- Email: [email protected]
Post-zygotic mutations and somatic mosaicism in androgen insensitivity syndrome
News & Comment
population, the analysis was repeated using individual pyramidal-like neurons. A small GTP-binding protein involved in vesicular trafficking, Rab3A, was significantly induced upon BDNF treatment both at the mRNA and the protein levels. Mice lacking Rab3A showed no increase in synaptic charge in response to BDNF at early time points. Because BDNF is involved in learning and memory, this study advances the genomics of memory formation at the level of single neurons. It is expected that dozens of genes critical for memory will be discovered using similar techniques, providing strategies against memory deficits. (Thakker-Varia, S. et al. [2001] J. Neurosci. 21, 6782–6790) AP
Picking your SNPs Single nucleotide polymorphisms (SNPs) allow researchers to generate a wealth of data when examining genetic variation and mapping the location of disease causing genes. A novel approach to SNPs analysis has been devised by Japanese researchers, Hangil Chang and Toshiro Fujita. They have constructed a catalog of SNPs where the polymorphism changes the amino acid sequence (called nonsynonymous or nsSNPs) and is therefore more likely to change the function of the protein in question. Chang and Fujita extracted 1 190 295 SNPs from the public databases. Of these, 3793 were found to be nsSNPs and could be classified into 1247 categories based on protein function. A search of protein domains and functional sites in the SwissProt database revealed that 495 annotations contained nsSNPs, including 38 in transmembrane regions. This public database can be accessed at http://picsnp.org. (Chang, H. and Fujita, T. [2001] Biochem. Biophys. Res. Commun. 287, 288–291) SG
TRENDS in Genetics Vol.17 No.11 November 2001
Genome sequencing in a day Billions of basepairs of DNA can be sequenced in hours using a nanopore technology being developed in a collaboration between Harvard University and Agilent Technologies Inc. The device is composed of a silicon nitride membrane with two-nm pores that allow long strands of DNA to pass through at 1000 bases per second. As the DNA strand goes through the membrane, subtle changes in electrical current can be measured that are related to the composition of the DNA. A prototype membrane allowed a DNA strand of 30 000 basepairs to pass through. Although this device cannot differentiate individual nucleotides yet, Daniel Branton, leader of the Harvard team is confident that this could be achieved within five years. If successfully completed, such a device could perhaps be used to provide a complete genomic profile to a doctor within a matter of hours – allowing a complete diagnosis or determining a patient’s susceptibility to drug side effects, for instance. (http://www.business2.com/articles/mag/ print/0,1643,16900,00.html) AP
A novel epilepsy gene Clues to the causes of epilepsy have come from work on Frings mice – spontaneously occurring mutants that are susceptible to seizures triggered by loud noises. Unlike many rodent models of audiogenic-seizure susceptibility (AGS) which are polygenic, recent work by Skradski and colleagues has revealed that the Frings phenotype is due to the autosomal recessive inheritance of a single gene, Mass1. Mutant mice are homozygous for a single basepair deletion in the Mass1 gene, resulting in premature termination of the encoded
627
protein product. This novel protein product provides a new mechanism for the cause of epilepsy. The MASS1 protein is likely to have a role in the metabolism of iron or other metals, oxidative stress and/or neurotransmitter processing, whereas to date most AGS genes have encoded ion channels. (Skradski, S.L. et al. [2001] Neuron 31, 537–544) AI
