Retroviruses

Retroviruses have their genetic information encoded in RNA and replicate through DNA by coding for an enzyme known as reverse transcriptase (p. 17), which makes a double-stranded DNA copy of the viral RNA. This DNA intermediate integrates into the host cell genome, allowing the appropriate proteins to be manufactured, resulting in repackaging of new progeny virions.

Naturally occurring retroviruses have only the three genes necessary to ensure replication: gag, encoding the structural proteins for the core antigens; pol, coding for reverse transcriptase; and env, the gene for the glycoprotein envelope proteins (Figure 14.1). Study of the virus responsible for the transmissible tumor in chickens, the so-called Rous sarcoma virus, identified a fourth gene that results in transformation of cells in culture, a model for malignancy in vivo. This viral gene, which transforms the host cell, is known as an oncogene.

FIGURE 14.1 Model for acquisition of transforming ability in retroviruses. A, Normal retroviral replication. B, The Rous sarcoma virus has integrated near a cellular oncogene. The transforming ability of this virus is due to the acquired homolog of the cellular oncogene, v-src. C, A defective transforming virus carries an oncogene similar to src but is defective in the structural genes, (e.g., Moloney murine sarcoma virus, which carries mos).

Identification of Oncogenes at Chromosomal Translocation Breakpoints

Chromosome aberrations are common in malignant cells, which often show marked variation in chromosome number and structure. Certain chromosomes seemed to be more commonly involved and it was initially thought that these changes were secondary to the transformed state rather than causal. This attitude changed when evidence suggested that chromosomal structural changes, often translocations (p. 44), resulted in rearrangements within or adjacent to proto-oncogenes. It has been found that chromosomal translocations can lead to novel chimeric genes with altered biochemical function or level of proto-oncogene activity. There are numerous examples of both types, of which chronic myeloid leukemia is an example of the former and Burkitt lymphoma an example of the latter.

Chronic myeloid leukemia

In 1960, investigators in Philadelphia were the first to describe an abnormal chromosome in white blood cells from patients with chronic myeloid leukemia (CML). The abnormal chromosome, referred to as the Philadelphia, or Ph¹, chromosome, is an acquired abnormality found in blood or bone marrow cells but not in other tissues from these patients. The Ph¹ is a tiny chromosome that is now known to be a chromosome 22 from which long arm material has been reciprocally translocated to and from the long arm of chromosome 9 (Figure 14.2), i.e., t(9;22)(q34;q11). This chromosomal rearrangement is seen in 90% of those with CML. This translocation has been found to transfer the cellular ABL (Abelson) oncogene from chromosome 9 into a region of chromosome 22 known as the breakpoint cluster, or BCR, region, resulting in a chimeric transcript derived from both the c-ABL (70%) and the BCR genes. This results in a chimeric gene expressing a fusion protein consisting of the BCR protein at the amino end and ABL protein at the carboxy end, which is associated with transforming activity.

FIGURE 14.2 Karyotype from a patient with chronic myeloid leukemia showing the chromosome 22 (arrow) or Philadelphia chromosome, which has material translocated to the long arm of one of the number 9 chromosomes (arrow).

Oncogene Amplification

Proto-oncogenes can also be activated by the production of multiple copies of the gene or what is known as gene amplification, a mechanism known to have survival value when cells encounter environmental stress. For example, when leukemic cells are exposed to the chemotherapeutic agent methotrexate, the cells acquire resistance to the drug by making multiple copies of the gene for dihydrofolate reductase, the target enzyme for methotrexate.

Gene amplification can increase the number of copies of the oncogene per cell up to several hundred times, leading to greater amounts of the corresponding oncoprotein. In mammals the amplified sequence of DNA in tumor cells can be recognized by the presence of small extra chromosomes known as double-minute chromosomes or homogeneously staining regions of the chromosomes. These changes are seen in approximately 10% of tumors and are often present more commonly in the later rather than the early stages of the malignant process.

