TRIM28 haploinsufficiency predisposes to Wilms tumor.

Two percent of patients with Wilms tumors have a positive family history. In many of these cases the genetic cause remains unresolved. By applying germline exome sequencing in two families with two affected individuals with Wilms tumors, we identified truncating mutations in TRIM28. Subsequent mutational screening of germline and tumor DNA of 269 children affected by Wilms tumor was performed, and revealed seven additional individuals with germline truncating mutations, and one individual with a somatic truncating mutation in TRIM28. TRIM28 encodes a complex scaffold protein involved in many different processes, including gene silencing, DNA repair and maintenance of genomic integrity. Expression studies on mRNA and protein level showed reduction of TRIM28, confirming a loss-of-function effect of the mutations identified. The tumors showed an epithelial-type histology that stained negative for TRIM28 by immunohistochemistry. The tumors were bilateral in six patients, and 10/11 tumors are accompanied by perilobar nephrogenic rests. Exome sequencing on eight tumor DNA samples from six individuals showed loss-of-heterozygosity (LOH) of the TRIM28-locus by mitotic recombination in seven tumors, suggesting that TRIM28 functions as a tumor suppressor gene in Wilms tumor development. Additionally, the tumors showed very few mutations in known Wilms tumor driver genes, suggesting that loss of TRIM28 is the main driver of tumorigenesis. In conclusion, we identified heterozygous germline truncating mutations in TRIM28 in 11 children with mainly epithelial-type Wilms tumors, which become homozygous in tumor tissue. These data establish TRIM28 as a novel Wilms tumor predisposition gene, acting as a tumor suppressor gene by LOH.

Somatic mosaic monosomy 7 and UPD7q in a child with MIRAGE syndrome caused by a novel SAMD9 mutation.

MIRAGE syndrome caused by mutations in SAMD9 is associated with potential loss of chromosome 7 (-7/7q-) and an increased risk to develop myelodysplastic syndrome (MDS). We report a case of MIRAGE syndrome, caused by a novel SAMD9 mutation p.Leu641Pro, leading to characteristic clinical features as well as to the coexistence of cells with monosomy 7 (20%) and with uniparental disomy of long arm of chromosome 7 (UPD7q). In contrast to previously reported MIRAGE patients with -7/7q- developing MDS, our patient achieved complete cytogenetic remission of monosomy 7. As UPD7q remained unchanged, it seems to be a protective factor against MDS.

Terminal 14q32.33 deletion as a novel cause of agammaglobulinemia.

Over the past decades, a pleiotropic spectrum of B-cell intrinsic defects leading to early onset agammaglobulinemia and absent B cells has been described. Herein we report terminal 14q32.33 deletion as a novel cause of agammaglobulinemia. We describe a 20-year old man with a 1MB terminal 14q32.33 deletion resulting in a loss of the entire Immunoglobulin heavy chain gene region of chromosome 14. The patient presented with absent serum immunoglobulin levels and absent circulating B cells since age 2. The clinical picture was dominated by severe episodes of recurrent upper respiratory tract infections. In the literature, the most prevalent features of terminal 14q32.33 deletions include mental disability, facial malformation, hypotonia, seizures, and visual problems with retinal abnormalities. Neither increased susceptibility to infections nor agammaglobulinemia have been described as a manifestation of terminal 14q32.33 deletion. Thus, our findings expand the known clinical spectrum of terminal 14q32.33 deletion to include susceptibility to infections.

The first Slovak Legius syndrome patient carrying the SPRED1 gene mutation.

Autosomal dominant disorder Legius syndrome (NF1- like syndrome) shows phenotype features that overlap with neurofibromatosis type 1 (NF1), such as CALMs, freckling, macrocephaly and learning disability. Mutation analysis provides an important tool in order to distinguish two entities that have different clinical implications. We analyzed SPRED1 gene by cDNA and/or gDNA sequencing in a cohort of 46 Slovak patients in whom previously NF1 mutation was excluded. In one case we identified a nonsense mutation c.46C>T (p.Arg16*) in exon 2 of SPRED1 gene, confirming diagnosis of Legius syndrome. This mutation was reported previously.

