Letter to the Editor

Coffin-Lowry Syndrome: Current Status

To the Editor:

Using a positional cloning approach we recentlyidentified in Xp22.2 the gene mutated in Coffin-Lowrysyndrome (CLS) patients. It encodes RSK2, a growth-factor-regulated serine-threonine proteine kinase[Trivier et al., 1996]. The human RSK2 cDNA has anopen reading frame of 2,220 bases which encodes a pro-tein of 740 amino acids; it is widely expressed [Bjor-baek et al., 1995]. A unique aspect of all RSK relativesis the presence of two nonidentical complete kinasecatalytic domains [Jones et al., 1988]. Our current re-sults indicate that the RSK2 transcript is organizedinto 22 exons. RSKs have been demonstrated to un-dergo activation in response to a variety of growth-promoting agents and mitogens which are implicatedin the activation of the ras-mitogene-activated protein(MAP) kinase cascade and the stimulation of cell pro-liferation (G0/G1 transition) or differentiation [Blenis,1993]. Known substrates for RSKs include the tran-scription factors encoded by immediate early genes, c-Fos, c-jun, as well as CREB [Blenis, 1993; Xing et al.,1996].

Up to now, 11 different RSK2 mutations have beenfound in CLS families [Trivier et al., 1996; Merienne etal., 1998]. All are unique and, as Figure 1 shows, arelocated throughout the protein without preference for adomain. Most of these changes (8 of 11) are predicted toresult in truncated RSK2 proteins, lacking variousparts of the C-terminus. These include two nonsenseand four splice site mutations, an intragenic deletion,and a 4-bp insertion. Western blotting with a commer-cially available anti-RSK2 antibody (Santa Cruz Bio-technology, Santa Cruz, CA), which is directed againstthe C-terminus of RSK2, was used to confirm trunca-tion in six patients [Trivier et al., 1996; Merienne et al.,1998]. Our preliminary results show that the extent ofprotein truncation does not determine the phenotype,since all of these mutations, including the 4-bp inser-

tion at the very 38 end, have been identified in patientswith very similar degrees of severe mental retardationand skeletal changes. The truncated proteins are pre-sumably unstable, resulting in a null allele, but thiscannot be assessed currently, no antibody toward theN-terminus being available.

To date, only three missense mutations have beenfound, all predicted to have detrimental effects on thetransduction function of the RSK2 protein. Threonine231 and serine 227 (mutated to isoleucine and alanine,respectively) are thought to be phosphorylation sites ofthe N-terminus kinase domain, critical for catalyticfunction. Glycine 75 (mutated to valine) is locatedwithin the putative ATP-binding site of the NH2-terminal kinase domain and plays an important role inATP binding. Indeed, two of these missense mutations(S227A and T231I) were shown to have a dramaticallyimpaired phosphotransferase activity when tested inan in vitro S6 kinase assay [Trivier et al., 1996; Meri-enne et al.,1998]. While the S227A and T231I changesare associated with a typical severe phenotype, the pa-tient bearing the G75V mutation appears only mildlymentally retarded (but with severe skeletal changes).Further examination of genotype/phenotype relation-ships and specific phenotypic consequences of particu-lar mutant alleles will consequently be of great inter-est. In four sporadic cases, for which samples wereavailable from the mothers of the propositi, we coulddemonstrate that the mutation arose de novo (Fig. 2).Thus, the high proportion of sporadic cases might in-deed reflect a high rate of new mutations in the RSK2gene. Since germline mosaicism could not be excluded,a prenatal diagnosis was performed in a male fetus inone of these four families. He did not carry the muta-tion (Fig. 2).

A common clinical problem is the need to decidewhether an isolated case of mental retardation, espe-cially when it is a young child, is due to Coffin-Lowrysyndrome. The widespread dispersion of mutationsthroughout the RSK2 gene together with the complexintron exon gene structure does not make geneticscreening easy. Thus, since most of RSK2 leads topremature termination of translation and/or to lossof phosphotransferase activity, we have been evaluat-ing immunoblot and RSK2 kinase assays as rapid(one day) and simple diagnostic tests for CLS (proto-cols described in Trivier et al. [1996]). Immunoblotanalysis allowed us recently to confirm the clinical di-agnosis of Coffin-Lowry syndrome in 6 patients out of13, and on the basis of the kinase assay in one addi-

Contract grant sponsor: Commission of the European Commu-nities; Contract grant sponsor: Ministere de la Recherche Fran-caise; Contract grant sponsor: Association Francaise contre lesMyopathies; Contract grant sponsor: CNRS; Contract grant spon-sor: INSERM; Contract grant sponsor: Hopital Universitaire deStrasbourg.