Eat less, live more Religious fasting might have positive health implications too. A study conducted by Stephen Spindler and colleagues at the University of California at Riverside compared the effects of caloric restriction on the expression patterns of more than 11 000 liver genes from young and old mice. They found that aging was accompanied by changes in expression of genes that are normally involved in processes such as inflammation and stress. These microarray analyses showed 46 genes to be differentially regulated during aging. However, both short-term and long-term caloric restriction reverted agerelated alterations in gene expression. Caloric restriction is the only intervention believed to prolong longevity in mammals, and these findings are important because they indicate that young and old states can be profiled at the genomic level. (Cao, S.X. et al. (2001) Proc. Natl. Acad. Sci. U. S. A. 98, 10630–10635) AP Steven G. Gray [email protected] Anthony Isles [email protected] Petros Ligoxygakis [email protected] Akhilesh Pandey [email protected]
Letters
Post-zygotic mutations and somatic mosaicism in androgen insensitivity syndrome In their recent review article, Gottlieb et al.1 discussed somatic mutations in various genetic diseases and their potential influence on the phenotype of http://tig.trends.com
affected individuals. In particular, the authors focused on somatic mosaicism in the X-linked androgen insensitivity syndrome (AIS) associated with mutations in the androgen receptor gene. However, crucial genetic facts concerning the discussed cases were either misinterpreted or cited incorrectly. This, in turn, might have serious consequences for genetic counseling and potential treatment of affected individuals and their families. The most obvious misinterpretation of our data is that Gottlieb et al. stated
somatic mosaicism in AIS would have originated from an initially inherited mutation being mutated back to ‘normal’. However, in the cited article of ours, we had no experimental evidence for such a reverse mutation2. In addition, we are not aware of any documented case in AIS where this mechanism has been suggested. For proof, the identification of an androgen receptor gene mutation in the mother’s germline and both the wild-type and the mutant allele in the affected child would be necessary. In fact, the essential
0168-9525/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.
628
News & Comment
TRENDS in Genetics Vol.17 No.11 November 2001
genetic conditions of somatic mosaicism in AIS in all cases we have investigated so far is the occurrence of a de novo mutation of the androgen receptor gene in the patient associated with the absence of a genetic alteration in the mother3,4. This is a very important difference to the statements of Gottlieb et al. In the case of a hypothetical reverse-mutation to the normal state, there would be a 50% chance for another XY child of the same mother to have AIS. In the case of a postzygotic de novo mutation, however, there is no such increased risk. Only if a de novo mutation occurred in the mother’s germline, would an increased risk for another offspring with AIS arise5. Interestingly, the occurrence of somatic mutations in AIS is not an unexpected event, as the seeming instability of X-chromosomal genes has been suspected for decades6. Indeed, the involvement of genetic instability in AIS was proposed eight years ago7 and was later proved by functional studies2,8. The clinical pattern in AIS of a discrepancy between the genotype and the phenotype should always lead to the suspicion of somatic mosaicism in a single-case family. Because somatic mosaicism in AIS is not exclusively associated with such discrepancies, genetic investigation of all single-case families is recommended. In those cases with de novo mutations of the androgen receptor gene, somatic mosaicism should be excluded3. On the molecular genetic level, the diagnosis of somatic mosaicism is based on the exclusion of a mutation in the mother in X-linked diseases and evidence of two different genomic DNA populations in the patients’ genomic DNA, preferably examined in more than one tissue (e.g. blood leukocytes and genital skin fibroblasts). Some genetic conditions in mosaic AIS patients offer the opportunity to distinguish two different androgen receptor protein populations by functional approaches2,8.
Albert O. Brinkmann Dept of Endocrinology and Reproduction, Erasmus University, Rotterdam, The Netherlands.
