Impact of routine clinic measurement of serum C-peptide in people with a clinician-diagnosis of type 1 diabetes.

AIMS/HYPOTHESIS: The aim of this study was to determine the impact of the routine use of serum C-peptide in an out-patient clinic setting on individuals with a clinician-diagnosis of type 1 diabetes. METHODS: In this single-centre study, individuals with type 1 diabetes of at least 3 years duration were offered random serum C-peptide testing at routine clinic review. A C-peptide ≥200 pmol/L prompted further evaluation of the individual using a diagnostic algorithm that included measurement of islet cell antibodies and genetic testing. Where appropriate, a trial of anti-diabetic co-therapies was considered. RESULTS: Serum C-peptide testing was performed in 859 individuals (90% of the eligible cohort), of whom 114 (13.2%) had C-peptide ≥200 pmol/L. The cause of diabetes was reclassified in 58 individuals (6.8% of the tested cohort). The majority of reclassifications were to type 2 diabetes (44 individuals; 5.1%), with a smaller proportion of monogenic diabetes (14 individuals; 1.6%). Overall, 13 individuals (1.5%) successfully discontinued insulin, while a further 16 individuals (1.9%) had improved glycaemic control following the addition of co-therapies. The estimated total cost of the testing programme was £23,262 (~€26,053), that is, £27 (~€30) per individual tested. In current terms, the cost of prior insulin therapy in the individuals with monogenic diabetes who successfully stopped insulin was approximately £57,000 (~€64,000). CONCLUSIONS/INTERPRETATION: Serum C-peptide testing can easily be incorporated into an out-patient clinic setting and could be a cost-effective intervention. C-peptide testing should be strongly considered in individuals with a clinician-diagnosis of type 1 diabetes of at least 3 years duration.

Cleavage of claspin by caspase-7 during apoptosis inhibits the Chk1 pathway.

Claspin is required for the phosphorylation and activation of the Chk1 protein kinase by ATR during DNA replication and in response to DNA damage. This checkpoint pathway plays a critical role in the resistance of cells to genotoxic stress. Here, we show that human Claspin is cleaved by caspase-7 during the initiation of apoptosis. In cells, induction of DNA damage by etoposide at first produced rapid phosphorylation of Chk1 at a site targeted by ATR. Subsequently, etoposide caused activation of caspase-7, cleavage of Claspin, and dephosphorylation of Chk1. In apoptotic cell extracts, Claspin was cleaved by caspase-7 at a single aspartate residue into a large N-terminal fragment and a smaller C-terminal fragment that contain different functional domains. The large N-terminal fragment was heavily phosphorylated in a human cell-free system in response to double-stranded DNA oligonucleotides, and this fragment retained Chk1 binding activity. In contrast, the smaller C-terminal fragment did not bind Chk1, but did associate with DNA and inhibited the DNA-dependent phosphorylation of Chk1 associated with its activation. These results indicate that cleavage of Claspin by caspase-7 inactivates the Chk1 signaling pathway. This mechanism may regulate the balance between cell cycle arrest and induction of apoptosis during the response to genotoxic stress.

DNA-dependent phosphorylation of Chk1 and Claspin in a human cell-free system.

Cell-cycle checkpoints induced by DNA damage or replication play critical roles in the maintenance of genomic integrity during cell proliferation. Biochemical analysis of checkpoint pathways has been greatly facilitated by the use of cell-free systems made from Xenopus eggs. In the present study, we describe a human cell-free system that reproduces a DNA-dependent checkpoint pathway acting on the Chk1 protein kinase. In this system, double-stranded DNA oligonucleotides induce the phosphorylation of Chk1 at activating sites targeted by ATR [ATM (ataxia telangiectasia mutated)- and Rad3-related] and ATM kinases. Phosphorylation of Chk1 is dependent on the interaction of Claspin, a protein first identified in Xenopus as a Chk1-binding protein. We show that the DNA-dependent binding of Chk1 to Claspin requires two phosphorylation sites, Thr916 and Ser945, which lie within the Chk1-binding domain of Claspin. Using a phosphopeptide derived from the consensus motif of these sites, we show that the interaction of Claspin with Chk1 is required for the ATR/ATM-dependent phosphorylation of Chk1. Using a panel of protein kinase inhibitors, we provide evidence that Chk1 is phosphorylated at an additional site in response to activation of the checkpoint response, probably by autophosphorylation. Claspin is phosphorylated in the Chk1-binding domain in an ATR/ATM-dependent manner and is also targeted by additional kinases in response to double-stranded DNA oligonucleotides. This cell-free system will facilitate further biochemical analysis of the Chk1 pathway in humans.