Diagnosis of Constitutional Mismatch Repair-Deficiency Syndrome Based on Microsatellite Instability and Lymphocyte Tolerance to Methylating Agents.

BACKGROUND & AIMS: Patients with bi-allelic germline mutations in mismatch repair (MMR) genes (MLH1, MSH2, MSH6, or PMS2) develop a rare but severe variant of Lynch syndrome called constitutional MMR deficiency (CMMRD). This syndrome is characterized by early-onset colorectal cancers, lymphomas or leukemias, and brain tumors. There is no satisfactory method for diagnosis of CMMRD because screens for mutations in MMR genes are noninformative for 30% of patients. MMR-deficient cancer cells are resistant to genotoxic agents and have microsatellite instability (MSI), due to accumulation of errors in repetitive DNA sequences. We investigated whether these features could be used to identify patients with CMMRD. METHODS: We examined MSI by PCR analysis and tolerance to methylating or thiopurine agents (functional characteristics of MMR-deficient tumor cells) in lymphoblastoid cells (LCs) from 3 patients with CMMRD and 5 individuals with MMR-proficient LCs (controls). Using these assays, we defined experimental parameters that allowed discrimination of a series of 14 patients with CMMRD from 52 controls (training set). We then used the same parameters to assess 23 patients with clinical but not genetic features of CMMRD. RESULTS: In the training set, we identified parameters, based on MSI and LC tolerance to methylation, that detected patients with CMMRD vs controls with 100% sensitivity and 100% specificity. Among 23 patients suspected of having CMMRD, 6 had MSI and LC tolerance to methylation (CMMRD highly probable), 15 had neither MSI nor LC tolerance to methylation (unlikely to have CMMRD), and 2 were considered doubtful for CMMRD based on having only 1 of the 2 features. CONCLUSION: The presence of MSI and tolerance to methylation in LCs identified patients with CMMRD with 100% sensitivity and specificity. These features could be used in diagnosis of patients.

Novel SCN1A variants in Dravet syndrome and evaluating a wide approach of patient selection.

Voltage-gated sodium channels are essential for generation and propagation of the action potential mainly in nerve and muscle cells. Causative variants in SCN1A gene which codes the main, pore-forming subunit of the channel expressed in central nervous system are associated predominantly with Dravet syndrome (DS), as well as with generalized epilepsy with febrile seizures plus (GEFS+) making it one of the most significant epilepsy gene. Our goal was to determine whether SCN1A screening is relevant in patients with a broad range of epileptic syndromes. 52 patients diagnosed with DS, generalized epilepsy with GEFS+ or similar types of epileptic syndromes were included. Sequencing of the protein coding parts of the gene complemented with MLPA analysis was carried out. One already described nonsense variant, four novel protein truncating variants and a deletion encompassing the whole SCN1A gene were revealed, all in heterozygous state. All identified variants were found in DS patients with 85.7% sensitivity, thus supporting the role of profound SCN1A gene variants in etiology of DS phenotype. No causative variants were identified in any of non-DS epileptic patients in our cohort, suggesting a minor, but not irrelevant role for SCN1A in patients with other types of childhood epilepsy.

High-resolution breakpoint analysis provides evidence for the sequence-directed nature of genome rearrangements in hereditary disorders.

Although most of the pertinent data on the sequence-directed processes leading to genome rearrangements (GRs) have come from studies on somatic tissues, little is known about GRs in the germ line of patients with hereditary disorders. This study aims at identifying DNA motifs and higher order structures of genome architecture, which can result in losses and gains of genetic material in the germ line. We first identified candidate motifs by studying 112 pathogenic germ-line GRs in hereditary colorectal cancer patients, and subsequently created an algorithm, termed recombination type ratio, which correctly predicts the propensity of rearrangements with respect to homologous versus nonhomologous recombination events.