Amplification of specific proto-oncogenes appears to be a feature of certain tumors and is frequently seen with the MYC family of genes. For example, N-MYC is amplified in approximately 30% of neuroblastomas, but in advanced cases the proportion rises to 50%, where gene amplification can be up to 1000-fold. Human small cell carcinomas of the lung also show amplification of MYC, N-MYC, and L-MYC.

Amplification of ERB-B2, MYC, and cyclin D1 is a feature in 20% of breast carcinomas, where it has been suggested that it correlates with a number of well-established prognostic factors such as lymph node status, estrogen and progestogen receptor status, tumor size, and histological grade.

Detection of Oncogenes by DNA Transfection Studies

The ability of DNA from a human bladder carcinoma cell line to transform a well established mouse fibroblast cell line called NIH3T3, as demonstrated by the loss of contact inhibition of the cells in culture, or what is known as DNA transfection, led to the discovery of the human sequence homologous to the ras gene of the Harvey murine sarcoma virus. The human RAS gene family consists of three closely related members, H-RAS, K-RAS, and N-RAS. The RAS proteins are closely homologous to their viral counterparts and differ from one another only near the carboxy termini. Oncogenicity of the ras proto-oncogenes has been shown to arise by acquisition of point mutations in the nucleotide sequence. In approximately 50% of colorectal cancers and 95% of pancreatic cancers, as well as in a proportion of thyroid and lung cancers, a mutation in a ras gene can be demonstrated. The RAS gene family has been shown to be the key pathway (RAS-MAPK) in neurofibromatosis type 1 (p. 298) and the Noonan/cardio-facio-cutaneous/Costello syndromes, all of which demonstrate some increased risk of tumor formation.

DNA transfection studies have also led to identification of other oncogenes that have not been demonstrated through retroviral studies. These include MET (hereditary papillary renal cell carcinoma), TRK, MAS, and RET (multiple endocrine neoplasia type 2, see Tables 14.5, 14.9).

Growth Factors

The transition of a cell from G₀ to the start of the cell cycle (p. 39) is governed by substances called growth factors. Growth factors stimulate cells to grow by binding to growth factor receptors. The best known oncogene that acts as a growth factor is the v-SIS oncogene, which encodes part of the biologically active platelet-derived growth factor B subunit. When v-SIS oncoprotein is added to the NIH 3T3 cultures, the cells are transformed, behaving like neoplastic cells; that is, their growth rate increases and they lose contact inhibition. In vivo they form tumors when injected into nude mice. Oncogene products showing homology to fibroblast growth factors include HST and INT-2, which are amplified in stomach cancers and in malignant melanomas, respectively.

Growth Factor Receptors

Many oncogenes encode proteins that form growth factor receptors, with tyrosine kinase activity possessing tyrosine kinase domains that allow cells to bypass the normal control mechanisms. More than 40 different tyrosine kinases have been identified and can be divided into two main types: those that span the cell membrane (growth factor receptor tyrosine kinases) and those located in the cytoplasm (non-receptor tyrosine kinases). Examples of tyrosine kinases include ERB-B, which encodes the epidermal growth factor receptor, and the related ERB-B2 oncogene. Mutations, rearrangements, and amplification of the ERB-B2 oncogene result in ligand-independent activation, which has been associated with cancer of the stomach, pancreas, and ovary. Mutations in KIT occur in the hereditary gastrointestinal stromal tumor syndrome. These oncogenes are not activated by translocation (as in Burkitt lymphoma) but rather by point mutations. When germline or inherited, the mutations are not lethal, nor are they sufficient by themselves to cause carcinogenesis. In the case of MET (located on chromosome 7), the papillary renal cell carcinoma tumors are trisomic for chromosome 7 and two of the three copies of MET are mutant. A ratio of one mutant to one wild-type copy of MET is not sufficient for carcinogenesis, but a 2:1 ratio is.

Intracellular Signal Transduction Factors

Two different types of intracellular signal transduction factor have been identified, proteins with GTPase activity and cytoplasmic serine threonine kinases.

