A non-synonymous single nucleotide polymorphism in SIRT6 predicts neurological severity in Friedreich ataxia.

Introduction: Friedreich ataxia (FRDA) is a recessive neurodegenerative disease characterized by progressive ataxia, dyscoordination, and loss of vision. The variable length of the pathogenic GAA triplet repeat expansion in the FXN gene in part explains the interindividual variability in the severity of disease. The GAA repeat expansion leads to epigenetic silencing of FXN; therefore, variability in properties of epigenetic effector proteins could also regulate the severity of FRDA. Methods: In an exploratory analysis, DNA from 88 individuals with FRDA was analyzed to determine if any of five non-synonymous SNPs in HDACs/SIRTs predicted FRDA disease severity. Results suggested the need for a full analysis at the rs352493 locus in SIRT6 (p.Asn46Ser). In a cohort of 569 subjects with FRDA, disease features were compared between subjects homozygous for the common thymine SIRT6 variant (TT) and those with the less common cytosine variant on one allele and thymine on the other (CT). The biochemical properties of both variants of SIRT6 were analyzed and compared. Results: Linear regression in the exploratory cohort suggested that an SNP (rs352493) in SIRT6 correlated with neurological severity in FRDA. The follow-up analysis in a larger cohort agreed with the initial result that the genotype of SIRT6 at the locus rs352493 predicted the severity of disease features of FRDA. Those in the CT SIRT6 group performed better on measures of neurological and visual function over time than those in the more common TT SIRT6 group. The Asn to Ser amino acid change resulting from the SNP in SIRT6 did not alter the expression or enzymatic activity of SIRT6 or frataxin, but iPSC-derived neurons from people with FRDA in the CT SIRT6 group showed whole transcriptome differences compared to those in the TT SIRT6 group. Conclusion: People with FRDA in the CT SIRT6 group have less severe neurological and visual dysfunction than those in the TT SIRT6 group. Biochemical analyses indicate that the benefit conferred by T to C SNP in SIRT6 does not come from altered expression or enzymatic activity of SIRT6 or frataxin but is associated with changes in the transcriptome.

Dmp1 physically interacts with p53 and positively regulates p53's stability, nuclear localization, and function.

The transcription factor Dmp1 is a Ras/HER2-activated haplo-insufficient tumor suppressor that activates the Arf/p53 pathway of cell-cycle arrest. Recent evidence suggests that Dmp1 may activate p53 independently of Arf in certain cell types. Here, we report findings supporting this concept with the definition of an Arf-independent function for Dmp1 in tumor suppression. We found that Dmp1 and p53 can interact directly in mammalian cells via the carboxyl-terminus of p53 and the DNA-binding domain of Dmp1. Expression of Dmp1 antagonized ubiquitination of p53 by Mdm2 and promoted nuclear localization of p53. Dmp1-p53 binding significantly increased the level of p53, independent of the DNA-binding activity of Dmp1. Mechanistically, p53 target genes were activated synergistically by the coexpression of Dmp1 and p53 in p53(-/-);Arf(-/-) cells, and genotoxic responses of these genes were hampered more dramatically in Dmp1(-/-) and p53(-/-) cells than in Arf(-/-) cells. Together, our findings identify a robust new mechanism of p53 activation mediated by direct physical interaction between Dmp1 and p53.

Analysis of echocardiograms in a large heterogeneous cohort of patients with friedreich ataxia.

Although Friedreich ataxia (FA) is associated with cardiomyopathy, the severity and evolution of cardiac disease is poorly understood. To identify factors predicting cardiomyopathy in FA, we assessed echocardiograms from a large heterogenous cohort and their relation to disease traits. The most recent echocardiograms from 173 subjects with FA were analyzed in a core laboratory to determine their relation to disease duration, subject age, age of onset, functional disability score, and GAA repeat length. Mean age of the cohort was 19.7 years, mean age of disease onset was 10.6 years, and mean shorter GAA length was 681 repeats. Echocardiograms collectively illustrated systolic dysfunction, diastolic dysfunction, and hypertrophy. Measurements of hypertrophy correlated moderately with each other (r = 0.39 to 0.79) but not with measurements of diastolic dysfunction (r <0.35). Diastolic measurements correlated poorly with each other, although 26% of the cohort had multiple diastolic abnormalities. The most common diastolic dysfunction classification was pseudonormalization. Classification of diastolic dysfunction was predicted by GAA repeat length but not by age or gender. Ejection fraction was below normal in 20% of the cohort. In linear regression analysis, increasing age predicted decreasing ejection fraction. Functional disability score, a measurement of neurologic ability, did not predict any echocardiographic measurements. In conclusion, hypertrophy and diastolic and systolic dysfunctions occur in FA and are substantially independent; diastolic dysfunction is the most common abnormality with most patients having an assigned diastolic dysfunction class of pseudonormalization.

Mortality in Friedreich ataxia.