Diagnostic criteria for constitutional mismatch repair deficiency syndrome: suggestions of the European consortium 'care for CMMRD' (C4CMMRD).

Constitutional mismatch repair deficiency (CMMRD) syndrome is a distinct childhood cancer predisposition syndrome that results from biallelic germline mutations in one of the four MMR genes, MLH1, MSH2, MSH6 or PMS2. The tumour spectrum is very broad, including mainly haematological, brain and intestinal tract tumours. Patients show a variety of non-malignant features that are indicative of CMMRD. However, currently no criteria that should entail diagnostic evaluation of CMMRD exist. We present a three-point scoring system for the suspected diagnosis CMMRD in a paediatric/young adult cancer patient. Tumours highly specific for CMMRD syndrome are assigned three points, malignancies overrepresented in CMMRD two points and all other malignancies one point. According to their specificity for CMMRD and their frequency in the general population, additional features are weighted with 1-2 points. They include multiple hyperpigmented and hypopigmented skin areas, brain malformations, pilomatricomas, a second childhood malignancy, a Lynch syndrome (LS)-associated tumour in a relative and parental consanguinity. According to the scoring system, CMMRD should be suspected in any cancer patient who reaches a minimum of three points by adding the points of the malignancy and the additional features. The diagnostic steps to confirm or refute the suspected diagnosis are outlined. We expect that application of the suggested strategy for CMMRD diagnosis will increase the number of patients being identified at the time when they develop their first tumour. This will allow adjustment of the treatment modalities, offering surveillance strategies for second malignancies and appropriate counselling of the entire family.

Human MSH6 deficiency is associated with impaired antibody maturation.

Ig class-switch recombination (Ig-CSR) deficiencies are rare primary immunodeficiencies characterized by defective switched isotype (IgG/IgA/IgE) production. Depending on the molecular defect, defective Ig-CSR may also be associated with impaired somatic hypermutation (SHM) of the Ig V regions. Although the mechanisms underlying Ig-CSR and SHM in humans have been revealed (at least in part) by studying natural mutants, the role of mismatch repair in this process has not been fully elucidated. We studied in vivo and in vitro Ab maturation in eight MSH6-deficient patients. The skewed SHM pattern strongly suggests that MSH6 is involved in the human SHM process. Ig-CSR was found to be partially defective in vivo and markedly impaired in vitro. The resolution of γH2AX foci following irradiation of MSH6-deficient B cell lines was also found to be impaired. These data suggest that in human CSR, MSH6 is involved in both the induction and repair of DNA double-strand breaks in switch regions.

Chronic pancreatitis associated with the p.G208A variant of PRSS1 gene in a European patient.

CONTEXT: The major etiologic factor of chronic pancreatitis in adults is excessive alcohol consumption, whereas among children structural anomalies, systemic and metabolic disorders, and genetic factors are prevalent. Mutations in the cationic trypsinogen gene (PRSS1) cause hereditary pancreatitis, while mutations in serine protease inhibitor Kazal type 1 (SPINK1), cystic fibrosis transmembrane conductance regulator (CFTR) and chymotrypsin C (CTRC) genes have been shown to associate with chronic pancreatitis as independent risk factors. CASE REPORT: We present a case of 13-year-old boy with idiopathic chronic pancreatitis. Given the unexplained attacks of pancreatitis since early childhood and despite the negative family history, molecular-genetic analysis of four pancreatitis susceptibility genes (PRSS1, SPINK1, CTRC and CFTR) was performed. The boy was found to carry the c.623G>C (p.G208A) mutation of the PRSS1 gene and the c.180C>T (p.G60G) mutation of the CTRC gene, both in heterozygous state. These mutations are considered as contributing risk factors for chronic pancreatitis. CONCLUSIONS: In children with idiopathic chronic pancreatitis genetic causes should be considered, even in absence of positive family history. To the best of our knowledge, this is the first description of a European patient with chronic pancreatitis associated with the p.G208A mutation of PRSS1 gene. This mutation was previously reported only in Asian subjects and is thought to be a unique genetic cause of pancreatitis in Asia.