DNA-Binding Nuclear Proteins

The FOS, JUN, and ERB-A oncogenes encode proteins that are specific transcription factors that regulate gene expression by activating or suppressing nearby DNA sequences. The function of MYC and related genes remains uncertain but appears to be related to alterations in control of the cell cycle. The MYC and MYB oncoproteins stimulate cells to progress from the G₁ into the S phase of the cell cycle (p. 39). Their overproduction prevents cells from entering a prolonged resting phase, resulting in persistent cellular proliferation.

Cell-Cycle Factors

Cancer cells can increase in number by increased growth and division, or accumulate through decreased cell death. In vivo, most cells are in a non-dividing state. Progress through the cell cycle (p. 39) is regulated at two points: one in G₁ when a cell becomes committed to DNA synthesis in the S phase, and another in G₂ for cell division in the M (mitosis) phase, through factors known as cyclin-dependent kinases. Abnormalities in regulation of the cell cycle through growth factors, growth factor receptors, GTPases or nuclear proteins, or loss of inhibitory factors lead to activation of the cyclin-dependent kinases, such as cyclin D1, resulting in cellular transformation with uncontrolled cell division. Alternatively, loss of the factors that lead to normal programmed cell death, a process known as apoptosis (p. 85), can result in the accumulation of cells through prolonged cell survival as a mechanism of development of some tumors. Activation of the bcl-2 oncogene through chromosomal rearrangements is associated with inhibition of apoptosis, leading to certain types of lymphoma.

Signal Transduction and Phakomatoses

Phakomatosis derives from the Greek phakos, meaning ‘lentil’ (in this context ‘lentil-shaped object’), and originally referred to three diseases that included scattered benign lesions: neurofibromatosis, tuberous sclerosis, and von Hippel-Lindau disease. To this list has now been added nevoid basal cell carcinoma (Gorlin) syndrome, Cowden disease, familial adenomatous polyposis, Peutz-Jegher syndrome, and juvenile polyposis. The genes for all of these conditions are now known and are normally active within intracellular signal transduction, and their protein products are tumor suppressors.

‘Two-Hit’ Hypothesis

In 1971, Knudson carried out an epidemiological study of a large number of cases of both types of Rb and advanced a ‘two-hit’ hypothesis to explain the occurrence of this rare tumor in patients with and without a positive family history. He proposed that affected individuals with a positive family history had inherited one non-functional gene that was present in all cells of the individual, known as a germline mutation, with the second gene at the same locus becoming inactivated somatically in a developing retinal cell (Figure 14.5). The occurrence of a second mutation was likely given the large number of retinal cells, explaining the autosomal dominant pattern of inheritance. This would also explain the observation that in hereditary Rb the tumors were often bilateral and multifocal. In contrast, in the non-heritable or sporadic form, two inactivating somatic mutations would need to occur independently in the same retinoblast cell (see Figure 14.5), which was much less likely to occur, explaining the fact that tumors in these patients were often unilateral and unifocal, and usually occurred at a later age than in the hereditary form. Hence, although the hereditary form of Rb follows an autosomal dominant pattern of inheritance, at the molecular level it is recessive because a tumor occurs only after the loss of both alleles.

FIGURE 14.5 Retinoblastoma and Knudson’s ‘two-hit’ hypothesis. All cells in the hereditary form (A) have one mutated copy of the gene, RB1 (i.e., the mutation is in the germline). In the non-hereditary form (B) a mutation in RB1 arises as a post-zygotic (somatic) event some time early in development. The retinoblastoma tumor occurs only when both RB1 genes are mutated—i.e., after a(nother) somatic event, which is more likely to be earlier in life in the hereditary form compared with the non-hereditary form; it is also more likely to give rise to bilateral and multifocal tumors.

It was also recognized, however, that approximately 5% of children presenting with Rb had other physical abnormalities along with developmental concerns. Detailed cytogenetic analysis of blood samples from these children revealed some of them to have an interstitial deletion involving the long arm of one of their number 13 chromosome pair. Comparison of the regions deleted revealed a common ‘smallest region of overlap’ involving the sub-band 13q14 (Figure 14.6). The detection of a specific chromosomal region involved in the etiology of these cases of Rb suggested that it could also be the locus involved in the autosomal dominant familial form of Rb. Family studies using a polymorphic enzyme, esterase D, which had previously been mapped to that region, rapidly confirmed linkage of the hereditary form of Rb to that locus.