BACKGROUND: Although cardiac dysfunction is widely accepted as the most common cause of mortality in Friedreich ataxia (FRDA), no studies have evaluated this since the advent of specific clinical and genetic diagnostic criteria. METHODS: We performed a retrospective study of FRDA patients to determine cause of death followed by a case-control analysis comparing characteristics of deceased patients with living, age- and sex-matched FRDA controls. RESULTS: Causes of death were cardiac dysfunction (59%), probable cardiac dysfunction (3.3%), non-cardiac (27.9%) or unknown (9.8%). Compared to non-cardiac deaths, cardiac deaths occurred earlier in the disease course (median 29 vs. 17years respectively). Congestive heart failure and arrhythmia were common causes of cardiac-related death. Compared to living, matched FRDA controls, deceased patients had longer triplet repeat lengths and higher rates of arrhythmia and dilated cardiomyopathy. The presence of hypertrophic cardiomyopathy did not differ between deceased and living patients. CONCLUSION: Cardiac dysfunction was the most frequent cause of death (59%), most commonly from congestive heart failure or arrhythmia. Arrhythmia and dilated cardiomyopathy were significantly more common in deceased patients compared to matched FRDA controls, while in contrast, the presence of cardiac hypertrophy did not differ. More research is needed to establish the clinical significance of hypertrophy in FRDA.

Idebenone in Friedreich ataxia cardiomyopathy-results from a 6-month phase III study (IONIA).

BACKGROUND: Friedreich ataxia (FRDA) is commonly associated with hypertrophic cardiomyopathy, but little is known about its frequency, severity, or treatment. In this 6-month randomized, double-blind, controlled study, we sought to determine whether idebenone improves cardiac measures in FRDA. METHODS: Seventy pediatric subjects were treated either with idebenone (450/900 mg/d or 1,350/2,250 mg/d) or with placebo. Electrocardiograms (ECGs) were assessed at each visit, and echocardiograms, at baseline and week 24. RESULTS: We found ECG abnormalities in 90% of the subjects. On echocardiogram, 81.4% of the total cohort had left ventricular (LV) hypertrophy, as measured by increased LV mass index-Dubois, and the mean ejection fraction (EF) was 56.9%. In linear regression models, longer PR intervals at baseline were marginally associated with longer GAA repeat length (P = .011). Similarly, GAA repeat length did not clearly predict baseline EF (P = .086) and LV mass by M-mode (P = .045). Left ventricular mass index, posterior wall thickness, EF, and ECG parameters were not significantly improved by treatment with idebenone. Some changes in echocardiographic parameters during the treatment phase correlated with baseline status but not with treatment group. CONCLUSIONS: Idebenone did not decrease LV hypertrophy or improve cardiac function in subjects with FRDA. The present study does not provide evidence of benefit in this cohort over a 6-month treatment period.

Broad overexpression of ribonucleotide reductase genes in mice specifically induces lung neoplasms.

Ribonucleotide reductase (RNR) catalyzes the rate-limiting step in nucleotide biosynthesis and plays a central role in genome maintenance. Although a number of regulatory mechanisms govern RNR activity, the physiologic effect of RNR deregulation had not previously been examined in an animal model. We show here that overexpression of the small RNR subunit potently and selectively induces lung neoplasms in transgenic mice and is mutagenic in cultured cells. Combining RNR deregulation with defects in DNA mismatch repair, the cellular mutation correction system, synergistically increased RNR-induced mutagenesis and carcinogenesis. Moreover, the proto-oncogene K-ras was identified as a frequent mutational target in RNR-induced lung neoplasms. Together, these results show that RNR deregulation promotes lung carcinogenesis through a mutagenic mechanism and establish a new oncogenic activity for a key regulator of nucleotide metabolism. Importantly, RNR-induced lung neoplasms histopathologically resemble human papillary adenocarcinomas and arise stochastically via a mutagenic mechanism, making RNR transgenic mice a valuable model for lung cancer.

Signal transduction involving the dmp1 transcription factor and its alteration in human cancer.

Dmp1 (cyclin D-interacting myb-like protein 1; also called Dmtf1) is a transcription factor that has been isolated in a yeast two-hybrid screen through its binding property to cyclin D2. Dmp1 directly binds to and activates the Arf promoter and induces Arf-p53-dependent cell cycle arrest in primary cells. D-type cyclins usually inhibit Dmp1-mediated transcription in a Cdk-independent fashion; however, Dmp1 shows synergistic effects with D-cyclins on the Arf promoter. Ras or Myc oncogene-induced tumor formation is accelerated in both Dmp1(+/-) and Dmp1(-/-) mice with no significant differences between Dmp1(+/-) and Dmp1(-/-). Thus, Dmp1 is haplo-insufficient for tumor suppression. Tumors from Dmp1(-/-) or Dmp1(+/-) mice often retain wild-type Arf and p53, suggesting that Dmp1 is a physiological regulator of the Arf-p53 pathway. The Dmp1 promoter is activated by oncogenic Ras-Raf signaling, while it is repressed by physiological mitogenic stimuli, overexpression of E2F proteins, and genotoxic stimuli mediated by NF-κB. The human DMP1 gene (hDMP1) is located on chromosome 7q21 and is hemizygously deleted in approximately 40% of human lung cancers, especially those that retain normal INK4a/ARF and P53 loci. Thus, hDMP1 is clearly involved in human carcinogenesis, and tumors with hDMP1 deletion may constitute a discrete disease entity.