[Familial Mediterranean fever - clinical picture, diagnosis and treatment]. / Familiárna stredomorská horúcka - klinický obraz, diagnóza a liecba.

Familial mediterranean fever (FMF) is the most prevalent genetically determined autoinflammatory disease. FMF significantly decreases the quality of life and limits life expectancy due to the development of amyloidosis in affected individuals. Prevalence of FMF is highest in the south-eastern Mediterraneans. In other parts of the world, its occurance is often restricted to high-risk ethnic goups. In Central Europe, experience with FMF is scarse. As for Slovakia, we have reported the first cases of FMF in ethnic Slovaks only recently. Along with their complicated fates, this has lead us to compile a comprehensive overview of the clinical picture, diagnosis and treatment of this elusive disease. Hereby we hope to be able to promote the awareness about this disease and possibly aid the diagnosis in new patients.

[Familial Mediterranean fever - first experiences in Slovakia]. / Familiárna stredomorská horúcka - prvé skúsenosti na Slovensku.

Familial Mediterranean fever (FMF) is the most prevalent genetically determined autoinflammatory disease. FMF significantly decreases the quality of life and limits life expectancy due to the development of amyloidosis in affected individuals. Prevalence of FMF is highest in the south-eastern Mediterraneans. In other parts of the world, its occurance is often restricted to high-risk ethnic groups. In Central Europe, experience with FMF is scarse to none, as in the case of Slovakia, where no cases have been reported, so far. Herein we report the first five patients (3 adults and 2 children, 4 native Slovaks) in whom the diagnosis of FMF could be confirmed in Slovakia. Our experience demonstrates that FMF does occur in low-risk populations in Central Europe. Due to low prevalence and lack of experience, FMF diagnosis may be significantly delayed (4.5-30 years) and undiagnosed cases are to be expected in our population.

Thirty-nine novel neurofibromatosis 1 (NF1) gene mutations identified in Slovak patients.

We performed a complex analysis of the neurofibromatosis type 1 (NF1) gene in Slovakia based on direct cDNA sequencing supplemented by multiple ligation dependent probe amplification (MLPA) analysis. All 108 patients had café-au-lait spots, 85% had axilary and/or inguinal freckling, 61% neurofibromas, 36% Lisch nodules of the iris and 31% optic pathway glioma, 5% suffered from typical skeletal disorders, and 51% of patients had family members with NF1. In 78 of the 86 (90.7%) index patients our analysis revealed the presence of NF1 mutations, 68 of which were small changes (87.2%), including 39 (50%) novel. Among the identified mutations the most prevalent were small deletions and insertions causing frameshift (42.3%), followed by nonsense (14.1%), missense (12.8%), and typical splicing (11.5%) mutations. Type 1 NF1 deletions and intragenic deletions/duplication were identified in five cases each (6.4%). Interestingly, in five other cases nontypical splicing variants were found, whose real effect on NF1 transcript would have remained undetected if using a DNA-based method alone, thus underlying the advantage of using the cDNA-based sequencing. We show that Slovak NF1 patients have a similar repertoire of NF1 germline mutations compared to other populations, with some prevalence of small deletions/insertions and a decreased proportion of nonsense mutations.

Comprehensive genetic characterization of hereditary breast/ovarian cancer families from Slovakia.

