The International Society for Mannosidosis & Related Diseases, Inc. presents
peeling the onion:
the genetics of alpha mannosidosis
 |
by Øivind Nilssen Senior Scientist Department of Medical Genetics University Hospital of Tromsø, Norway |
as published in Pathways
Winter 2000 edition (vol. 2, no 3)
see also our comprehensive section on Alpha-Mannosidosis
Back in 1991, the Department of Medical Genetics in Tromsø, Norway, was newly established and we were looking for worthwhile projects that would benefit patients and increase our skills and knowledge. Having experience from a related disorder, Aspartylglucosaminuria (AGU), and being strongly encouraged by Dr. Dag Malm, we decided to concentrate on characterizing the molecular basis of Alpha-Mannosidosis. Knowledge about Alpha-Mannosidosis and its gene was very sparse at that time. In order to clone and characterize the gene we depended on the pure alpha-mannosidase enzyme. Ole K. Tollersrud carried out purification of the enzyme in a neighboring laboratory. Dr. Malm explained the design of this work, in a previous issue of Pathways (Vol.2, #1 2000).
From the pure enzyme we obtained amino acid sequences. DNA encodes amino acid sequences and, hence, small pieces of DNA can be synthesized that match amino acid sequences specific to alpha-mannosidase. By using a combination of several techniques these small pieces of synthetic DNA were then used as probes or ”pathfinders” in order to trap the alpha-mannosidase gene. The gene was then cloned into a small virus that infects bacteria. By this strategy the gene could be multiplied along with the virus, in amounts sufficient for DNA sequencing analysis. The alpha-mannosidase gene was sequenced in bits and pieces and then the sequence was put together revealing a coding sequence of more than 3000 base pairs with the capacity to encode a protein of 1011 amino acids, the enzyme alpha-mannosidase. Including the intervening DNA, which is always contained within mammalian genes, the real size of the gene was found to be 21.500 base pairs.
It was a big moment for all of us the day we reported the cloning and sequencing of the alpha- mannosidase gene at the annual meeting of the American Society of Human Genetics in Minneapolis, in 1995. Later, the gene sequence was published in scientific journals by us and by other research groups.
Mutations
Since cloning the gene, one of our major goals has been to investigate the spectrum of mutations in alpha-mannosidosis. Most of all we wanted to develop diagnostic tools to be applied at the molecular genetic level. This would allow us to offer high quality and specific diagnosis to families at risk. Furthermore, it was of interest to investigate whether the clinical variability of Alpha-Mannosidosis was caused by mutational heterogeneity in the alpha-mannosidase gene. As a part of this effort we collected blood samples, fibroblast cells and DNA from patients from all over Europe. In addition, samples were obtained from North Africa, the island of Reunion in the Indian ocean, Australia, New Zealand, North America and recently also from Russia. By the efforts of our dedicated PhD students, Hilde M. F. Riise, Thomas Berg and Gaute M. Hansen, the spectrum of 21 disease-causing mutations were published in 1999. The mutations were indeed different in nature! Some were expected to be severe as they deleted parts of the gene, whereas others appeared to cause only minor alterations of the enzyme. However, we found that all mutations completely abolished alpha-mannosidase function and there appeared to be no correlation between the types of mutations and the clinical manifestations. Hence, clinical variation must be explained by other mechanisms such as environmental factors and contribution of other genes.
Thanks to the contribution and the skills of bioengineer Helle Klenow, among others at our laboratory, we have discovered more than 30 new disease-causing mutations the last couple of years. Specific diagnostic methods are being developed to detect each one of them!
Most Alpha-Mannosidosis mutations are rare and private, belonging to one or only a few families, and only a few mutations are found in more than one country. However, one specific mutation, named R750W, appears to account for a large fraction of Alpha-Mannosidosis disease genes occurring in Turkey, Italy, France, Holland, Germany, Poland, Finland, Sweden, Russia, England, New Zealand, Australia and North America.
It was of interest to see if this wide geographic distribution results from recurrent mutational events or whether it results from one single mutation that has spread by migration. By studying specific genetic markers within alpha-mannosidase genes, containing the R750W mutation, we have come to the preliminary conclusion that the R750W mutation results from one, or perhaps two, mutational events that occurred sufficiently long ago to allow this particular mutant gene to spread across Europe by migration. In the last 400-500 years, R750W has spread to the New World by the same mechanisms. This is a common phenomenon called Founder Effect.
Are all R750W carriers related? Some carriers are of course related directly as siblings or cousins. It is also reasonable to believe that most R750W carriers have identical alpha-mannosidase genes by descent. However, R750W is just one out of thousands of "genetic markers" that are inherited from the past. The implication is that in most cases R750W carriers cannot necessarily be considered as distant relatives in the way the term "distant relative" is normally used. Rather, in most cases, R750W carriers have in common that they share a tiny fragment on chromosome 19 that originates from one (or a few) common founder mutation(s) hundreds, or even thousands of years ago.
Incidence
There is some uncertainty with regard to the incidence of Alpha-Mannosidosis. This could result for a number of reasons: a bias of ascertainment regarding recognition of mild cases by the health care system; insufficient availability of diagnostic tools around the world; the diagnosis “Lysosomal Storage Disorder” might be regarded as a satisfactory explanation alone or there might simply be a lack of awareness about the disease in some countries. However, a rough estimate of the incidence would be 1:500,000 – 1:1,000,000. This would correspond to a carrier frequency of 1:350–1:500 (the square root of the incidence multiplied by 2 applies for rare recessive disorders). This means that of 60,000 inhabitants in the town where I live (Tromsø, Norway) there would be about 120 to 170 carriers. Surprised? The high carrier frequency relates to the fact that the large bulk of recessive disease genes in the population are hidden in normal carriers. Actually, every one of us carries at least several genes for autosomal recessive disorders that would be very damaging and even lethal if homozygous, or expressed.
Conclusions
There are so many clinical and molecular aspects of Alpha-Mannosidosis that deserve further investigation. The good news is that recently several research groups in Europe have become interested in Alpha-Mannosidosis. We have joined our forces and will continue our research in a collaborative manner!
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