Laura P.W. Ranum, Ph.D. Professor, Department of Genetics, Cell Biology and Development E-mail: ranum001@umn.edu

Myotonic Dystrophy:
Myotonic dystrophy (DM) is a multisystemic disease and the most common form of muscular dystrophy in adults. In 1992, DM was shown to be caused by an expanded CTG repeat in the 3' untranslated region of the myotonin protein kinase gene (DMPK) on chromosome 19. Although several theories were proposed to explain how the CTG expansion causes the broad spectrum of clinical features associated with DM, it was not understood how this mutation, which does not alter the protein coding region of a gene, causes an effect at the cellular level. Eight years ago Dr. John Day identified a five-generation family (MN1) with a genetically distinct form of myotonic dystrophy. Affected members exhibit remarkable clinical similarity to DM (myotonia, proximal and distal limb weakness, frontal balding, cataracts and cardiac arrhythmias) but do not have the chromosome 19 CTG expansion. Since 1995 Dr. Day and I have collaborated to clinically and genetically characterize the MN1 family. We mapped the disease locus (DM2) for the MN1 family to a 10 cM region of chromosome 3q [Nature Genetics 19:196-198 (1998)]. We then used positional cloning and linkage disequilibrium strategies to isolate the gene and demonstrated that the disease is caused by a CCTG tetranucleotide expansion in intron 1 of the zinc finger protein 9 gene [Science, 293:864-867(2001)]. Similar to the DM1 CUG repeat, RNA containing the expanded DM2 CCUG repeat tract accumulates in muscle nuclei. Our results clarify previous models of DM pathogenesis demonstrating that expansions in RNA can themselves be pathogenic and cause the multisystemic features common to both diseases. Future directions include generating a murine model to better understand the disease process.

Spinocerebellar Ataxias:
Another area of our research is directed towards understanding the molecular causes of ataxia. The ataxias are a group of neurodegenerative diseases that to varying degrees affect the cerebellum, brainstem, and spinocerebellar tracts. Patients lose their ability to coordinate movements causing difficulty with gait, speech and swallowing. We have collected DNA samples from patients representing 400 different ataxia kindreds with dominant, recessive, or sporadic forms of adult-onset ataxia. Among the 178 dominant kindreds represented, the ataxia for approximately 60% of the families is caused by a known CAG trinucleotide repeat expansion in one of the ataxia genes that have been identified to date. In general these CAG expansions are translated into polyglutamine tracts in the corresponding proteins. To speed the identification of additional ataxia genes we developed a novel method to clone potentially pathogenic trinucleotide repeat expansions from the genomic DNA of single affected individuals. Using our method of repeat analysis pooled isolation and detection (RAPID) cloning we isolated two novel ataxia genes for spinocerebellar ataxia types 7 and 8 (SCA7 & SCA8) [Nature Genetics 18:72-75 (1998) and Nature Genetics 21:379-384 (1999)].

Spinocerebellar ataxia type 8.
The inheritance pattern of SCA8, though generally dominant, is complicated, showing reduced penetrance and a strong maternal penetrance bias. Surprisingly, molecular analyses of this expansion revealed that, unlike other SCA CAG mutations, the SCA8 expansion is not translated as a polyglutamine tract. Rather, the expanded repeat is an untranslated CTG expansion near the 3' end of a naturally occurring antisense transcript. Myotonic dystrophy type 1 is the only other disease known to be caused by an untranslated CTG expansion.
SCA8 has the clinical features typical of spinocerebellar ataxia, whereas the untranslated CTG expansion responsible for this disease has the molecular characteristics that had previously only been seen for myotonic dystrophy. We are generating mouse models to address the pathogenesis of SCA8. One of these models has developed a progressive and lethal neurological phenotype. We are currently investigating the CNS pathology of these mice. Further clinical and molecular characterization of both DM2 and SCA8 and the correlation of these findings to those from both DM and the polyglutamine SCAs should prove to be a fruitful means of more fully understanding the pathophysiology of both ataxia and myotonic dystrophy.

Spinocerebellar ataxia type 5 (SCA5).
We are also working to identify the mutation that causes spinocerebellar ataxia type 5, whose gene I mapped to chromosome 11 a number of years ago [Nature Genetics 8:280-284]. We have recently identified and collected an additional branch of the family and through that work have identified a large number of additional patients that we plan to visit and collect this fall. We have made significant progress in recent months in narrowing the gene interval and in establishing a collaborative effort with investigators with SCA5 families from France and Australia.

Amyotrophic Lateral Sclerosis (ALS).
Amyotrophic lateral sclerosis (ALS) is a devastating rapidly fatal disease that shows a clear family history in 10% of cases. While one genetic cause of typical ALS has been identified, it is present in only 10% of familial cases and the molecular mechanisms of the change are not understood. Although there is wide agreement among ALS investigators that the field will be advanced by the identification of additional genetic mutations the rapid disease course results in families with few living affected individuals, making the genetics of these familial disorders hard to study. During the past ten years I have collected blood samples from members of a large family with dominantly-inherited ALS (the ALS-1 family). We are pursuing a novel method to improve the efficiency of linkage analysis and have recently completed a genome screen that identified several candidate regions.

