Letter to the Editor

Long Runs of Adenines and Human Mutations

To the Editor:

In their paper ‘‘Familial colorectal cancer in Ashken-azim due to a hypermutable tract in APC,’’ Laken et al.[1997] characterized a novel APC gene mutation caus-ing familial adenomatous polyposis. This mutation wasa T to A transversion in exon 15 of the APC gene thatgenerated a stretch of eight consecutive adenines (As).The stretch of eight consecutive As created a ‘‘hot spot’’for subsequent somatic mutations. One-quarter of thetumors had nine consecutive As (rather than the ex-pected eight) within the genomic DNA, a +1 frameshiftthat yields a truncated, dysfunctional APC protein(Fig. 1a). Thus, slippage during DNA replication—resulting in the insertion of an extra adenine into thestretch of eight consecutive As—was a relatively com-mon cause of tumors. This work was heralded as alandmark study in a recent editorial commentary inScience [Editorial, 1997]. We would like to point outthat slippage along a long stretch of As can occur dur-ing gene transcription as well.

Linton et al. [1992] recently analyzed a case of famil-ial hypobetalipoproteinemia caused by the deletion of asingle cytosine within an exon of the apolipoprotein Bgene. This mutation yielded a stretch of eight consecu-tive As and resulted in the synthesis of a truncatedapo-B protein (Fig. 1b). However, a major product ofthe mutant allele was actually a full-length apo-B pro-tein. Of note, somatic DNA mutations at the stretch ofeight consecutive As were not detectable. However, ap-proximately 11% of the apo-B transcripts containednine consecutive As at the site of mutation, rather thaneight. Thus, transcriptional slippage by RNA polymer-ase added an extra adenine within the stretch of eightconsecutive As, ‘‘correcting’’ the −1 frameshift muta-tion in the genomic DNA and accounting for the pro-duction of the full-length apo-B protein. Transcrip-tional slippage at long stretches of As also has beendocumented very convincingly in Escherichia coli[Wagner et al., 1990].

The studies by Laken et al. [1997] and Linton et al.[1992] indicate that long stretches of As are ‘‘slippery’’sequences, both for DNA replication and for transcrip-

tion. One might therefore predict that evolution wouldselect against long stretches of As within protein-coding sequences. Long stretches of As within proteinswould code for lysines. In human proteins, AAA codesfor 40.6% of the lysines, while the remainder (59.4%)are specified by AAG. We have analyzed the nucleotidesequences coding for 150 Lys-Lys-Lys motifs in humanproteins [Linton et al., 1997]. Lys-Lys-Lys was encodedby sequences with short (two to five) stretches of As ata higher-than-predicted frequency and by sequenceswith eight or nine consecutive As at a much lower-than-predicted frequency (P <0.004 by x2). Thus, it ap-pears that evolution has avoided slippery runs of Aswithin protein-coding sequences. In the case of tumorsuppressor genes, such as APC, the absence of longstretches of As may help to prevent cancer.

REFERENCES

Editorial (1997): New kind of cancer mutation found. Science 277:1201–1202.

Laken SJ, Petersen GM, Gruber SB, Oddoux C, Ostrer H, Giardiello FM,Hamilton SR, Hampel H, Markowitz A, Klimstra D, Jhanwar S,Winawer S, Offit K, Luce MC, Kinzler KW, Vogelstein B (1997): Fa-milial colorectal cancer in Ashkenazim due to a hypermutable tract inAPC. Nature Genet 17:79–83.

Linton MF, Pierotti V, Young SG (1992): Reading-frame restoration withan apolipoprotein B gene frameshift mutation. Proc Natl Acad Sci USA89:11431–11435.

*Correspondence to: Dr. Martin Raabe, The Gladstone Insti-tute of Cardiovascular Disease, P.O. Box 419100, San Francisco,CA 94141-9100. E-mail: martin raabe@quickmail.ucsf.edu

Received 29 October 1997; Accepted 7 November 1997

Fig. 1. Addition of an extra adenine to a stretch of eight consecutiveadenines can occur during DNA replication or gene transcription. a: In theAPC mutation characterized by Laken et al. [1997], a T to A transversionwithin exon 15 created a stretch of eight consecutive adenines, which rep-resented a hot spot for subsequent somatic mutations. The most frequentsomatic mutation, an insertion of an extra adenine, is caused by slippageduring DNA replication. b: Linton et al. [1992] analyzed an APOB muta-tion associated with eight consecutive adenines and found that an extraadenine was occasionally be added during gene transcription. The additionof an extra adenine, whether it occurs during DNA replication or genetranscription, creates a frameshift. We suspect that both slippage mecha-nisms potentially apply whenever eight or more consecutive adenines occurwithin protein-coding sequences.

American Journal of Medical Genetics 76:101–102 (1998)

© 1998 Wiley-Liss, Inc.

Linton MF, Raabe M, Pierotti V, Young SG (1997): Reading-frame resto-ration by transcriptional slippage at long stretches of adenine residuesin mammalian cells. J Biol Chem 272:14127–14132.

Wagner LA, Weiss RB, Driscoll R, Dunn DS, Gesteland RF (1990): Tran-scriptional slippage occurs during elongation at runs of adenine orthymine in Escherichia coli. Nucl Acids Res 18:3529–3535.

Martin Raabe*The Gladstone Institute of

Cardiovascular DiseaseSan Francisco, CaliforniaCardiovascular Research InstituteUniversity of CaliforniaSan Francisco, California

MacRae F. LintonDepartments of Medicine

and PharmacologyVanderbilt University School of MedicineNashville, Tennessee

Stephen G. YoungThe Gladstone Institute of

Cardiovascular DiseaseSan Francisco, CaliforniaDepartment of MedicineCardiovascular Research InstituteUniversity of CaliforniaSan Francisco, California

102 Raabe et al.