Paul-Martin Holterhus Dept of Pediatrics, Medical University of Lübeck, Ratzenberger Allee 160, 23538 Lübeck, Germany. e-mail: [email protected]
Holterhus et al. (this issue) correctly point out that we did indeed misinterpret part of their results concerning the possibility that some cases of androgen insensitivity syndrome (AIS) could be due to somatic mosaicism caused by the back mutation of mutant androgen receptors to wild-type in some androgen responsive tissues. However, although back mutations have
Hennie T. Brüggenwirth Dept of Ophthalmology, Erasmus University, Rotterdam, The Netherlands. http://tig.trends.com
Olaf Hiort Department of Pediatrics, Medical University of Lübeck, Germany. References 1 Gottlieb, B. et al. (2001) Somatic mosaicism and variable expressivity. Trends Genet. 17, 79–82 2 Holterhus, P.M. et al. (1997) Mosaicism due to a somatic mutation of the androgen receptor gene determines phenotype in androgen insensitivity syndrome. J. Clin. Endocrinol. Metab. 82, 3584–3589 3 Hiort, O. et al. (1998) Inherited and de novo androgen receptor gene mutations: investigation of single-case families. J. Pediatr. 132, 939–943 4 Holterhus, P.M. et al. (1999) Clinical and molecular spectrum of somatic mosaicism in androgen insensitivity syndrome. Pediatr. Res. 46, 684–690 5 Boehmer, A.L. et al. (1997) Germ-line and somatic mosaicism in the androgen insensitivity syndrome: implications for genetic counseling. Am. J. Hum. Genet. 60, 1003–1006 6 Haldane, J.B.S. (1935) The rate of spontaneous mutation of a human gene. J. Genet. 31, 317–326 7 Hiort, O. et al. (1993) Single strand conformation polymorphism analysis of androgen receptor gene mutations in patients with androgen insensitivity syndromes: application for diagnosis, genetic counseling, and therapy. J. Clin. Endocrinol. Metab. 77, 262–266 8 Holterhus, P.M. et al. (1999) Expression of two functionally different androgen receptors in a patient with androgen insensitivity. Eur. J. Pediatr. 158, 702–706
Post-zygotic mutations and somatic mosaicism in androgen insensitivity syndrome Response from Bruce Gottlieb, Lenore K. Beitel and Mark Trifiro
not yet been proven to be a source of somatic mosaicism in cases of AIS, there are a number of other cases in the literature where back mutations do indeed result in somatic mosaicism1–3. We are also grateful that they chose to reinforce the importance of such information for genetic counseling, particularly because the occurrence of somatic mosaicism has recently been shown to be more than just an isolated series of events in a number of genetic disorders. In these conditions, which include hemophilia A (Ref. 4), hemophilia B (Ref. 5), retinoblastoma6 and tuberous sclerosis complex7, somatic mosaicism can occur at a frequency of 10–20%. Furthermore, these studies also note that somatic mosaicism is a possible source not only of variable expressivity, but also of altered penetrance. These observations add weight to our hypothesis that, with the recently acquired ability to sequence specific tissues easily and efficiently, somatic mosaicism is increasingly likely to be observed as a source of variable expressivity in many disorders and diseases. Bruce Gottlieb* Lenore K. Beitel Mark Trifiro Lady Davis Institute for Medical Research, 3755 Cote Ste-Catherine Road, Montreal, Quebec, Canada H3T 1E2. *e-mail: bruce.gottlieb@ mcgill.ca References 1 Wahn, V. et al. (1998) Reverse mutations – spontaneous amelioration or cure of inherited disorders? Eur. J. Pediatr. 157, 613–617 2 Jonkman, M.F. (1999) Revertant mosaicism in human genetic disorders. Am. J. Med. Genet. 85, 361–364 3 Ellis, N.A. et al. (2001) Back mutations can produce phenotype reversion in Bloom syndrome somatic cells. Hum. Genet. 108, 167–173 4 Leuer, M. et al. (2001) Somatic mosaicism in Hemophilia A: a fairly common event. Am. J. Hum. Genet. 69, 75–87 5 Ketterling, R.P. et al. (1999) Germline origins in the human F9 gene: frequent G:C→A:T mosaicism and increased mutations with advanced maternal age. Hum. Genet. 105, 629–640 6 Sippel, K.C. et al. (1998) Frequency of somatic and germ-line mosaicism in Retinoblastoma: implications for genetic counseling. Am. J. Hum. Genet. 62, 610–619 7 Verhoef, S. et al. (1999) High rate of mosaicism in tuberous sclerosis complex. Am. J. Hum. Genet. 64, 1632–1637
0168-9525/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.
