination, transvaginal ultrasound, Pipelle curettage, urine cytology, and dipstick tests annually from age of 20 years (20). Stoffel et al. recommend that patients with LS should undergo colonoscopy every 1–2 years, urine analysis annually from age of 25–30 years, endometrial biopsy every 1–2 years from age of 30–35 years, and EGD every 3–5 years from age of 40 years (21).

In this study, four patients with CMMRD syndrome had café-au-lait spots since birth, but they were not recognized as having hereditary syndrome until their tumors were diagnosed. Most of them had their first tumors before the age of 10 years, the average delay in diagnosing CMMRD syndrome was 5.38 years. One case of desmoid fibroma was diagnosed 2 months after birth due to a family history of cancer, and the diagnosis was confirmed by genetic testing. However, in the one LS patient, the first tumor occurred at the age of 10 years, and the patient was subsequently diagnosed with LS by genetic testing 3 years later because of a positive family history; thus, the delay in the diagnosis of CMMRD syndrome was 13 years.

Most patients with CMMRD syndrome have CALMs, Mongolian spots, and different degrees of pigmentation; however, the majority of patients with CMMRD syndrome are not recognized at birth. Indeed, they are not recognized until the first tumor develops. Improving the awareness of clinicians of the differential diagnosis of different CALM morphologies and genetic syndromes is therefore important.

Because NF1 is the most common disease with CALMs, and the skin manifestations of CALMs, axillary freckles, and inguinal floss in CMMRD syndrome patients can mimic those of NF1, these diagnoses can be easily mixed up. For patients with LS, the first tumor usually develops in the adult stage and patients seldom have benign skin manifestations, which frequently leads to a long delay in diagnosis. The absence of a family history of cancer can also lead to a delay in diagnosis. Both CMMRD syndrome and LS are tumor-prone syndromes, some patients can remain tumor-free for life. Zhang et al. performed gene sequencing for 1,120 children and adolescents, and found 58 patients with tumor susceptibility mutations, among whom 35 (60%) had no family history of cancer (22). Therefore, if there are no other tumor patients in the proband’s family, germline gene testing is usually not considered, and the diagnosis may be delayed until the tumor develops.

Additionally, CMMRD syndrome is not easy to diagnose. It was not until 2021 that experts from the International Consensus Working Group developed seven diagnostic criteria for CMMRD syndrome for the first time. Of the seven diagnostic criteria, four provide strong evidence of CMMRD syndrome, and three provide moderate evidence. The criteria were established using three components: (I) MMR germline results; (II) ancillary tests; and (III) clinical manifestations. Since genetic testing may not provide information, ancillary tests and clinical manifestations can help confirm a diagnosis. Ancillary test methods include immunohistochemistry (IHC) of non-tumor tissues, germline microsatellite instability (MSI), a combination of methylating agents tolerance and ex vivo MSI, in vitro repair assays, and next-generation sequencing (NGS) to detect low levels of MSI in tissues (1). In our Case 4, no germline genetic testing was performed, but MSH6 protein deficiency in normal tissue adjacent to the malignant tumor was detected by IHC. A clinical diagnosis of CMMRD syndrome can be made based on a combination of clinical manifestations of multiple CALMs, gliomas, and LS (the MSH6 pathogenic germline mutation) in first-degree relatives.

To facilitate the differential diagnosis of various genetic syndromes with possible tumors using CALM morphological characteristics, we summarize several common genetic syndromes associated with CALMs, and compare related genes, tumors, CALM morphological characteristics, and incidence rates (Table 3, Figure 1). CALM usually presents at birth or in early infancy. Isolated CALM is common in the general population, while multiple CALMs are associated with a variety of genetic syndromes. NF1 is the most well-known form of cancer predisposing syndrome caused by loss of function mutations in the NF1 tumor suppressing gene. It is characterized by CALMs, which have homogeneous pigmentation with smooth borders similar to the outline of the California coast. Multiple CALMs (≥6 greater than 15 mm) indicate NF1. Neurofibromatosis 2 (NF2) is much less common than NF1 and is rarely associated with CALMs. CALMs in NF2 are usually well defined, flat, pigmented, and often solitary and inconspicuous. Unlike CALMs in NF1, CALMs in CMMRD syndrome often have irregular margins or serrated borders, and patients usually also have Mongolian spots with varying degrees of pigmentation.

