Letters to the Editor 395 pregnancies would result in the identification, each year, is known about the mutant protein, the range of muta- of about 300,000 couples in which one or both part- tions possible, and the new mutation rate before popu- ners is a likely CF carrier. Who will be responsible for lation screening to identify CF carriers is attempted. counseling couples in advance of the testing, and who It is also necessary that the questions raised by the pos- will interpret the results and discuss further testing op- sibility of general population screening for any genetic tions for such couples? Does one limit counseling post- disorder- including plans for the delivery of counsel- testing to couples in which both partners are identified ing, guidelines for counseling, quality control for test- as carriers of the defined mutation, or should all cou- ing, levels of reimbursement for testing, and medical ples in which one partner is identified as a likely carrier liability- be discussed in the genetics community be- be offered amniocentesis for microvillar enzyme analy- fore that possibility becomes a reality. sis (an approach to CF prenatal diagnosis with its own FRED GILBERT false-positive and false-negative rates [Mulivor et al. Genetics 1987]). Is it sufficient to screen couples for the pres- Cornell University Medical College ence/absence of the defined mutation, or should the New York study be extended to include probes for closely linked markers? Given that only some 70% of CF chromo- somes are identifiable by assaying for the defined mu- References tation (Kerem et al. 1989), only some 44% of CF- Beaudet AL, Feldman GL, Fernbach SD, Buffone GJ, O'Brien homozygous affecteds would be prenatally detectable WE (1989) Linkage disequilibrium, cystic fibrosis, and using this approach (Stewart 1989). Over one-half of genetic counseling. Am J Hum Genet 44:319-326 all CF affecteds would, therefore, be missed using this Cutting GR, Antonarakis SE, Buetow KH, Kasch LM, Rosen- approach alone for prenatal diagnosis (adding microvil- stein BJ, Kazazian HH Jr (1989) Analysis of DNA poly- lar enzyme analysis to prenatal testing would increase morphism haplotypes linked to the cystic fibrosis locus in the detection rate of affecteds to 90% or greater). How North American black and Caucasian families supports will liability for false negatives (missed affecteds) be the existence of multiple mutations of the cystic fibrosis handled? Can the cost to patients and medical insurers gene. Am J Hum Genet 44:307-318 for all ofthe testing (including amniocentesis) be justified? Estivill X, Farrall M, Williamson R, Ferrari M, Seia M, Guinta The opportunity to immediately offer population AM, Novelli G, et al (1988) Linkage disequilibrium be- tween cystic fibrosis and linked DNA polymorphisms in screening, via probes specific for the common muta- Italian familes: a collaborative study. Am J Hum Genet tion and other linked markers, to identify a significant 43:23-28 fraction of CF carriers and CF affecteds in utero, must Goodfellow P (1989) Steady steps to the gene. Nature 341: be balanced against the logistical problems of the test- 102-103 ing itself (screening millions with the gene probes, Kerem B-S, Rommens J, Duchanan H, Markiewicz D, Cox hundreds of thousands with other linked probes, and T, Chakravarti A, Buchwald M, et al (1989) Identification performing perhaps hundreds of thousands of amnio- of the cystic fibrosis gene: genetic analysis. Science 245: centeses for microvillar enzyme analysis), the compli- 1073-1080 cated counseling required (for which available person- Mulivor RA, Cook D, Muller F, Boue A, Gilbert F, Menutti nel may be insufficient), and the reality of an appreciable M, Pergament E, et al (1987) Analysis of fetal intestinal false-negative rate. Might not the whole cumbersome enzymes in amniotic fluid for the prenatal diagnosis ofcys- tic fibrosis. Am J Hum Genet 40:131-146 process so turn off patients and obstetricians that even Stewart A (1989) Screening for cystic fibrosis. Nature 341:696 when all CF mutations are characterized and the predic- tive accuracy of the testing approaches 100%, interest in population screening for CF will have dissipated? And a bad experience with CF testing may deter at- tempts in the immediate future to introduce popula- Am. J. Hum. Genet. 