SPring-8, the large synchrotron radiation facility

Release Date

11 Feb, 2010

Nara Institute of Science and Technology

1. Background of research
Premature aging syndromes are rare diseases in which overall aging accelerates when the patients are still young. Recently, Hutchinson-Gilford syndrome (also called progeria, in which children begin to age rapidly immediately after birth) has become known to the public because overseas patients suffering from this syndrome were the subject of documentary programs. However, another genetic disease called Werner's syndrome is more prevalent in Japanese people. While it is estimated that there are only several thousand patients with Werner's syndrome worldwide, 70% of those patients are Japanese. In Werner's syndrome, patients age at double the normal rate because of an abnormality in a protein called the Werner helicase. The Werner helicase is known to unwind DNA having unique structures (four-stranded intermediates called Holliday junctions and telomeres located at the ends of chromosomes,^*3 which regulate the cell lifetime; Fig. 2A) that cannot be unwound by ordinary helicases. However, the mechanism of unwinding by the Werner helicase had not been elucidated.

2. Experimental methods and results
To elucidate the relationship between proteins and premature aging syndromes, the researchers examined the effect of the Werner helicase on the DNA in a healthy person by X-ray crystal structure analysis.^*4 The shapes of molecules were observed by crystallizing the central part (RecQ domain) of the Werner helicase bound to double-stranded DNA and exposing the obtained crystals to the high-brilliance X-ray of the Structural Biology I Beamline (BL41XU) and the Macromolecular Assemblies Beamline (BL44XU) in SPring-8.

As a result, the state of the Werner helicase as it begins to unwind the DNA was elucidated three dimensionally (Fig. 1). The Werner helicase has a protruding structure named a "DNA-unwinding knife" and unwinds the double helix of the DNA while rotating around it, similarly to the peeling of an apple with a knife. Moreover, this unwinding knife has an elongated shape protruding from the molecular surface, which is the most suitable shape for unwinding DNA with a complicated structure (Fig. 2B). It is considered that the Werner helicase prevents the occurrence of premature aging syndromes by unwinding DNA structures such as Holliday junctions and telomeres with the DNA-unwinding knife.

3. Future expectations
Even in a healthy person, genes are damaged in daily life and such damaged genes cause aging and may cause cancer. In particular, telomeres are important parts of chromosomes that regulate the cell lifetime. Because telomeres have a complicated loop structure, it is considered to be difficult to maintain their length by the function of ordinary proteins alone. In patients with Werner's syndrome, the telomeres tend to become shorter because of mutation of the Werner helicase, resulting in the accelerated aging of the body. There is still no technique for curing patients with shortened telomeres. However, the continued examination of the shapes of proteins may lead to the discovery of a means of maintaining the original length of telomeres, namely, a means of keeping cells young. Also, it has been reported that inhibition of the Werner helicase activity suppresses the growth of cancer. These research achievements are expected to be applied to the development of anticancer drugs for the general public.

<Figure>

Fig. 1 DNA-unwinding knife of Werner helicase observed in this study
It has the shape of a knife being held by a hand.

Fig. 2 Unwinding process of unique DNA structures by Werner helicase

(A) The DNA-unwinding knife of the Werner helicase is so fine that it can be inserted into narrow gaps in the DNA structure, which ordinary helicases cannot enter. Owing to this feature, the DNA-unwinding knife appears to be used for unwinding complicated DNA structures.
(B) The Werner helicase unwinding a Holliday junction (simulation model on the basis of this study). The RecQ domains (blue) consisting of two molecules insert the DNA-unwinding knife into the narrow branch points of the Holliday junction, and each domain starts to unwind the DNA structures on the left and right.

<Glossary>

*1 Werner's syndrome
This is a premature aging syndrome named after its discoverer Otto Werner. While the symptoms of progeria appear immediately after birth, in the case of Werner's syndrome, overall aging starts to accelerate rapidly when the patients are in their mid-teens. The mean life expectancy of a patient with Werner's syndrome is 46 years, and no treatment for this disease has yet been discovered. It is estimated that one out of several hundred Japanese people is a latent carrier of Werner's syndrome, namely, a carrier of the mutation of the Werner gene, even if he/she does not develop the disease.

*2 DNA helicases
DNA helicases are proteins that separate the double helix of DNA into single strands. They can be classified into various types. The amino-acid sequence of the Werner helicase is similar to that of the Bloom helicase, which causes Bloom syndrome (a rare disease in which the patients frequently develop cancer in their childhood) and that of the RECQL4 helicase, which causes Rothmund-Thomson syndrome. These helicases are generally called RecQ family helicases. The results of this study indicated that the Bloom helicase also has a similar DNA-unwinding knife.

*3 Telomeres
Telomeres are the regions of DNA at the ends of chromosomes that shorten every time a cell divides. They are sometimes described as an "aging clock." Telomeres were the subject of the research awarded the 2009 Nobel Prize in Physiology or Medicine.

*4 X-ray crystal structure analysis
This is a method of determining the three-dimensional structure of a protein or DNA molecule by crystallizing the molecules and exposing the crystals to an X-ray beam.