A Hypomorphic PALB2 Allele Gives Rise to an Unusual Form of FA-N Associated with Lymphoid Tumour Development.

Patients with biallelic truncating mutations in PALB2 have a severe form of Fanconi anaemia (FA-N), with a predisposition for developing embryonal-type tumours in infancy. Here we describe two unusual patients from a single family, carrying biallelic PALB2 mutations, one truncating, c.1676_1677delAAinsG;(p.Gln559ArgfsTer2), and the second, c.2586+1G>A; p.Thr839_Lys862del resulting in an in frame skip of exon 6 (24 amino acids). Strikingly, the affected individuals did not exhibit the severe developmental defects typical of FA-N patients and initially presented with B cell non-Hodgkin lymphoma. The expressed p.Thr839_Lys862del mutant PALB2 protein retained the ability to interact with BRCA2, previously unreported in FA-N patients. There was also a large increased chromosomal radiosensitivity following irradiation in G2 and increased sensitivity to mitomycin C. Although patient cells were unable to form Rad51 foci following exposure to either DNA damaging agent, U2OS cells, in which the mutant PALB2 with in frame skip of exon 6 was induced, did show recruitment of Rad51 to foci following damage. We conclude that a very mild form of FA-N exists arising from a hypomorphic PALB2 allele.

Growth retardation and growth hormone deficiency in patients with Ataxia telangiectasia.

BACKGROUND: Ataxia telangiectasia (A-T) is a devastating human recessive disorder characterised by progressive cerebellar ataxia, immunodeficiency, genetic instability, and cancer susceptibility. In addition, many patients suffer from growth failure. METHODS: We analyzed growth and IGF-1/BP3 levels of 24 A-T-patients compared with an age-matched group of healthy controls (n = 36). RESULTS: Ten (41.7%) A-T patients and none of healthy controls had an IGF-1 level below the 3rd percentile for age. The growth hormone (GH) stimulation tests revealed a severe GH deficiency with no increase of >5 ng/ml in six of the ten A-T patients. The IGF-1 generation tests revealed normal increases in IGF-1 values in all patients. CONCLUSION: Our results show that a disturbance in the GH/IGF-1 axis was present in 58.3% of A-T patients. Low levels of GH were the result of reduced central GH secretion. GH treatment may be a therapeutic option for A-T patients with severe growth failure.

Hypomorphic PCNA mutation underlies a human DNA repair disorder.

Numerous human disorders, including Cockayne syndrome, UV-sensitive syndrome, xeroderma pigmentosum, and trichothiodystrophy, result from the mutation of genes encoding molecules important for nucleotide excision repair. Here, we describe a syndrome in which the cardinal clinical features include short stature, hearing loss, premature aging, telangiectasia, neurodegeneration, and photosensitivity, resulting from a homozygous missense (p.Ser228Ile) sequence alteration of the proliferating cell nuclear antigen (PCNA). PCNA is a highly conserved sliding clamp protein essential for DNA replication and repair. Due to this fundamental role, mutations in PCNA that profoundly impair protein function would be incompatible with life. Interestingly, while the p.Ser228Ile alteration appeared to have no effect on protein levels or DNA replication, patient cells exhibited marked abnormalities in response to UV irradiation, displaying substantial reductions in both UV survival and RNA synthesis recovery. The p.Ser228Ile change also profoundly altered PCNA's interaction with Flap endonuclease 1 and DNA Ligase 1, DNA metabolism enzymes. Together, our findings detail a mutation of PCNA in humans associated with a neurodegenerative phenotype, displaying clinical and molecular features common to other DNA repair disorders, which we showed to be attributable to a hypomorphic amino acid alteration.

Variant ataxia telangiectasia: clinical and molecular findings and evaluation of radiosensitive phenotypes in a patient and relatives.

Variant ataxia telangiectasia (A-T) may be an underdiagnosed entity. We correlate data from radiosensitivity and kinase assays with clinical and molecular data from a patient with variant A-T and relatives. The coding region of ATM was sequenced. To evaluate the functional effect of the mutations, we performed kinase assays and developed a novel S-G2 micronucleus test. Our patient presented with mild dystonia, moderately dysarthric speech, increased serum α-fetoprotein but no ataxia nor telangiectasias, no nystagmus or oculomotor dyspraxia. She has a severe IgA deficiency, but does not have recurrent infections. She is compound heterozygote for ATM c.8122G>A (p.Asp2708Asn) and c.8851-1G>T, leading to in frame loss of 63 nucleotides at the cDNA level. A trace amount of ATM protein is translated from both alleles. Residual kinase activity is derived only from the p.Asp2708Asn allele. The conventional G0 micronucleus test, based on irradiation of resting lymphocytes, revealed a radiosensitive phenotype for the patient, but not for the heterozygous relatives. As ATM is involved in homologous recombination and G2/M cell cycle checkpoint, we optimized an S-G2 micronucleus assay, allowing to evaluate micronuclei in lymphocytes irradiated in the S and G2 phases. This test showed increased radiosensitivity for both the patient and the heterozygous carriers. Intriguingly, heterozygous carriers of c.8851-1G>T (mutation associated with absence of kinase activity) showed a stronger radiosensitive phenotype with this assay than heterozygous carriers of p.Asp2708Asn (mutation associated with residual kinase activity). The modified S-G2 micronucleus assay provided phenotypic insight into complement the diagnosis of this atypical A-T patient.

