Three NewBLMGene Mutations Associated with Bloom Syndrome

Coexistence of Bloom Syndrome and Kostmann Disease and a Novel Mutation

Bloom syndrome (BS) is a rare autosomal recessive inherited disorder. Patients with BS have photosensitivity, telangiectatic facial erythema, and stunted growth. They usually have mild microcephaly, and distinctive facial features such as a narrow, slender face, micrognathism, and a prominent nose. Kostmann disease (KD) is a subgroup of severe congenital neutropenias. The diagnosis of severe congenital neutropenia is based on clinical symptoms, bone marrow findings, and genetic mutation. Here, we report a female patient with a triangular face, nasal prominence, and protruding ears presenting with recurrent infections and severe neutropenia. Molecular genetic testing revealed a compound heterozygous variant in the HCLS-1-associated protein X-1 gene [(c.130_131insA) p.(trp44*), c.430 dup(p.Val144fs)] and a new homozygous variant in Bloom Syndrome RecQ like helicase gene [c.2074+2T>C p.(?)]. She was diagnosed with both BS and KD. To the best of our knowledge, this is the first case of coexisting BS and KD in a patient ever reported.

[Progeroid syndromes : Aging, skin aging, and mechanisms of progeroid syndromes]

Progeroid syndromes (PSs) are characterized by the premature onset of age-related pathologies. PSs display a wide range of heterogeneous pathological symptoms that also manifest during natural aging, including vision and hearing loss, atrophy, hair loss, progressive neurodegeneration, and cardiovascular defects. Recent advances in molecular pathology have led to a better understanding of the underlying mechanisms of these diseases. The genetic mutations underlying PSs are functionally linked to genome maintenance and repair, supporting the causative role of DNA damage accumulation in aging. While some of those genes encode proteins with a direct involvement in a DNA repair machinery, such as nucleotide excision repair (NER), others destabilize the genome by compromising the stability of the nuclear envelope, when lamin A is dysfunctional in Hutchinson-Gilford progeria syndrome (HGPS) or regulate the DNA damage response (DDR) such as the ataxia telangiectasia-mutated (ATM) gene. Understanding the molecular pathology of progeroid diseases is crucial in developing potential treatments to manage and prevent the onset of symptoms. This knowledge provides insight into the underlying mechanisms of premature aging and could lead to improved quality of life for individuals affected by progeroid diseases.

Phenotypic spectrum of BLM- and RMI1-related Bloom syndrome

Bloom syndrome (BS) is an autosomal recessive disorder with characteristic clinical features of primary microcephaly, growth deficiency, cancer predisposition, and immunodeficiency. Here, we report the clinical and molecular findings of eight patients from six families diagnosed with BS. We identified causative pathogenic variants in all families including three different variants in BLM and one variant in RMI1. The homozygous c.581_582delTT;p.Phe194* and c.3164G>C;p.Cys1055Ser variants in BLM have already been reported in BS patients, while the c.572_573delGA;p.Arg191Lysfs*4 variant is novel. Additionally, we present the detailed clinical characteristics of two cases with BS in which we previously identified the biallelic loss-of-function variant c.1255_1259delAAGAA;p.Lys419Leufs*5 in RMI1. All BS patients had primary microcephaly, intrauterine growth delay, and short stature, presenting the phenotypic hallmarks of BS. However, skin lesions and upper airway infections were observed only in some of the patients. Overall, patients with pathogenic BLM variants had a more severe BS phenotype compared to patients carrying the pathogenic variants in RMI1, especially in terms of immunodeficiency which should be considered as one of the most important phenotypic characteristics of BS. This article is protected by copyright. All rights reserved.

Bloom syndrome and the underlying causes of genetic instability

Autosomal hereditary recessive diseases characterized by genetic instability are often associated with cancer predisposition. Bloom syndrome (BS), a rare genetic disorder, with <300 cases reported worldwide, combines both. Indeed, patients with Bloom's syndrome are 150 to 300 times more likely to develop cancers than normal individuals. The wide spectrum of cancers developed by BS patients suggests that early initial events occur in BS cells which may also be involved in the initiation of carcinogenesis in the general population and these may be common to several cancers. BS is caused by mutations of both copies of the BLM gene, encoding the RecQ BLM helicase. This review discusses the different aspects of BS and the different cellular functions of BLM in genome surveillance and maintenance through its major roles during DNA replication, repair, and transcription. BLM's activities are essential for the stabilization of centromeric, telomeric and ribosomal DNA sequences, and the regulation of innate immunity. One of the key objectives of this work is to establish a link between BLM functions and the main clinical phenotypes observed in BS patients, as well as to shed new light on the correlation between the genetic instability and diseases such as immunodeficiency and cancer. The different potential implications of the BLM helicase in the tumorigenic process and the use of BLM as new potential target in the field of cancer treatment are also debated.

Missing heritability in Bloom syndrome: First report of a deep intronic variant leading to pseudo-exon activation in the BLM gene

Pathogenic biallelic variants in the BLM/RECQL3 gene cause a rare autosomal recessive disorder called Bloom syndrome (BS). This syndrome is characterized by severe growth delay, immunodeficiency, dermatological manifestations and a predisposition to a wide variety of cancers, often multiple and very early in life. Literature shows that the main mode of BLM inactivation is protein translation termination. We expanded the molecular spectrum of BS by reporting the first deep intronic variant causing intron exonisation. We describe a patient with a clinical phenotype of BS and a strong increase in sister chromatid exchanges (SCE), who was found to be compound heterozygous for a novel nonsense variant c.3379C>T, p.(Gln1127Ter) in exon 18 and a deep intronic variant c.3020-258A>G in intron 15 of the BLM gene. The deep intronic variant creates a high-quality de novo donor splice site, which leads to retention of two intron segments. Both pseudo-exons introduce a premature stop codon into the reading frame and abolish BLM protein expression, confirmed by Western Blot analysis. These findings illustrate the role of non-coding variation in Mendelian disorders and herewith highlight an unmet need in routine testing of Mendelian disorders, being the added value of RNA-based approaches to provide a complete molecular diagnosis.

