Normal expression of the Fanconi anemia proteins FAA and FAC and sensitivity to mitomycin C in two patients with Seckel syndrome

Central nervous system vasculopathy and Seckel syndrome: case illustration and systematic review

PURPOSE

To systematically review reported cases of Seckel syndrome (SS) and point out cases associated with central nervous system (CNS) vasculopathy and provide a summary of their clinical presentation, management, and outcomes including our illustrative case.

METHODS

We conducted a search on the MEDLINE, PubMed, Google Scholar, and Cochrane databases using the keywords "Seckel + syndrome." We identified 127 related articles reporting 252 cases of SS apart from our case. Moreover, we searched for SS cases with CNS vasculopathies from the same databases. We identified 7 related articles reporting 7 cases of CNS vasculopathies in SS patients.

RESULTS

The overall rate of CNS vasculopathy in SS patients is 3.16% (n = 8/253), where moyamoya disease (MMD) accounted for 1.97%. The mean age is 13.5 years (6-19 years), with equal gender distribution (M:F, 1:1). The most common presenting symptoms were headache and seizure followed by weakness or coma. Aneurysms were mostly located in the basilar artery, middle cerebral artery, and internal carotid artery, respectively. Regardless of the management approach, 50% of the cases sustained mild-moderate neurological deficit, 37.5% have died, and 12.5% sustained no deficit.

CONCLUSION

A high index of suspicion should be maintained in (SS) patients, and MMD should be part of the differential diagnosis. Prevalence of CNS vasculopathy in SS is 3.16% with a much higher prevalence of MMD compared to the general population. Screening for cerebral vasculopathy in SS is justifiable especially in centers that have good resources. Further data are still needed to identify the most appropriate management plan in these cases.

Seckel syndrome exhibits cellular features demonstrating defects in the ATR-signalling pathway

To date, the only reported genetic defect identified in the developmental disorder, Seckel syndrome, is a mutation in ataxia telangiectasia and Rad3-related protein (ATR). Seckel syndrome is clinically and genetically heterogeneous and whether defects in ATR significantly contribute to Seckel syndrome is unclear. Firstly, we characterize ATR-Seckel cells for their response to DNA damage. ATR-Seckel cells display impaired phosphorylation of ATR-dependent substrates, impaired G2/M checkpoint arrest and elevated micronucleus (MN) formation following exposure to UV and agents that cause replication stalling. We describe a novel phenotype, designated nuclear fragmentation (NF), that occurs following replication arrest. Finally, we report that ATR-Seckel cells have an endogenously increased number of centrosomes in mitotic cells demonstrating a novel role for ATR in regulating centrosome stability. We exploit these phenotypes to examine cell lines derived from additional unrelated Seckel syndrome patients. We show that impaired phosphorylation of ATR-dependent substrates is a common but not invariant feature of Seckel syndrome cell lines. In contrast, all cell lines displayed defective G2/M arrest, increased levels of NF and MN formation following exposure to agents that cause replication stalling. All the Seckel syndrome cell lines examined showed increased endogenous centrosome numbers. Though ATR cDNA can complement the defects in ATR-Seckel cells, it failed to complement any of the additional cell lines. We conclude that Seckel syndrome represents a further damage response disorder that is uniquely associated with defects in the ATR-signalling pathway resulting in failed checkpoint arrest following exposure to replication fork stalling.

Chromosomal instability at common fragile sites in Seckel syndrome

Seckel syndrome (SCKL) is a rare, genetically heterogeneous disorder, with dysmorphic facial appearance, growth retardation, microcephaly, mental retardation, variable chromosomal instability, and hematological disorders. To date, three loci have been linked to this syndrome, and recently, the gene encoding ataxia-telangiectasia and Rad3-related protein (ATR) was identified as the gene mutated at the SCKL1 locus. The ATR mutation affects splicing efficiency, resulting in low levels of ATR in affected individuals. Elsewhere, we reported increased instability at common chromosomal fragile sites in cells lacking the replication checkpoint gene ATR. Here, we tested whether cells from patients carrying the SCKL1 mutation would show increased chromosome breakage following replication stress. We found that, compared with controls, there is greater chromosomal instability, particularly at fragile sites, in SCKL1-affected patient cells after treatment with aphidicolin, an inhibitor of DNA polymerase alpha and other polymerases. The difference in chromosomal instability between control and patient cells increases at higher levels of aphidicolin treatment, suggesting that the low level of ATR present in these patients is not sufficient to respond appropriately to replication stress. This is the first human genetic syndrome associated with increased chromosome instability at fragile sites following replication stress, and these findings may be related to the phenotypic findings in patients with SCKL1.