Selected Publications:

Saito T, Amakusa Y, Kimura T, Yahara O, Aizawa H, Ikeda Y, Day JW, Ranum LP, Ohno K, Matsuura T. Myotonic dystrophy type 2 in Japan: ancestral origin distinct from Caucasian families. Neurogenetics. 2007 Dec 5.

Martins S, Calafell F, Gaspar C, Wong VC, Silveira I, Nicholson GA, Brunt ER, Tranebjaerg L, Stevanin G, Hsieh M, Soong BW, Loureiro L, Dürr A, Tsuji S, Watanabe M, Jardim LB, Giunti P, Riess O, Ranum LP, Brice A, Rouleau GA, Coutinho P, Amorim A, Sequeiros J. Asian origin for the worldwide-spread mutational event in Machado-Joseph disease. Arch Neurol. 2007 Oct;64(10):1502-8.

Ikeda Y, Daughters RS, Ranum LP. Bidirectional expression of the SCA8 expansion mutation: One mutation, two genes. Cerebellum. 2007 Jun 5;:1-9.

Lorenzo DN, Forrest SM, Ikeda Y, Dick KA, Ranum LP, Knight MA.   Spinocerebellar ataxia type 20 is genetically distinct from spinocerebellar ataxia type 5. Neurology. 2006 Dec 12;67(11):2084-5.

Moseley ML, Zu T, Ikeda Y, Gao W, Mosemiller AK, Daughters RS, Chen G, Weatherspoon MR, Clark HB, Ebner TJ, Day JW, Ranum LP. Bidirectional expression of CUG and CAG expansion transcripts and intranuclear polyglutamine inclusions in spinocerebellar ataxia type 8. Nat Genet. 2006 Jul;38(7):758-69.

Ranum LP, Cooper TA. RNA-mediated neuromuscular disorders. Annu Rev Neurosci. 2006;29:259-77.

Udd B, Meola G, Krahe R, Thornton C, Ranum LP, Bassez G, Kress W, Schoser B, Moxley R. 140th ENMC International Workshop: Myotonic Dystrophy DM2/PROMM and other myotonic dystrophies with guidelines on management. Neuromuscul Disord. 2006 Jun;16(6):403-13.

Margolis JM, Schoser BG, Moseley ML, Day JW, Ranum LP. DM2 intronic expansions: evidence for CCUG accumulation without flanking sequence or effects on ZNF9 mRNA processing or protein expression. Hum Mol Genet. 2006 Jun 1;15(11):1808-15.

Ikeda Y, Dick KA, Weatherspoon MR, Gincel D, Armbrust KR, Dalton JC, Stevanin G, Durr A, Zuhlke C, Burk K, Clark HB, Brice A, Rothstein JD, Schut LJ, Day JW, Ranum LP. Spectrin mutations cause spinocerebellar ataxia type 5. Nat Genet. 2006 Feb;38(2):184-90.

Trudeau MM, Dalton JC, Day JW, Ranum LP, Meisler MH. Heterozygosity for a protein truncation mutation of sodium channel SCN8A in a patient with cerebellar atrophy, ataxia, and mental retardation.
J Med Genet. 2006 Jun;43(6):527-30.

Chen DH, Cimino PJ, Ranum LP, Zoghbi HY, Yabe I, Schut L, Margolis RL, Lipe HP, Feleke A, Matsushita M, Wolff J, Morgan C, Lau D, Fernandez M, Sasaki H, Raskind WH, Bird TD. The clinical and genetic spectrum of spinocerebellar ataxia 14. Neurology. 2005 Apr 12;64(7):1258-60.

Day JW, Ranum LP. Genetics and molecular pathogenesis of the myotonic dystrophies. Curr Neurol Neurosci Rep. 2005 Feb;5(1):55-9.

Day JW, Ranum LP. RNA pathogenesis of the myotonic dystrophies. Neuromuscul Disord. 2005 Jan;15(1):5-16.

Recent Reviews:

Ranum, L.P.W., and J.W. Day. (2002) Dominant non-coding microsatellite expansion disorders. Current Opinions in Genetics and Development 12:266-271.

Ranum, L.P.W. and J.W. Day (2002) Myotonic dystrophy: Clinical and molecular parallels between DM1 and DM2. Current Neurology and Neuroscience Reports 2:465-470.

Mosemiller, A.K., J. C., Dalton, J. W. Day, L. P. W. Ranum. (2003) Molecular genetics of spinocerebellar ataxia type 8. Cytogenetic and Genome Research 100:175-83.

Ranum, L.P.W. and J. W. Day (2004) Myotonic dystrophy: RNA pathogenesis comes into focus. American Journal of Human Genetics 74:793-804.

Ranum, L.P.W. and J. W. Day (2004) Pathogenic RNA repeats and their expanding role in genetic disease. Trends in Genetics (in press).

Current Graduate Students:

Anne Mosemiller (Neuroscience, University of Minnesota).