Germline mutations in the BRCA1/2 genes account for the majority of hereditary breast ovarian cancer (HBOC). Identification of causal mutations may have significant impact on clinical management of such families. Despite high mutation detection rate, many HBOC cases remain without identified cause. These cases warrant use of several analysis methods, such as those for large genomic rearrangements and DNA copy number changes, or analysis other genes, shown to be associated with increased HBOC risk. We assessed 585 Slovak HBOC for the presence of mutations in BRCA genes. Sequencing revealed mutations in 100 families, representing 17.1% (88 and 12% of mutations were located in BRCA1 and BRCA2, respectively). Four of the mutations, c.80+4del4, c.1938_1947del10 and c.1166delG in BRCA1 and c.6589delA in BRCA2 gene have been described only in Slovak population. Using MLPA analysis, we detected two large genomic rearrangements in three families, a deletion of exons 21 and 22, and a rare deletion of a whole BRCA1 gene. Twenty-seven different variants of uncertain clinical effect (four novel) and 14 distinct SNP BRCA1 haplotypes were detected. Their potential effect was considered using the prediction software packages Align-GVGD, Pmut and Polyphen. We observed that the best clinical criterion for the initiation of BRCA1 analysis is the presence of breast cancer at 40 years of age in the association with the presence of ovarian cancer diagnosed around the age of 50. Conversely, the best clinical criterion for starting with BRCA2 analysis is the presence of breast cancer diagnosed in older age (above 50), or the presence of breast cancer in conjunction with carcinomas at different sites e.g., prostate, colorectum, ovary and uterus. Finally we have seen that the analyses of other HBOC risk gene TP53 and specific mutation in CHEK2*c.1100delC in Slovak HBOC families were not efficient since no mutations were found in these genes.

High-grade brain tumors in siblings with biallelic MSH6 mutations.

Biallelic germline mutations of Constitutional mismatch repair-deficiency syndrome (CMMR-D) genes, MLH1, MSH2, MSH6, and PMS2 are characterized by increased risk of childhood malignancy. We report a case with CMMR-D caused by novel homozygous MSH6 mutations leading to gliomatosis cerebri and T-ALL in an 11-year-old female and glioblastoma multiforme in her 10-year-old brother, both with rapid progression of the diseases. A literature review on brain tumors in CMMR-D families shows that they are treatment-resistant and lead to early death. Identification of patients with CMMR-D is critical, and specific cancer screening programs with early surgery are recommended.

Myhre Syndrome Associated With Dunbar Syndrome and Urinary Tract Abnormalities: A Case Report.

Myhre syndrome is a rare condition caused by a mutation in the SMAD4 gene, which leads to a defective TGF-ß/BMP signaling, resulting in the proliferation of abnormal fibrous tissues. Clinically, patients with Myhre syndrome manifest with defects of connective tissue (skin, muscles, joints), and cardiovascular and neurological impairment. In our report, we present a case of a 16-year-old female with skeletal abnormalities, reduced articular mobility, skin, and muscular hypertrophy and cardiovascular defects characteristic of Myhre syndrome. Long-term pulmonary hypertension and arterial hypertension were persistent in spite of antihypertensive treatment. Our patient was also diagnosed with chronic kidney disease and Dunbar syndrome, which is an external compression of the coeliac trunk or coeliac artery by the surrounding tissues. Until now, only a few cases of renal complications in Myhre syndrome have been published. We describe for the first time a female patient with genetically confirmed Myhre syndrome caused by the p.Ile500Val SMAD4 mutation presenting with an unusual occurrence of congenital vesicoureteral reflux, proteinuria with a decreased renal function, and a condition recognized as Dunbar syndrome.

Chronic pancreatitis with polycystic kidney disease: A rare coincidence?