The Emerging Role of Cytidine Deaminase in Human Diseases: A New Opportunity for Therapy?

The recycling activity of cytidine deaminase (CDA) within the pyrimidine salvage pathway is essential to DNA and RNA synthesis. As such, CDA deficiency can lead to replicative stress, notably in Bloom syndrome. Alternatively, CDA also can deaminate cytidine and deoxycytidine analog-based therapies, such as gemcitabine. Thus, CDA overexpression is often associated with lower systemic, chemotherapy-related, adverse effects but also with resistance to treatment. Considering the increasing interest of CDA in cancer chemoresistance, the aims of this review are to describe CDA structure, regulation of expression, and activity, and to report the therapeutic strategies based on CDA expression that recently emerged for tumor treatment.

Bloom syndrome does not always present with sun-sensitive facial erythema

Bloom syndrome is an autosomal recessive condition characterized by severe pre- and postnatal growth deficiency, immunodeficiency, an increased risk for malignancies, craniofacial dysmorphisms, and "typical" erythematous sun-sensitive skin lesions of the face. This facial rash has a butterfly-shaped distribution around the nose and is usually observed for the first time during the early years of life. Though reported as being a main feature of Bloom syndrome, there seems to be phenotypic variability regarding this facial skin rash among patients. It has been previously reported that in some individuals with Bloom syndrome these sun-sensitive lesions are less prominent or even absent. In this report we describe a 36 year old woman with short stature, microcephaly, several dysmorphisms, congenital hypothyroidism and premature ovarian failure. She was diagnosed with nasopharyngeal carcinoma at 36 years of age, only a few months after her consultation at the department of Clinical Genetics. Whole Exome Sequencing demonstrated that she had Bloom syndrome caused by a compound heterozygous mutation in BLM (c.2207_2212delinsTAGATTC; p.(Tyr736Leufs*5) and c.3681del; p.(Lys1227Asnfs*52)). She did not have facial sun-sensitive erythematous rash during childhood nor adulthood. We conclude that Bloom syndrome does not always present with erythematous sun-sensitive skin lesions of the face. We would like to underline that phenotypic variation regarding this "hallmark" feature of Bloom syndrome exists. Being aware of this might prevent a delay in diagnosing this rare short-stature syndrome and, subsequently, its potential clinical implications.

A rigorous approach for selection of optimal variant sets for carrier screening with demonstration of clinical utility

Carrier screening for certain diseases is recommended by major medical and Ashkenazi Jewish (AJ) societies. Most carrier screening panels test only for common, ethnic-specific variants. However, with formerly isolated ethnic groups becoming increasingly intermixed, this approach is becoming inadequate. Our objective was to develop a rigorous process to curate all variants, for relevant genes, into a database and then apply stringent clinical validity classification criteria to each in order to retain only those with clear evidence for pathogenicity. The resulting variant set, in conjunction with next-generation DNA sequencing (NGS), then affords the capability for an ethnically diverse, comprehensive, highly specific carrier-screening assay. The clinical utility of our approach was demonstrated by screening a pan-ethnic population of 22,864 individuals for Bloom syndrome carrier status using a BLM variant panel comprised of 50 pathogenic variants. In addition to carriers of the common AJ founder variant, we identified 57 carriers of other pathogenic BLM variants. All variants reported had previously been curated and their clinical validity documented, or were of a type that met our stringent, preassigned validity criteria. Thus, it was possible to confidently report an increased number of Bloom’s syndrome carriers compared to traditional, ethnicity-based screening, while not reducing the specificity of the screening due to reporting variants of unknown clinical significance.

A novel frameshift mutation in BLM gene associated with high sister chromatid exchanges (SCE) in heterozygous family members

The Bloom syndrome (BS) is an autosomic recessive disorder comprising a wide range of abnormalities, including stunted growth, immunodeficiency, sun sensitivity and increased frequency of various types of cancer. Bloom syndrome cells display a high level of genetic instability, including a 10-fold increase in the sister chromatid exchanges (SCE) level. Bloom syndrome arises through mutations in both alleles of the BLM gene, which was identified as a member of the RecQ helicase family. In this study, we screened a Tunisian family with three BS patients. Cytogenetic analysis showed several chromosomal aberrations, and an approximately 14-fold elevated SCE frequency in BS cells. A significant increase in SCE frequency was observed in some family members but not reaching the BS patients values, leading to suggest that this could be due to the heterozygous profile. Microsatellite genotyping using four fluorescent dye-labeled microsatellite markers revealed evidence of linkage to BLM locus and the healthy members, sharing higher SCE frequency, showed heterozygous haplotypes as expected. Additionally, the direct BLM gene sequencing identified a novel homozygous frameshift mutation c.3617-3619delAA (p.K1207fsX9) in BS patients and a heterozygous BLM mutation in the family members with higher SCE frequency. Our findings suggest that this latter mutation likely leads to a reduced BLM activity explaining the homologous recombination repair defect and, therefore, the increase in SCE. Based on the present data, the screening of this mutation could contribute to the rapid diagnosis of BS. The genetic confirmation of the mutation in BLM gene provides crucial information for genetic counseling and prenatal diagnosis.