population, the analysis was repeated using individual pyramidal-like neurons. A small GTP-binding protein involved in vesicular trafficking, Rab3A, was significantly induced upon BDNF treatment both at the mRNA and the protein levels. Mice lacking Rab3A showed no increase in synaptic charge in response to BDNF at early time points. Because BDNF is involved in learning and memory, this study advances the genomics of memory formation at the level of single neurons. It is expected that dozens of genes critical for memory will be discovered using similar techniques, providing strategies against memory deficits. (Thakker-Varia, S. et al. [2001] J. Neurosci. 21, 6782–6790) AP
Picking your SNPs Single nucleotide polymorphisms (SNPs) allow researchers to generate a wealth of data when examining genetic variation and mapping the location of disease causing genes. A novel approach to SNPs analysis has been devised by Japanese researchers, Hangil Chang and Toshiro Fujita. They have constructed a catalog of SNPs where the polymorphism changes the amino acid sequence (called nonsynonymous or nsSNPs) and is therefore more likely to change the function of the protein in question. Chang and Fujita extracted 1 190 295 SNPs from the public databases. Of these, 3793 were found to be nsSNPs and could be classified into 1247 categories based on protein function. A search of protein domains and functional sites in the SwissProt database revealed that 495 annotations contained nsSNPs, including 38 in transmembrane regions. This public database can be accessed at http://picsnp.org. (Chang, H. and Fujita, T. [2001] Biochem. Biophys. Res. Commun. 287, 288–291) SG
TRENDS in Genetics Vol.17 No.11 November 2001
Genome sequencing in a day Billions of basepairs of DNA can be sequenced in hours using a nanopore technology being developed in a collaboration between Harvard University and Agilent Technologies Inc. The device is composed of a silicon nitride membrane with two-nm pores that allow long strands of DNA to pass through at 1000 bases per second. As the DNA strand goes through the membrane, subtle changes in electrical current can be measured that are related to the composition of the DNA. A prototype membrane allowed a DNA strand of 30 000 basepairs to pass through. Although this device cannot differentiate individual nucleotides yet, Daniel Branton, leader of the Harvard team is confident that this could be achieved within five years. If successfully completed, such a device could perhaps be used to provide a complete genomic profile to a doctor within a matter of hours – allowing a complete diagnosis or determining a patient’s susceptibility to drug side effects, for instance. (http://www.business2.com/articles/mag/ print/0,1643,16900,00.html) AP
A novel epilepsy gene Clues to the causes of epilepsy have come from work on Frings mice – spontaneously occurring mutants that are susceptible to seizures triggered by loud noises. Unlike many rodent models of audiogenic-seizure susceptibility (AGS) which are polygenic, recent work by Skradski and colleagues has revealed that the Frings phenotype is due to the autosomal recessive inheritance of a single gene, Mass1. Mutant mice are homozygous for a single basepair deletion in the Mass1 gene, resulting in premature termination of the encoded
627
protein product. This novel protein product provides a new mechanism for the cause of epilepsy. The MASS1 protein is likely to have a role in the metabolism of iron or other metals, oxidative stress and/or neurotransmitter processing, whereas to date most AGS genes have encoded ion channels. (Skradski, S.L. et al. [2001] Neuron 31, 537–544) AI