INTRODUCTION: In children, chronic pancreatitis (CP) is usually associated with anatomical anomalies of the pancreas and biliary tract or is genetically determined. Autosomal dominant polycystic kidney disease (ADPKD) may present with extrarenal cyst formation, sometimes involving the pancreas. Large enough, these cysts may cause pancreatitis in ADPKD patients. CASE PRESENTATION: Herein, we present a case of a 12-year-old Caucasian girl with recurrent pancreatitis with no identifiable traumatic, metabolic, infectious, drug, or immunologic causes. Structural anomalies of the pancreas, including cysts, were ruled out by imaging. However, bilateral cystic kidneys were found as an incidental finding. Her family history was negative for pancreatitis, but positive for polycystic kidney disease. Molecular analysis of ADPKD-causing mutations revealed a novel c.9659C>A (p.Ser3220*) mutation in the PKD1 gene confirming the clinical suspicion of ADPKD. Although CP may rarely occur as an extrarenal manifestation of ADPKD with pancreatic cysts, it is unusual in their absence. Thus, molecular analysis of pancreatitis susceptibility genes was performed and a homozygous pathologic c.180C>T (p.G60=) variant of the CTRC gene, known to increase the risk of CP, was confirmed. CONCLUSION: This is the first reported case of a pediatric patient with coincidence of genetically determined CP and ADPKD. Occurrence of pancreatitis in children with ADPKD without pancreatic cysts warrants further investigation of CP causing mutations.

Partial loss of heterozygosity events at the mutated gene in tumors from MLH1/MSH2 large genomic rearrangement carriers.

BACKGROUND: Depending on the population studied, large genomic rearrangements (LGRs) of the mismatch repair (MMR) genes constitute various proportions of the germline mutations that predispose to hereditary non-polyposis colorectal cancer (HNPCC). It has been reported that loss of heterozygosity (LOH) at the LGR region occurs through a gene conversion mechanism in tumors from MLH1/MSH2 deletion carriers; however, the converted tracts were delineated only by extragenic microsatellite markers. We sought to determine the frequency of LGRs in Slovak HNPCC patients and to study LOH in tumors from LGR carriers at the LGR region, as well as at other heterozygous markers within the gene to more precisely define conversion tracts. METHODS: The main MMR genes responsible for HNPCC, MLH1, MSH2, MSH6, and PMS2, were analyzed by MLPA (multiplex ligation-dependent probe amplification) in a total of 37 unrelated HNPCC-suspected patients whose MLH1/MSH2 genes gave negative results in previous sequencing experiments. An LOH study was performed on six tumors from LGR carriers by combining MLPA to assess LOH at LGR regions and sequencing to examine LOH at 28 SNP markers from the MLH1 and MSH2 genes. RESULTS: We found six rearrangements in the MSH2 gene (five deletions and dup5-6), and one aberration in the MLH1 gene (del5-6). The MSH2 deletions were of three types (del1, del1-3, del1-7). We detected LOH at the LGR region in the single MLH1 case, which was determined in a previous study to be LOH-negative in the intragenic D3S1611 marker. Three tumors displayed LOH of at least one SNP marker, including two cases that were LOH-negative at the LGR region. CONCLUSION: LGRs accounted for 25% of germline MMR mutations identified in 28 Slovakian HNPCC families. A high frequency of LGRs among the MSH2 mutations provides a rationale for a MLPA screening of the Slovakian HNPCC families prior scanning by DNA sequencing. LOH at part of the informative loci confined to the MLH1 or MSH2 gene (heterozygous LGR region, SNP, or microsatellite) is a novel finding and can be regarded as a partial LOH. The conversion begins within the gene, and the details of conversion tracts are discussed for each case.

Multiplex SNaPshot genotyping for detecting loss of heterozygosity in the mismatch-repair genes MLH1 and MSH2 in microsatellite-unstable tumors.

