Novel DYT11 gene mutation in patients without dopaminergic deficit (SWEDD) screened for dystonia.

OBJECTIVE: To test the hypothesis that adult-onset primary dystonia may be the underlying etiology of tremulous patients with clinical diagnosis of Parkinson disease (PD) but without evidence of dopaminergic deficit at nigrostriatal SPECT imaging. METHODS: We retrospectively reviewed clinical and imaging data of patients with clinical diagnosis of PD assessed at our tertiary movement disorder clinic, who underwent dopamine transporter SPECT imaging consecutively between 2002 and 2011. Molecular screening for DYT1, DYT5, DYT6, DYT11, and DYT16 dystonia genes was performed in all cases who met the following criteria at the time of SPECT scan: (1) clinical diagnosis of PD; (2) normal dopamine transporter SPECT; (3) asymmetric rest tremor, with or without postural/kinetic component; (4) ≥ 12-month follow-up; and (5) normal brain MRI. We excluded subjects with (6) overt dystonic features, and (7) head or voice tremor. RESULTS: Twenty-three subjects were eligible for molecular analysis. Positive family history for tremor or PD was present in 45% of probands. We found one patient with a novel heterozygous frameshift mutation in the DYT11 gene (c.1058-1062 delCACCA/p.Gln352fsX376). Electrophysiologic study of tremor revealed that the main contributor was 5- to 6-Hz pseudo-rhythmic myoclonus, primarily involving extensor muscles. In 2 brothers, we found a missense variant in the DYT5 gene (c.334A>G; p.Thr112Ala) of uncertain pathogenicity in humans. CONCLUSION: Our findings provide further support to the hypothesis that adult-onset monogenic dystonia may underlie a "PD look-alike" clinical phenotype. In addition to dystonic tremor, pseudo-rhythmic myoclonus may be mischaracterized as "rest tremor."

To trim or not to trim: progression and control of DSB end resection.

DNA double-strand breaks (DSBs) are the most cytotoxic form of DNA damage, since they can lead to genome instability and chromosome rearrangements, which are hallmarks of cancer cells. To face this kind of lesion, eukaryotic cells developed two alternative repair pathways, homologous recombination (HR) and non-homologous end joining (NHEJ). Repair pathway choice is influenced by the cell cycle phase and depends upon the 5'-3' nucleolytic processing of the break ends, since the generation of ssDNA tails strongly stimulates HR and inhibits NHEJ. A large amount of work has elucidated the key components of the DSBs repair machinery and how this crucial process is finely regulated. The emerging view suggests that besides endo/exonucleases and helicases activities required for end resection, molecular barrier factors are specifically loaded in the proximity of the break, where they physically or functionally limit DNA degradation, preventing excessive accumulation of ssDNA, which could be threatening for cell survival.

Dynamics of Rad9 chromatin binding and checkpoint function are mediated by its dimerization and are cell cycle-regulated by CDK1 activity.

Saccharomyces cerevisiae Rad9 is required for an effective DNA damage response throughout the cell cycle. Assembly of Rad9 on chromatin after DNA damage is promoted by histone modifications that create docking sites for Rad9 recruitment, allowing checkpoint activation. Rad53 phosphorylation is also dependent upon BRCT-directed Rad9 oligomerization; however, the crosstalk between these molecular determinants and their functional significance are poorly understood. Here we report that, in the G1 and M phases of the cell cycle, both constitutive and DNA damage-dependent Rad9 chromatin association require its BRCT domains. In G1 cells, GST or FKBP dimerization motifs can substitute to the BRCT domains for Rad9 chromatin binding and checkpoint function. Conversely, forced Rad9 dimerization in M phase fails to promote its recruitment onto DNA, although it supports Rad9 checkpoint function. In fact, a parallel pathway, independent on histone modifications and governed by CDK1 activity, allows checkpoint activation in the absence of Rad9 chromatin binding. CDK1-dependent phosphorylation of Rad9 on Ser11 leads to specific interaction with Dpb11, allowing Rad53 activation and bypassing the requirement for the histone branch.

