Hots pots

phosphorylation. Mitochondria1 DNA does not un- dergo recombination and it has a much higher mutation ratecompared to nuclear DNA. Mitochon- drial DNA is also more likely to survive in an ancient specimen, as there is a far greater copy number of mitochondrial compared with nudear DNA. Taken together these properties make the mitochondria particularly attractive as a tool for deducing evolu- tionary relationships between species. The principle behind using mitochondria as a ‘molecular clock’ is simple and well established. The more distantly related two species are, the greater the number of different mutations between them (Fig. 2).

To clarify historically controversial issues, forensic genetics has been successfully used to identify re- mains of historically important persons, such as the remains of Tzar Nicolas of Russia. Such endeavors require the extraction of DNA from bone and the successful amplification of the DNA. However, such feats are technically challenging both in terms of deriving sufficient intact DNA and in particular avoiding the major problem of human contamina- tion. These difficulties are dramatically enhanced when it comes to the analyses of ancient DNA. The field of ancient DNA analyses has been plagued by unreliable data. Modem human contamination to- gether with PCR artifacts generated when amplifying miniscule amounts of DNA remain formidable hur- dles.

Nevertheless, Krings et al. have in a truly convinc- ing manner managed to amplify and clone ancient mitochondria DNA from Neandertal bone. Their incredibly strict protocol, together with their obvious expertise in recovering and handling ancient DNA were paramount to the success of this landmark achievement.

They sequenced 378 nucleotides of mitochondrial DNA from a Neandertal fossil and compared it with

Fig. 3.

that of modern humans and chimpanzees. As ex- pected sequence variation was greatest between chimpanzee and humans who are thought to have diverged between 4 and 5 million years ago. Sequence variation amongst modern humans, indicated that modern humans themselves had a common ancestor a little over 100000 years ago. One of the most exciting findings presented in this paper is that modem humans and Neandertals diverged approx- imately 550 000-690 000 years ago. Furthermore, Neandertals became extinct without contributing mitochondrial DNA to modern humans. Modern humans they conclude arose recently out of Africa as a separate species distinct from the Neandertals and while they coexisted for a period, modem humans eventually replaced them.

This study is a technical masterpiece in the art of contamination prevention during PCR. For those geneticists involved in amplification of particularly small amounts of DNA such as in preimplantation diagnosis, forensic genetics and single sperm analyses (currently for research purposes) the rigorousness described here is highly instructive.

This study underscores the power of molecular genetics to contribute to the understanding of a variety of fields seemingly unrelated to clinical genetics. Indeed, numerous controversies of anthro- pological, zoological and historical importance have been resolved via molecular genetic technology.

M u It if a cto ria I ag e-re la t ed macular degeneration allelic with autosomal recessive Stargardt macular dystrophy Mutation of the Stargardt disease gene (ABCR) in age-related macular dystrophy Allikmets et al. (1997) Science 277: 1805-1807

The macula is that part of the retina that enables us to see fine detail. Age-related macular degeneration (AMD) is the most common cause of acquired visual impairment in the elderly and is associated with a variety of lifestyle related exogenous risk factors including smoking, diet and cholesterol lev- els. AMD is a multifactorial disorder and genetic factors are also thought to play a role in the etiology of this disease.

The search for the genetic contribution to any multifactorial disorder is difficult owing to the complex nature of the disease and is often further compounded by late onset of symptoms. The ap- proach adopted here is that of screening likely

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candidate genes for mutations in patients with the disease. Selection of a good candidate gene for AMD would require that it is specifically expressed in the retina. In addition, genes for other retinal disorders sharing clinical features with AMD would be excellent candidate genes. To date, however, no mutations in the tissue inhibitor of metallo- proteinase-3 gene (Sorsby’s fundus dystrophy), pe- ripherin nor rhodopsin genes have been described in persons with AMD.