Table 3

Café-au-lait macules and associated genetic syndromes

Genetic syndrome	Associated genes	Associated tumors	CALM characteristics	Incidence of CALM	Figures
NF1	NF1	Neurofibromas; optic pathway gliomas; malignant peripheral nerve sheath tumors	Uniform hyperpigmentation with smooth borders	Almost all
NF2	NF2	Acoustic schwannoma, meningiomas	Well-defined flat, hyperpigmentation, often singular and inconspicuous	Less common
CMMRD syndrome	MSH2, MSH6, MLH1, PMS2	Glioma; hematological cancer; GI adenocarcinoma	Vary in pigmentation, have irregular and jagged borders	62–97% (23)
MAS	GNAS1	Polyostotic fibrous dysplasia, bone degeneration occurs in 1% cases	Single large and with jagged borders, rarely extend over the midline	Common
Watson syndrome	NF1	Neurofibromas	Similar characteristics to NF1	Frequently present
Bloom syndrome	BLM, WRN, RECQL4	Any type of cancer	Hypopigmentation or hyperpigmentation respectively	Frequently present
LEOPARD syndrome	PTPN11, BRAF, MAP2K, RAF1	Neuroblastoma, AML	Widely spread small size (<0.5 cm), well-demarcated, round, brown and black macules (lentigines)	More than 90%

CALM, café-au-lait macule; NF1, neurofibromatosis type 1; NF2, neurofibromatosis type 2; CMMRD, constitutional mismatch repair deficiency; GI, gastrointestinal; MAS, McCune-Albright syndrome; AML, acute myelogenous leukemia.

Figure 1 Flow chart on differential diagnosis of CALM-related syndromes. GI, gastrointestinal; IHC, immunohistochemistry; dMMR, deficiency of mismatch repair; MSI, microsatellite instability; NF1, neurofibromatosis type 1; NF2, neurofibromatosis type 2; MAS, McCune-Albright syndrome; CMMRD, constitutional mismatch repair deficiency; AML, acute myelogenous leukemia.

McCune-Albright syndrome is a very rare condition. The classical triad of this syndrome is a pigmented skin patch, which is characterized by a distinct morphological distribution of large, jagged borders, similar to the coastal border in Maine, and these spots rarely extends across the midline. Watson syndrome, which is related to the NF1 gene mutation, is also rare. CALMs in Watson syndrome have similar characteristics to NF1, and neurofibromas may also occur. Clinical manifestations, such as pulmonary stenosis, mental retardation, and short stature, may remind clinicians to consider this syndrome. Bloom syndrome is a rare condition in which skin lesions are characterized by a rash after exposure to sunlight, with patches of skin that are shallower or deeper than the surrounding area. LEOPARD syndrome, also known as Noonan syndrome with multiple lentigines, is a very rare genetic disease. The most common feature of this syndrome is lentigines, which are characterized by diffused multiple round or oval brownish black spots on the skin. All of the above seven genetic syndromes have specific genetic changes, and the related tumors also differ. Combining different tumors and specific genetic changes with the different morphological characteristics of CALMs, it is easier to arrive at a correct diagnosis of genetic syndrome.

Conclusions

CMMRD syndrome and LS are considered cancer-prone syndromes in childhood and adulthood, respectively. However, both can occur across a wide range of ages, and are associated with various types of cancers, or other benign manifestations. The benign pigmented skin lesions may lead to confusion between CMMRD syndrome and NF1. More than half of patients with hereditary syndromes may have no family history of these syndromes, and clinicians may not be aware of CMMRD syndrome-related CALMs, which can lead to the delayed or missed diagnosis of genetic syndromes. CMMRD syndrome-related CALMs differ from the skin manifestations of other genetic syndromes such as NF1. It is important to combine germline genetic testing, auxiliary investigations, and clinical phenotypes for the diagnosis of cancer susceptibility syndrome caused by MMR gene mutations.

Acknowledgments

These cases have been presented at the VIVA-Asia Rare Tumor Board conference, and we would like to thank the following experts for their opinions: Uri Tabori, Anirban Das, Dora Lai-Wan Kwong, and Kenneth Kak-Yuen Wong.

Funding: None.

Footnote

Reporting Checklist: The authors have completed the AME Case Series reporting checklist. Available at https://tp.amegroups.com/article/view/10.21037/tp-24-343/rc

Peer Review File: Available at https://tp.amegroups.com/article/view/10.21037/tp-24-343/prf

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://tp.amegroups.com/article/view/10.21037/tp-24-343/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All procedures performed in this study were in accordance with the ethical standards of the institutional research committee and with the Helsinki Declaration (as revised in 2013). Written informed consent for publication of this case series and accompanying images was not obtained from the patients or the relatives after all possible attempts were made.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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(English Language Editor: L. Huleatt)