Eat less, live more Religious fasting might have positive health implications too. A study conducted by Stephen Spindler and colleagues at the University of California at Riverside compared the effects of caloric restriction on the expression patterns of more than 11 000 liver genes from young and old mice. They found that aging was accompanied by changes in expression of genes that are normally involved in processes such as inflammation and stress. These microarray analyses showed 46 genes to be differentially regulated during aging. However, both short-term and long-term caloric restriction reverted agerelated alterations in gene expression. Caloric restriction is the only intervention believed to prolong longevity in mammals, and these findings are important because they indicate that young and old states can be profiled at the genomic level. (Cao, S.X. et al. (2001) Proc. Natl. Acad. Sci. U. S. A. 98, 10630–10635) AP Steven G. Gray [email protected] Anthony Isles [email protected] Petros Ligoxygakis [email protected] Akhilesh Pandey [email protected]
Letters
Post-zygotic mutations and somatic mosaicism in androgen insensitivity syndrome In their recent review article, Gottlieb et al.1 discussed somatic mutations in various genetic diseases and their potential influence on the phenotype of http://tig.trends.com
affected individuals. In particular, the authors focused on somatic mosaicism in the X-linked androgen insensitivity syndrome (AIS) associated with mutations in the androgen receptor gene. However, crucial genetic facts concerning the discussed cases were either misinterpreted or cited incorrectly. This, in turn, might have serious consequences for genetic counseling and potential treatment of affected individuals and their families. The most obvious misinterpretation of our data is that Gottlieb et al. stated
somatic mosaicism in AIS would have originated from an initially inherited mutation being mutated back to ‘normal’. However, in the cited article of ours, we had no experimental evidence for such a reverse mutation2. In addition, we are not aware of any documented case in AIS where this mechanism has been suggested. For proof, the identification of an androgen receptor gene mutation in the mother’s germline and both the wild-type and the mutant allele in the affected child would be necessary. In fact, the essential
0168-9525/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.
628
News & Comment
TRENDS in Genetics Vol.17 No.11 November 2001
genetic conditions of somatic mosaicism in AIS in all cases we have investigated so far is the occurrence of a de novo mutation of the androgen receptor gene in the patient associated with the absence of a genetic alteration in the mother3,4. This is a very important difference to the statements of Gottlieb et al. In the case of a hypothetical reverse-mutation to the normal state, there would be a 50% chance for another XY child of the same mother to have AIS. In the case of a postzygotic de novo mutation, however, there is no such increased risk. Only if a de novo mutation occurred in the mother’s germline, would an increased risk for another offspring with AIS arise5. Interestingly, the occurrence of somatic mutations in AIS is not an unexpected event, as the seeming instability of X-chromosomal genes has been suspected for decades6. Indeed, the involvement of genetic instability in AIS was proposed eight years ago7 and was later proved by functional studies2,8. The clinical pattern in AIS of a discrepancy between the genotype and the phenotype should always lead to the suspicion of somatic mosaicism in a single-case family. Because somatic mosaicism in AIS is not exclusively associated with such discrepancies, genetic investigation of all single-case families is recommended. In those cases with de novo mutations of the androgen receptor gene, somatic mosaicism should be excluded3. On the molecular genetic level, the diagnosis of somatic mosaicism is based on the exclusion of a mutation in the mother in X-linked diseases and evidence of two different genomic DNA populations in the patients’ genomic DNA, preferably examined in more than one tissue (e.g. blood leukocytes and genital skin fibroblasts). Some genetic conditions in mosaic AIS patients offer the opportunity to distinguish two different androgen receptor protein populations by functional approaches2,8.
Albert O. Brinkmann Dept of Endocrinology and Reproduction, Erasmus University, Rotterdam, The Netherlands.