FIGURE 14.6 Two homologs of chromosome 13 from a patient with retinoblastoma showing an interstitial deletion of 13q14 in the right-hand homolog, as indicated.

Loss of Heterozygosity

Analyses of the DNA sequences in this region of chromosome 13 in the peripheral blood and in Rb tumor material from children who had inherited the gene for Rb showed them to have loss of an allele at the Rb locus in the tumor material, known as loss of heterozygosity (LOH), or sometimes as loss of constitutional heterozygosity. An example of this is shown in Figure 14.7, A, in which the mother transmits the Rb gene along with allele 2 at a closely linked marker locus. The father is homozygous for allele 1 at this same locus, with the result that the child is constitutionally an obligate heterozygote at this locus. Analysis of the tumor tissue reveals apparent homozygosity for allele 2. In fact, there has been loss of the paternally derived allele 1 (i.e., LOH in the tumor material). This LOH is consistent with the ‘two-hit’ hypothesis leading to development of the malignancy as proposed by Knudson.

FIGURE 14.7 A, Diagrammatic representation of the loss of heterozygosity (LOH) in the development of a tumor. The mother (M) and father (F) are both homozygous for different alleles at the same locus, 2–2 and 1–1, respectively. The child (C) will therefore be constitutionally heterozygous, 1–2. If an analysis of DNA from a tumor at that locus reveals only a single allele, 2, this is consistent with LOH. B, Diagrammatic representations of the mechanisms by causing the ‘second hit’ leading to the development of retinoblastoma.

LOH can occur through several mechanisms, which include loss of a chromosome through mitotic non-disjunction (p. 43), a deletion on the chromosome carrying the corresponding allele, or a crossover between the two homologous genes leading to homozygosity for the mutant allele (Figure 14.7, B). Observation of consistent cytogenetic rearrangements in other malignancies has led to demonstration of LOH in a number of other cancers (Table 14.2). Subsequent to the observation of LOH, linkage studies of familial cases can be carried out to determine whether the familial cases of a specific type of malignancy are due to mutations at the same locus and thus lead to the identification of the gene responsible, as occurred with the isolation of the RB1 gene.

Table 14.2 Syndromes and Cancers that Show Loss of Heterozygosity and Their Chromosomal Location

Function of Tumor Suppressor Genes

Although familial Rb was classically considered to be an autosomal dominant trait, demonstration of the action of the Rb gene as a tumor suppressor gene is consistent with it being a recessive trait, as originally suggested in the somatic cell hybridization studies carried out by Harris and colleagues. In other words, absence of the gene product in the homozygous state leads to the development of this particular tumor. In contrast to oncogenes, tumor suppressor genes are a class of cellular genes whose normal function is to suppress inappropriate cell proliferation (i.e., the development of a malignancy is due to a loss of function mutation, see p. 26).

The tumor suppressor activity of the Rb gene has been demonstrated in vitro in cancer cells. In addition, further support for the RB1 gene acting as a tumor suppressor gene comes from the recognition that individuals with the hereditary form of Rb have an increased risk of developing second new malignancies later in life, including osteosarcoma, fibrosarcoma, and chondrosarcoma.

The RB1 Gene/p110^RB Protein

The RB1 gene specifies a 4.7-kilobase (kb) transcript that encodes a nuclear protein called p110^RB, which associates with DNA and is involved in the regulation of the cell cycle. Fortuitously, research on the mechanism of action of the E1A oncogene of human adenovirus demonstrated that p110^RB forms a complex with E2F-1, which is an E1A oncogene-regulated inhibitor of the transcription factor E2F. The complex so formed interferes with the ability of E2F to activate transcription of some key proteins required for DNA synthesis. When p110^RB is in a hyperphosphorylated state, it does not interact readily with E2F-1, so permitting the cell cycle to proceed into the S phase (p. 39). Retinoblasts fail to differentiate normally in the presence of mutant p110^RB.

p110^RB interacts with several viral oncoproteins, such as the transforming proteins of simian virus (SV) 40 (large T antigen) and papilloma virus (E7 protein), and is inactivated, thereby liberating cells from normal growth constraints. These findings yield insight into the mechanisms of interaction between oncogenes, tumor suppressor genes, and the cell cycle.