Very mild presentation in adult with classical cellular phenotype of ataxia telangiectasia.

BACKGROUND: The major clinical feature of ataxia telangiectasia (A-T) is severe progressive neurodegeneration with onset in infancy. This classical A-T phenotype is caused by biallelic null mutations in the ATM gene, leading to the absence of ATM protein and increased cellular radiosensitivity. We report an unusual case of A-T in a 41-year-old mother, A-T210, who had very mild neurological symptoms despite complete loss of ATM protein. METHODS: A neurological examination was performed, cellular radiosensitivity was assessed, and the ATM gene was sequenced. Skin fibroblasts and a lymphoblastoid cell line (LCL) were assayed for ATM protein expression and kinase activity. RESULTS: Patient A-T210 showed mild chorea, dystonia, and gait ataxia, walked independently, and drove a car. LCL and skin fibroblasts were radiosensitive and did not express ATM protein. Two ATM-null mutations were identified. CONCLUSIONS: The severe neurodegeneration resulting from loss of ATM can be mitigated in some circumstances.

Presence of ATM protein and residual kinase activity correlates with the phenotype in ataxia-telangiectasia: a genotype-phenotype study.

Ataxia-telangiectasia (A-T) is an autosomal recessive neurodegenerative disorder with multisystem involvement and cancer predisposition, caused by mutations in the A-T mutated (ATM) gene. To study genotype-phenotype correlations, we evaluated the clinical and laboratory data of 51 genetically proven A-T patients, and additionally measured ATM protein expression and kinase activity. Patients without ATM kinase activity showed the classical phenotype. The presence of ATM protein, correlated with slightly better immunological function. Residual kinase activity correlated with a milder and essentially different neurological phenotype, absence of telangiectasia, normal endocrine and pulmonary function, normal immunoglobulins, significantly lower X-ray hypersensitivity in lymphocytes, and extended lifespan. In these patients, cancer occurred later in life and generally consisted of solid instead of lymphoid malignancies. The genotypes of severely affected patients generally included truncating mutations resulting in total absence of ATM kinase activity, while patients with milder phenotypes harbored at least one missense or splice site mutation resulting in expression of ATM with some kinase activity. Overall, the phenotypic manifestations in A-T show a continuous spectrum from severe classical childhood-onset A-T to a relatively mild adult-onset disorder, depending on the presence of ATM protein and kinase activity. Each patient is left with a tremendously increased cancer risk.

Bmi-1 is induced by the Epstein-Barr virus oncogene LMP1 and regulates the expression of viral target genes in Hodgkin lymphoma cells.

Polycomb group (PcG) proteins are chromatin modifiers that are necessary for the maintenance and renewal of embryonic and adult stem cells. However, overexpression of the PcG protein, Bmi-1, causes lymphoma in transgenic mice. We show that Bmi-1 is up-regulated in Hodgkin lymphoma (HL) cells by the Epstein-Barr virus (EBV) oncogene latent membrane protein-1 (LMP1) and that this up-regulation is mediated by NF-kappaB signaling. We also show that Bmi-1 is up-regulated by NF-kappaB in EBV-negative HL cells. Down-regulation of LMP1 and Bmi-1 decreased the survival of HL cells, suggesting that Bmi-1 may mediate the prosurvival effects of LMP1-induced NF-kappaB signaling in HL cells. Transcriptional targets of Bmi-1 were identified after its knockdown in an HL cell line. We show here that Bmi-1 and LMP1 down-regulate the ataxia telangiectasia-mutated (ATM) tumor suppressor and conclude that Bmi-1 contributes to LMP1-induced oncogenesis in HL.

Rhabdomyosarcoma in Nijmegen breakage syndrome: strong association with perianal primary site.

Nijmegen breakage syndrome (NBS) is a rare autosomal recessive disorder resulting from mutations in the NBS1 gene, which encodes for the DNA double strand break repair protein nibrin. NBS is clinically characterized by microcephaly, dysmorphic features, immunodeficiency, and increased susceptibility to malignancy, mainly of lymphoid origin. Here, we describe a 7-year-old girl with NBS who is homozygous for the NBS1 698del4 mutation. She had been diagnosed with perianal rhabdomyosarcoma (RMS) and experienced severe toxicity from chemotherapy. RMS arising perianally is extremely uncommon but has been previously described in two cases with NBS. The strong association of perianal RMS with NBS should, therefore, be considered when confronted with a perianal RMS, as this carries important clinical implications in terms of potential need for therapy modification and follow up investigations. In addition, it suggests a role for the NBS1 gene and the nibrin dependent pathway in the pathogenesis of RMS, especially those arising perianally.

Adult-onset ataxia telangiectasia due to ATM 5762ins137 mutation homozygosity.

Ataxia telangiectasia (A-T) is an autosomal recessive neurodegenerative disorder that arises because of mutations in the ATM gene. The 5762ins137 A-->G point mutation activates a cryptic splice donor site resulting in a 137 bp intronic insert being aberrantly spliced into the ATM transcript. However, normal ATM transcript also is produced from this affected allele, albeit at significantly reduced levels. An exceptionally mild A-T phenotype occurs as a result of homozygosity for the 5762ins137 mutation because of relative preservation of ATM protein expression/kinase activity.