BACKGROUND: In the workup of patients with suspected hereditary nonpolyposis colorectal cancer (HNPCC), detection of loss of heterozygosity (LOH) could help pinpoint the mismatch-repair (MMR) gene carrying the germline mutation, but analysis of microsatellite markers has proved unreliable for this purpose. We developed a simple, low-cost method based on single-nucleotide polymorphism (SNP) genotyping and capillary electrophoresis for the assessment of LOH at 2 MMR loci simultaneously. METHODS: We used the Applied Biosystems SNaPshot Multiplex Kit with meticulously selected primers to assess 14 common SNPs in MLH1 [mutL homolog 1, colon cancer, nonpolyposis type 2 (E. coli)] and MSH2 [mutS homolog 2, colon cancer, nonpolyposis type 1 (E. coli)] and optimized the protocol for DNA isolated from peripheral blood and fresh/frozen or archival microsatellite-unstable tumors from patients with confirmed (n = 42) or suspected (n = 25) HNPCC. The 42 tumors from patients with confirmed MLH1 or MSH2 germline mutations were used to validate the method's diagnostic accuracy against results obtained with DNA sequencing or multiplex ligation-dependent probe amplification. RESULTS: The SNaPshot assay provided better detection of certain SNPs than DNA sequencing. The MLH1 and MSH2 SNP marker sets were informative in 82% and 76% of the 67 cases analyzed, respectively. The new assay displayed 100% specificity for detecting LOH and predicted the location of the germline mutation in 40% of the cases (54% of those involving MLH1, 22% in MSH2). CONCLUSIONS: Our SNP-based method for detecting LOH in MLH1 and MSH2 is simple to perform with instruments available in most clinical genetics laboratories. It can be a valuable addition to protocols now used to guide mutational screening of patients with suspected HNPCC.

No Overt Clinical Immunodeficiency Despite Immune Biological Abnormalities in Patients With Constitutional Mismatch Repair Deficiency.

Immunoglobulin class-switch recombination (CSR) and somatic hypermutations (SHMs) are prerequisites for antibody and immunoglobulin receptor maturation and adaptive immune diversity. The mismatch repair (MMR) machinery, consisting of homologs of MutSα, MutLα, and MutSß (MSH2/MSH6, MLH1/PMS2, and MSH2/MSH3, respectively) and other proteins, is involved in CSR, primarily acting as a backup for nonhomologous end-joining repair of activation-induced cytidine deaminase-induced DNA mismatches and, furthermore, in addition to error-prone polymerases, in the repair of SHM-induced DNA breaks. A varying degree of antibody formation defect, from IgA or selective IgG subclass deficiency to common variable immunodeficiency and hyper-IgM syndrome, has been detected in a small number of patients with constitutional mismatch repair deficiency (CMMRD) due to biallelic loss-of-function mutations in one of the MMR genes (PMS2, MSH6, MLH1, or MSH2). To elucidate the clinical relevance of a presumed primary immunodeficiency (PID) in CMMRD, we systematically collected clinical history and laboratory data of a cohort of 15 consecutive, unrelated patients (10 not previously reported) with homozygous/compound heterozygous mutations in PMS2 (n = 8), MSH6 (n = 5), and MLH1 (n = 2), most of whom manifested with typical malignancies during childhood. Detailed descriptions of their genotypes, phenotypes, and family histories are provided. Importantly, none of the patients showed any clinical warning signs of PID (infections, immune dysregulation, inflammation, failure to thrive, etc.). Furthermore, we could not detect uniform or specific patterns of laboratory abnormalities. The concentration of IgM was increased in 3 out of 12, reduced in 3 out of 12, and normal in 6 out of 12 patients, while concentrations of IgG and IgG subclasses, except IgG4, and of IgA, and specific antibody formation were normal in most. Class-switched B memory cells were reduced in 5 out of 12 patients, and in 9 out of 12 also the CD38hiIgM- plasmablasts were reduced. Furthermore, results of next generation sequencing-based analyses of antigen-selected B-cell receptor rearrangements showed a significantly reduced frequency of SHM and an increased number of rearranged immunoglobulin heavy chain (IGH) transcripts that use IGHG3, IGHG1, and IGHA1 subclasses. T cell subsets and receptor repertoires were unaffected. Together, neither clinical nor routine immunological laboratory parameters were consistently suggestive of PID in these CMMRD patients, but previously shown abnormalities in SHM and rearranged heavy chain transcripts were confirmed.