A New Type of Ataxia, Spinocerebellar Ataxia Type 12 (SCA12), may be Caused by an Unusual Trinucleotide Repeat Expansion Mutation.

The Laboratory of Genetic Neurobiology (directed by Dr. Russell L. Margolis and located within the Division of Neurobiology of the Department of Psychiatry of the Johns Hopkins University School of Medicine), is attempting to identify the genetic mutations that cause hereditary movement disorders, including the ataxias. Our hope is that such work will 1) improve the diagnosis, and eventually the treatment, for individuals with hereditary diseases affecting movement, 2) provide additional understanding into the mechanisms by which disease damages or kills nerve cells, and 3) enable us to better understand the relationship between brain disease and problems with emotion, thinking, and memory. The work in our laboratory complements ongoing investigations in other laboratories of the Division of Neurobiology, under the overall leadership of Dr. Christopher A. Ross, into the biology of Huntington's disease, dentatorubral pallidoluysian atrophy (DRPLA), and spinocerebellar ataxia type 3 (SCA3, Machado-Joseph Disease, or MJD). Our rationale is that knowledge gained about any one of these diseases improves our understanding of all related diseases.

We have recently identified a new type of spinocerebellar ataxia (SCA12) in a single large pedigree (Holmes et al., Nature Genetics, December 1999, p 391-392). To preserve the confidentiality of the family, we arbitrarily refer to the family as pedigree R. Most members of the family live in the Mid-Atlantic region of the United States, but the family originally immigrated to the United States from Southern Germany in the early 19th-Century. The age of disease onset in this family ranges from 8 to 55 years, though determining the precise age of onset has been difficult. Most individuals begin to show symptoms in middle age, usually beginning with upper extremity tremor, and progressing over several decades to include head tremor, gait ataxia, dysmetria, increased reflexes, a general decrease in movements, abnormal eye movements, and, in the eldest subjects, difficulties with thinking and memory. Dr. O'Hearn is preparing a manuscript that will describe the clinical features of SCA12 in more detail.

We used methods similar to those developed by Drs. Ranum and Koob (described by Amy Durand in the Fall 1999 issue of Generations) to find a genetic mutation that may cause the disease in this family. As in the case of SCA8, the mutation that we found is a trinucleotide repeat expansion, though with some intriguing differences. In the case of SCA12, the expansion consists of too many repeating units of the nucleotides (the building blocks of DNA) C, A, and G, found within a gene known as PR55B or PPP2R2B. In 394 individuals without a neurological disease, the CAG repeat in this gene varied from 7 to 29 triplets in length. All affected family members of pedigree R had an expanded repeat containing 66-78 triplets. In seven older members of pedigree R without neurological disease, the repeat was of the normal length. One unaffected family member had an expended repeat; this individual may develop the disease at a later age, or it is possible, as in SCA8, that not everyone with the genetic mutation will develop the disease.

Our statistical analyses indicate that the repeat expansion is most likely the cause of SCA12, but the final proof that the mutation is the cause of the disease rather than a coincidence must await the discovery of additional families with the same mutation and experiments to determine the effect of the mutation in cultured cells and in mice. We did not detect abnormally long repeats in 1100 unrelated individuals from other families with neurological disease (including 748 individuals with ataxia), indicating that this particular expanded repeat is rare.

As Amy Durand explained in her article last year, genes provide each cell in the body with instructions for making proteins, and it is the proteins that do most of the work of the cell. Part of each gene specifies (encodes) the exact protein to be made, and another part of each gene (the promoter) helps to specify how much of the protein should be made and when the protein should be made. Up to this point, all of the CAG repeats that expand to cause neurological disorders (Huntington's disease, DRPLA, and SCAs 1, 2, 3, 6, and 7) have been located in the part of the genes that encodes the protein. CAG is the genetic code for the amino acid glutamine, and it is believed that damage to brain cells arises when too many glutamines are incorporated into a protein.

However, the CAG repeat in the PR558 gene appears to be located in the promoter region of this gene, rather than in the protein coding region. We speculate that the repeat expansion changes the instructions about the quantity of protein to be made from the PR55B gene, so that an abnormal amount of the gene is translated into protein. This is of particular interest, since it is known that the protein encoded by the PR55B gene helps to regulate an important enzyme, known as protein phosphatase 2A (PP2A). An enzyme is a protein, or a cluster of proteins, that carries out chemical reactions in the body. The enzyme PP2A is an important regulator of many cellular activities, including cell division and cell death. PR55B regulates PP2A in the brain. We hypothesize that the CAG expansion in PR55B changes the amount of protein made from the PR55B gene, which may in turn alter the function of PP2A and lead to injury or death of neurons in the brain. We are beginning a series of biological experiments to test this possibility. If true, it suggests that PP2A is part of an important biochemical pathway for regulation of the health of brain cells, and could have implications for understanding other brain diseases.

It is important to note that this work was only possible with the help of many collaborators in the United States and in Germany. In addition, the work would not have been possible without the assistance of Athena Diagnostics, which provided DNA samples (after obtaining informed consent) for our efforts. Funding for our work came from the National Institute of Neurological Diseases and Stroke (NH16375) and the National Institute of Mental Health (MH01275 and MH50763), with additional funds from the Huntington's Disease Society of America.

Individuals with familial ataxias of unknown cause who are interested in participating in this study should contact Dr. Margolis by e-mail ([e-mail]) or by regular mail (Johns Hopkins University School of Medicine, Meyer 2-181, 600 North Wolfe Street, Baltimore, MD 21287).

Under a licensing agreement between the Johns Hopkins University and Athena Diagnostics, Dr. Holmes, Dr. Ross, and Dr. Margolis are entitled to a share of royalty received by the University on sales of products described in this summary. The terms of this arrangement are being managed by the Johns Hopkins University in accordance with its conflict of interest policies.

Russell L. Margolis, M.D.
Associate Professor of
Psychiatry
Division of Neurobiology,
Department of Psychiatry
Johns Hopkins University
School of Medicine
Meyer 2-181,
600 N. Wolfe Street
Baltimore, Maryland 21287