Paul-Martin Holterhus Dept of Pediatrics, Medical University of Lübeck, Ratzenberger Allee 160, 23538 Lübeck, Germany. e-mail: [email protected]
Holterhus et al. (this issue) correctly point out that we did indeed misinterpret part of their results concerning the possibility that some cases of androgen insensitivity syndrome (AIS) could be due to somatic mosaicism caused by the back mutation of mutant androgen receptors to wild-type in some androgen responsive tissues. However, although back mutations have
Hennie T. Brüggenwirth Dept of Ophthalmology, Erasmus University, Rotterdam, The Netherlands. http://tig.trends.com
Olaf Hiort Department of Pediatrics, Medical University of Lübeck, Germany. References 1 Gottlieb, B. et al. (2001) Somatic mosaicism and variable expressivity. Trends Genet. 17, 79–82 2 Holterhus, P.M. et al. (1997) Mosaicism due to a somatic mutation of the androgen receptor gene determines phenotype in androgen insensitivity syndrome. J. Clin. Endocrinol. Metab. 82, 3584–3589 3 Hiort, O. et al. (1998) Inherited and de novo androgen receptor gene mutations: investigation of single-case families. J. Pediatr. 132, 939–943 4 Holterhus, P.M. et al. (1999) Clinical and molecular spectrum of somatic mosaicism in androgen insensitivity syndrome. Pediatr. Res. 46, 684–690 5 Boehmer, A.L. et al. (1997) Germ-line and somatic mosaicism in the androgen insensitivity syndrome: implications for genetic counseling. Am. J. Hum. Genet. 60, 1003–1006 6 Haldane, J.B.S. (1935) The rate of spontaneous mutation of a human gene. J. Genet. 31, 317–326 7 Hiort, O. et al. (1993) Single strand conformation polymorphism analysis of androgen receptor gene mutations in patients with androgen insensitivity syndromes: application for diagnosis, genetic counseling, and therapy. J. Clin. Endocrinol. Metab. 77, 262–266 8 Holterhus, P.M. et al. (1999) Expression of two functionally different androgen receptors in a patient with androgen insensitivity. Eur. J. Pediatr. 158, 702–706
Post-zygotic mutations and somatic mosaicism in androgen insensitivity syndrome Response from Bruce Gottlieb, Lenore K. Beitel and Mark Trifiro
not yet been proven to be a source of somatic mosaicism in cases of AIS, there are a number of other cases in the literature where back mutations do indeed result in somatic mosaicism1–3. We are also grateful that they chose to reinforce the importance of such information for genetic counseling, particularly because the occurrence of somatic mosaicism has recently been shown to be more than just an isolated series of events in a number of genetic disorders. In these conditions, which include hemophilia A (Ref. 4), hemophilia B (Ref. 5), retinoblastoma6 and tuberous sclerosis complex7, somatic mosaicism can occur at a frequency of 10–20%. Furthermore, these studies also note that somatic mosaicism is a possible source not only of variable expressivity, but also of altered penetrance. These observations add weight to our hypothesis that, with the recently acquired ability to sequence specific tissues easily and efficiently, somatic mosaicism is increasingly likely to be observed as a source of variable expressivity in many disorders and diseases. Bruce Gottlieb* Lenore K. Beitel Mark Trifiro Lady Davis Institute for Medical Research, 3755 Cote Ste-Catherine Road, Montreal, Quebec, Canada H3T 1E2. *e-mail: bruce.gottlieb@ mcgill.ca References 1 Wahn, V. et al. (1998) Reverse mutations – spontaneous amelioration or cure of inherited disorders? Eur. J. Pediatr. 157, 613–617 2 Jonkman, M.F. (1999) Revertant mosaicism in human genetic disorders. Am. J. Med. Genet. 85, 361–364 3 Ellis, N.A. et al. (2001) Back mutations can produce phenotype reversion in Bloom syndrome somatic cells. Hum. Genet. 108, 167–173 4 Leuer, M. et al. (2001) Somatic mosaicism in Hemophilia A: a fairly common event. Am. J. Hum. Genet. 69, 75–87 5 Ketterling, R.P. et al. (1999) Germline origins in the human F9 gene: frequent G:C→A:T mosaicism and increased mutations with advanced maternal age. Hum. Genet. 105, 629–640 6 Sippel, K.C. et al. (1998) Frequency of somatic and germ-line mosaicism in Retinoblastoma: implications for genetic counseling. Am. J. Hum. Genet. 62, 610–619 7 Verhoef, S. et al. (1999) High rate of mosaicism in tuberous sclerosis complex. Am. J. Hum. Genet. 64, 1632–1637
0168-9525/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.