*Correspondence to: Andre Hanauer, Ph.D., I.G.B.M.C., B.P.163, F-67404 Illkirch Cedex, France.

Received 22 August 1997; Accepted 9 March 1999

American Journal of Medical Genetics 85:214–215 (1999)

© 1999 Wiley-Liss, Inc.

tional patient. Mutations in the RSK2 gene were sub-sequently found in all seven patients [Merienne et al.,1998].

RSK2 is the first member of the RSK family to beimplicated in causing a genetic disease. Therefore, thestudy of RSK2 in the context of its normal function andin the CLS phenotype should provide important infor-mation about the roles RSK proteins play in develop-ment.

NOTE ADDED IN PROOF

Since acceptance of this letter the RSK2 gene struc-ture and additional mutations have been reported inJacquot et al. [1998].

ACKNOWLEDGMENTS

This work was supported by grants from the Com-mission of the European Communities, the Ministere

de la Recherche Francaise, the Association Francaisecontre les Myopathies, the CNRS, the INSERM, andthe Hopital Universitaire de Strasbourg.

REFERENCES

Bjorbaek C, Vik TA, Echwald SM, Yang PY, Vestergaard H, Wang JP,Webb GC, Richmond K, Hansen T, Erikson RL, Gabor Miklos GL,Cohen PTW, Pederson O. 1995. Cloning of a human insulin-stimulatedprotein kinase (ISPK-1) gene and analysis of coding regions and mRNAlevels of the ISPK-1 and the protein phosphatase-1 genes in musclefrom NIDDM patients. Diabetes 44:90–97.

Blenis J. 1993. Signal transduction via the MAP kinases: Proceed at yourown RSK. Proc Natl Acad Sci USA 90:5889–5892.

Jacquot S, Merienne K, De Cesare D, Pannetier S, Mandel JL, Sassone-Corsi P, Hanauer A. 1998. Mutation analysis of the RSK2 gene inCoffin-Lowry patients: Extensive allelic heterogeneity and a high rateof de novo mutations. Am J Hum Genet 63:1631–1640.

Jones SW, Erikson E, Blenis J, Maller J, Erikson RL. 1988. A Xenopusribosomal protein S6 kinase has two apparent kinase domains that areeach similar to distinct protein kinases. Proc Natl Acad Sci USA 85:3377–3381.

Merienne K, Jacquot S, Trivier E, Pannetier S, Rossi A, Scott C, SchinzelA, Castelan C, Kress W, Hanauer A. 1998. Rapid immunoblot andkinase assay tests for a syndromal form of X-linked mental retardation:Coffin-Lowry syndrome. J Med Genet 35:890–894.

Trivier E, De Cesare D, Jacquot S, Pannetier S, Zackai E, Young I, MandelJL, Sassone-Corsi P, Hanauer A. 1996. Mutations in the kinase Rsk-2associated with Coffin-Lowry syndrome. Nature 384:567–570.

Xing J, Ginty DD, Greenberg ME. 1996. Coupling of the RAS-MAPK path-way to gene activation by RSK2, a growth-factor-regulated CREB ki-nase. Science 273:959–963.

Sylvie JacquotKarine MerienneElisabeth TrivierMaria ZeniouSolange PannetierAndre Hanauer*Institut de Genetique et de Biologie Moleculaire

et CellulaireCNRS/INSERM/ULPIllkirch, France

Fig. 1. Schematic representations of the RSK2 protein showing the localization of 11 mutations so far detected in Coffin-Lowry syndrome patients.

Fig. 2. SSCP analysis in three CLS sporadic case families with identi-fied RSK2 mutations. A: 326-1G→C. B: 2104+GTGC. C: 487-1G→A. Prim-ers and conditions used for RT-PCR amplification and SSCP analysis havebeen described previously [Trivier et al., 1996]. The three mothers and thefetus in the third family had only normal bands, demonstrating that theydo not carry the mutation of the propositus.

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