Checkpoint mechanisms at the intersection between DNA damage and repair.

In response to genomic insults cells trigger a signal transduction pathway, known as DNA damage checkpoint, whose role is to help the cell to cope with the damage by coordinating cell cycle progression, DNA replication and DNA repair mechanisms. Accumulating evidence suggests that activation of the first checkpoint kinase in the cascade is not due to the lesion itself, but it requires recognition and initial processing of the lesion by a specific repair mechanism. Repair enzymes likely convert a variety of physically and chemically different lesions to a unique common structure, a ssDNA region, which is the checkpoint triggering signal. Checkpoint kinases can modify the activity of repair mechanisms, allowing for efficient repair, on one side, and modulating the generation of the ssDNA signal, on the other. This strategy may be important to allow the most effective repair and a prompt recovery from the damage condition. Interestingly, at least in some cases, if the damage level is low enough the cell can deal with the lesions and it does not need to activate the checkpoint response. On the other hand if damage level is high or if the lesions are not rapidly repairable, checkpoint mechanisms become important for cell survival and preservation of genome integrity.

Phosphorylation of the budding yeast 9-1-1 complex is required for Dpb11 function in the full activation of the UV-induced DNA damage checkpoint.

Following genotoxic insults, eukaryotic cells trigger a signal transduction cascade known as the DNA damage checkpoint response, which involves the loading onto DNA of an apical kinase and several downstream factors. Chromatin modifications play an important role in recruiting checkpoint proteins. In budding yeast, methylated H3-K79 is bound by the checkpoint factor Rad9. Loss of Dot1 prevents H3-K79 methylation, leading to a checkpoint defect in the G(1) phase of the cell cycle and to a reduction of checkpoint activation in mitosis, suggesting that another pathway contributes to Rad9 recruitment in M phase. We found that the replication factor Dpb11 is the keystone of this second pathway. dot1Delta dpb11-1 mutant cells are sensitive to UV or Zeocin treatment and cannot activate Rad53 if irradiated in M phase. Our data suggest that Dpb11 is held in proximity to damaged DNA through an interaction with the phosphorylated 9-1-1 complex, leading to Mec1-dependent phosphorylation of Rad9. Dpb11 is also phosphorylated after DNA damage, and this modification is lost in a nonphosphorylatable ddc1-T602A mutant. Finally, we show that, in vivo, Dpb11 cooperates with Dot1 in promoting Rad9 phosphorylation but also contributes to the full activation of Mec1 kinase.

Histone methyltransferase Dot1 and Rad9 inhibit single-stranded DNA accumulation at DSBs and uncapped telomeres.

Cells respond to DNA double-strand breaks (DSBs) and uncapped telomeres by recruiting checkpoint and repair factors to the site of lesions. Single-stranded DNA (ssDNA) is an important intermediate in the repair of DSBs and is produced also at uncapped telomeres. Here, we provide evidence that binding of the checkpoint protein Rad9, through its Tudor domain, to methylated histone H3-K79 inhibits resection at DSBs and uncapped telomeres. Loss of DOT1 or mutations in RAD9 influence a Rad50-dependent nuclease, leading to more rapid accumulation of ssDNA, and faster activation of the critical checkpoint kinase, Mec1. Moreover, deletion of RAD9 or DOT1 partially bypasses the requirement for CDK1 in DSB resection. Interestingly, Dot1 contributes to checkpoint activation in response to low levels of telomere uncapping but is not essential with high levels of uncapping. We suggest that both Rad9 and histone H3 methylation allow transmission of the damage signal to checkpoint kinases, and keep resection of damaged DNA under control influencing, both positively and negatively, checkpoint cascades and contributing to a tightly controlled response to DNA damage.