46:395-396, 1990 tion testing for other disorders. It would seem most appropriate at this time to limit Rapid Nonradioactive Detection of the Major testing to determine CF status to those with a family Cystic Fibrosis Mutation history of CF (with or without a living CF-affected rel- ative for comparison) and to the spouses of CF carriers To the Editor: or affecteds. It would be prudent to wait until more The gene for cystic fibrosis (CF) has recently been 396 Letters to the Editor Ml 1 2 3 4 5 6 M2 tween a fetus that is homozygous for the deletion and one that is homozygous without the deletion. Such a distinction is critical in prenatal diagnosis. To differen- tiate the two opposite genotypes, PCR product of the normal sequence may be added to each ofthe test sam- ples after the reaction. If mutant sequence exists in the Het DNA sample, heteroduplexes should form after heat denturation and renaturation (fig. lb). These procedures, therefore, eliminate the need for M radioactive labeling or any enzyme-linked immuno- i ~N N/YM reaction in the detection ofthe 3-bp deletion in CF car- riers and patients and reduce the amount of time re- quired for an accurate diagnosis. Figure I Analysis ofPCR product by polyacrylamide-gel elec- trophoresis. DNA was extracted from the blood ofindividuals known JOHANNA ROMMENS,* BAT-SHEVA KEREM,* to be heterozygous (lanes 1 and 4) and homozygous (lanes 2 and WENDA GREER,* PATRICIA CHANG,§ 5) for the 3-bp deletion (AF508) located within the CF gene and AND from an individual homozygous for the normal allele (lanes 3 and LAP-CHEE TsuI,*'t PETER RAY* 6). The region surrounding the deletion was amplified using condi- *Department of Genetics, Hospital for Sick tions and oligonucleotide primers described elsewhere (Kerem et al. Children, and tDepartments of Medical 1989). a, PCR products examined directly on a 20-cm 12% high- Genetics and Medical Biophysics, resolution polyacrylamide gel in TBE. b, PCR products mixed with University of Toronto, Toronto; an equivalent amount of material derived by amplifying DNA of a and known noncarrier of the mutation (lane 5). Following denaturation §Department of Pediatrics, (5 min at 940C) and reannealing (5 min at 650C) the DNA was ex- McMaster University, amined on an 8-cm 12% polyacrylamide minigel run in TBE at 150 Hamilton, Ontario V for 20 min. References Kerem B, Rommens JM, Buchanan J, Markiewicz D, Cox identified (Rommens et al. 1989). Analysis of the CF T, Chakravarti A, Buchwald M, et al (1989) Identification patient population indicated that the major mutation of the cystic fibrosis gene: genetic analysis. Science 245: corresponds to a 3-bp deletion (AF508), although the 1073-1080 frequency ofthis mutation varies in different geographic Nagamine CM, Chan K, Lau Y-F (1989). A PCR artifact: populations (Kerem et al. 1989). An immediate appli- generation of heteroduplexes. Am J Hum Genet 45: cation of the knowledge about the mutation is in the 337-339 area ofgenetic diagnosis at the DNA level. The specific RommensJM, lannuzzi MC, Kerem B, Drumm ML, Melmer DNA sequence alteration could be easily detected by G, Dean M, Rozmahel R, et al (1989) Identification of the sequence-specific oligonucleotide hybridization against cystic fibrosis gene: chromosome walking and jumping. DNA samples amplified by the polymerase chain reac- Science 245:1059-1065 tion (PCR) (Kerem et al. 1989). We describe here a sim- ple procedure which is based on the electrophoretic separation ofthe normal and mutant sequences on poly- acrylamide gels and on the formation ofheteroduplexes Am. J. Hum. Genet. 46:396-397, 1990 between these two sequences (Nagamine et al. 1989). As shown in figure la, the PCR products of the nor- mal and mutant alleles are clearly distinguishable from each other in samples prepared from heterozygous in- Histograms: Multimodal or Poorly Constructed? dividuals. In addition, a heteroduplex between the nor- mal and mutant sequences is detected as extra bands To the Editor: of slower mobility in this gel system, providing a con- Graphical displays are not only indispensable tools for venient test for identifying heterozygous individuals. the mere presentation or summarization of data but However, because of the small separation of mutant also are useful during the initial stages of the analysis and normal fragments, it is difficult to discriminate be- of phenotype distributions. Biometrical geneticists in-.