Chromosome instability induced in vitro with mitomycin C in five Seckel syndrome patients

Seckel syndrome (SS) is an autosomal recessive entity characterized by proportionate pre- and post-natal growth retardation, microcephaly, typical facial appearance with beak-like protrusion, and severe mental retardation. A heterogeneous basis for SS was proposed since around 25% of SS patients have hematological anomalies, suggesting a subgroup of SS with chromosome instability and hematological disorders. Chromosome instability induced by mitomycin C (MMC) has been observed in previous reports. The purpose of this study is to report cytogenetic features in five patients with SS. The patients had low birth weight (mean 1,870 g), short stature (SD = 6.36), microcephaly (OFC, SD = 8.1), typical facial appearance, and multiple articular dislocations. None of them had anemia at the time of examination. In all cases their parents were healthy and non-consanguineous. Lymphocytes of SS patients and a control group (n = 9) matched by age and sex were cultured with and without MMC, and harvested at 72 and 96 hr. Chromosomal aberrations (chromatid and chromosomal gaps and breaks, deletions, fragments, and exchanges) were scored in 100 metaphases per culture. A statistical increase of chromosomal aberrations was observed in 96 hr MMC cultures in all patients (40.2% vs. 2.8%). Sister chromatid exchanges were also performed with no differences between groups. Clinical and cytogenetic findings support the idea that SS may correspond to a chromosome instability syndrome.

The ATR-p53 pathway is suppressed in noncycling normal and malignant lymphocytes

Chronic lymphocytic leukaemia (CLL) results from the accumulation of apoptosis-resistant clonal B cells that are arrested in G0/G1, and is heterogeneous with respect to clinical outcome. An aggressive form of the disease is identified by an impaired p53 response to ionizing radiation (IR). This is associated with inactivating mutations of either p53 or ATM, a regulator of p53 activated by IR-induced DNA damage. Since other forms of DNA damage activate p53 via ATR, a kinase closely related to ATM, abnormalities of the ATR-p53 pathway also have the potential to result in p53 dysfunction. We therefore tested cases of CLL for abnormal p53 responses to ultraviolet irradiation (UVC), a known activator of ATR, to screen for additional forms of p53 dysfunction. CLL cells and normal peripheral blood mononuclear cell (PBMC) preparations (predominantly noncycling lymphocytes) were treated with UVC and assessed for p53 responses. In all of the CLL cases and PBMC preparations tested, we were unable to detect p53 accumulation, phosphorylation or transcriptional consequences in response to UVC-induced DNA damage. The most likely explanation for the absence of UVC-induced p53 activation in CLL and normal lymphocytes was that, in contrast to other cell types, the UVC-induced ATR pathway was inactive. This notion was confirmed by showing that ATR protein was absent or undetectable in all of the cases of CLL and normal PBMCs screened. This was an unexpected finding because ATR was thought to be essential for the viability of somatic cells and for normal human and murine embryonic development. An obvious difference between the cell lines used as positive controls for ATR antibodies and the CLL cells/PBMCs was that the former were actively cycling while the latter were quiescent. We therefore hypothesized that the ATR-p53 pathway is selectively downregulated in noncycling lymphocytes. To test this, we induced cycling in the T-cell fraction of PBMC preparations and demonstrated that ATR protein expression was restored. Furthermore, p53 was upregulated and phosphorylated in response to UVC in these cells. Our data support the conclusion that the ATR-p53 pathway is suppressed in noncycling lymphocytes via ATR downregulation. We tentatively suggest that this repressed DNA damage response may have evolved to protect quiescent lymphocytes from the potential for p53-dependent apoptosis in the face of some forms of endurable genotoxic stress. If this is the case, DNA repair and genome stability might be compromised in quiescent lymphocytes with potentially negative consequences.

High risk of malignancy in mosaic variegated aneuploidy syndrome

Fourteen cases of mosaic variegated aneuploidy (MVA) syndrome have been reported in the last 10 years. The phenotype of this rare condition has been quite consistent: severe microcephaly, growth deficiency, mild physical anomalies, and mental retardation. We describe here a young boy in whom MVA syndrome is associated to myelodysplasia with a monosomy 7 bone marrow clone. At the age of 3 years, myelodysplasia progressed to an acute lymphoblastic leukemia, and the patient died soon after. Several syndromes with short stature and severe microcephaly, such as the Seckel and Nijmegen syndromes, comprise hematological findings and chromosome instability. However, chromosome instability was not confirmed in our patient. MVA with hematological findings has not been reported before, but 3 patients of 14 (21%) have developed a malignancy (rhabdomyosarcoma, acute lymphoblastic leukemia, and nephroblastoma). Therefore, we propose that MVA is a condition predisposing to neoplasia.