The gene for Stargardt disease which is an auto- soma1 recessive juvenile macular dystrophy with early blindness, was recently identified. The gene is a member of the ATP-binding cassette (ABC) trans- porter family. These proteins transport molecules across the cell membrane using ATP. Defective ABC proteins have been implicated in several genetic diseases including cystic fibrosis (cystic fibrosis transmembrane conductance protein) and Zellweger syndrome (peroxins). Since the Stargardt disease gene, called ABCR (ABC-retina), has photorecptor specific expression and when mutated culminates in a similar (but more severe) phenotype to AMD, it represented a good candidate gene for AMD.

To assess the role of ABCR defects in AMD, 167 unrelated patients were screened for mutations in all 51 exons of the gene. Remarkably, mutations were detected in (26/167) - 16% of AMD patients. Most of these were missense but there were also frameshift, deletions and splice site mutations. These were distributed across the entire coding region of the ABCR gene.

One exciting conclusion is that while homozygous disruption of the ABCR gene produces the rapid macular degeneration seen in Stargardt disease, a heterozygous defect in the same gene enhances susceptibility to another slower macular degenera- tive disorder, AMD. In other words, an autosomal recessive disease (Stargardt) and an increased sus- ceptibility to disease (AMD) differ in the dose of the gene that is rendered abnormal (Fig. 3).

These findings present an excellent opportunity to prevent blindness. We now have ability to screen the population for a mutation which will identify a proportion of those at risk for AMD. Unlike predic- tive testing for dominant disorders such Huntington disease where at present no effective therapy is available, those found-to harbor the disease suscep- tibility mutations could embark on lifestyle modifi- cations. It is hoped that early cessation of smoking and dietary manipulation could alter the natural history of AMD.

Furthermore, this study provides new insight into identifying disease susceptibility genes in general. Good candidate susceptibility genes for multifacto- rial disorders could be genes for autosomal recessive

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diseases with a more severe but overlapping clinical phenotype.

Diabetes and pancreatic agenesis Early onset type-I1 diabetes mellitus (MODY) linked to IPFl Stoffers et al. (1997) Nature Genetics 17: 138- 139

Maturity onset diabetes in the young (MODY) is a form of early onset autosomal dominant type I1 (non-insulin dependent) diabetes. In contrast to type I diabetes, type I1 is not linked to a specific HLA haplotype nor does it have an autoimmune basis. The genetic basis of MODY is however, rapidly unfolding in an exciting way providing novel insights into pancreatic function. Transcrip- tion factors HNFla and HNF4a have been impli- cated in MODY3 and MODY 1, respectively, while mutations in the enzyme glucokinase are responsi- ble for MODYZ. Stoffers et al. now add to this growing list of MODY genes by identifying muta- tions in the insulin promotor factor-1 (IPFl) in patients with MODY.

IPFl is a pancreatic transcription factor which plays a pivotal role in pancreatic development. In humans, homozygo-us inactivating mutations in IPFl result in pancreatic agenesis. Knockout stud- ies in mice provide further corroborating evidence for this: targeted disruption of the Ipf i results in pancreatic agenesis. Since homeodomain related proteins are critical in early embryonic segmenta- tion and the establishment of body pattern, the involvement in pancreatic agenesis of IPF 1, a homeodomain pancreatic master switch gene, is not surprising.

Stoffers et al. studied a 6-generation family with age dependent autosomal dominant transmission of diabetes. Although the average age of onset was 35 years (range 17-67), somewhat older than that seen in other forms of MODY, this still meets the age criteria for this disorder (age of onset in at least one family member <25 years). Linkage analysis excluded the three known loci for MODY. In view of IPFl’s role in pancreatic agenesis, Stof- fers et al. decided to test for IPFl mutations in this family. Mutation analysis of the IPFI gene uncov- ered an inactivating mutation (Pro63fsdelC = cy- tosine deletion at codon 63 causing a frameshift 59 codons downstream) which segregated with early onset diabetes in this large 5-generation family. Remarkably, this is the identical mutation previ- ously described in the homozygous state in a per-