Li-Fraumeni Syndrome

Because mutations in TP53 appear to be a common event in the genesis of many cancers, an inherited or germline mutation of TP53 would be expected to have serious consequences. This hypothesis was substantiated with the discovery of such a defect in persons with Li-Fraumeni syndrome. Members of families with this rare syndrome (p. 225), which is inherited as an autosomal dominant trait, are highly susceptible to developing a variety of malignancies at an early age, including sarcomas, adrenal carcinomas, and breast cancer. Point mutations in highly conserved regions of the TP53 gene (codons 245 to 258) have been identified in the germline of family members, with analysis of the tumor revealing loss of the normal allele.

Multistage Process of Carcinogenesis

The majority of colorectal cancers are thought to develop from ‘benign’ adenomas. Conversely, only a small proportion of adenomas proceed to invasive cancer. Histologically, adenomatous polyps smaller than 1 cm in diameter rarely contain areas of carcinomatous change, whereas the risk of carcinomatous change increases to 5% to 10% when an adenoma reaches 2 cm in diameter. The transition from a small adenomatous polyp to an invasive cancer is thought to take between 5 and 10 years. Adenomatous polyps less than 1 cm in diameter have mutations in the ras gene in less than 10% of cases. As the size of the polyp increases to between 1 and 2 cm, the prevalence of ras gene mutations is in the region of 40%, rising to approximately 50% in full-blown colorectal cancers.

Similarly, allele loss of chromosome 5 markers occurs in approximately 40% of adenomatous polyps and 70% of carcinomas. Deletions on chromosome 17p in the region containing the TP53 gene occur in more than 75% of carcinomas, but this is an uncommon finding in small or intermediate-sized polyps. A region on 18q is deleted in about 10% of small adenomas, rising to almost 50% when the adenoma shows foci of invasive carcinoma, and in more than 70% of carcinomas (Figure 14.10). Genes at this locus include DCC (deleted in colorectal cancer), SMAD2, and SMAD4, the latter being part of the transforming growth factor-β (TGF-β) pathway. In some colorectal cancers mutations in the TGF-β receptor gene have been identified.

FIGURE 14.10 The development of colorectal cancer is a multistage process of accumulating genetic errors in cells. The red arrows represent a new critical mutation event, followed by clonal expansion. At the stage of carcinoma, the proliferating cells contain all the genetic errors that have accumulated.

It appears, therefore, that mutations of the RAS and TP53 genes and LOH on 5q and 18q accumulate during the transition from a small ‘benign’ adenoma to carcinoma. The accumulation of alterations, rather than the order, appears to be more important in the development of carcinoma. More than one of these four alterations is seen in only 7% of small, early adenomas. Two or more such alterations are seen with increasing frequency when adenomas progress in size and show histological features of malignancy. More than 90% of carcinomas show two or more such alterations, and approximately 40% show three.

The multistage process of the development of cancer is likely, of course, to be an oversimplification. The distinction between oncogenes and tumor suppressor genes (Table 14.3) has not always been clear-cut—e.g., the RET oncogene and MEN2 (p. 93, Table 14.5). In addition, the same mutation in some of the inherited cancer syndromes (p. 225) can result in cancers at various sites in different individuals, perhaps as a consequence of the effect of interactions with inherited polymorphic variation in a number of other genes or a variety of environmental agents.

Table 14.3 Some Familial Cancers or Cancer Syndromes due to Tumor Suppressor Mutations

Further insight into the processes involved in the development of colorectal cancer came from a rare cause of familial colonic cancer known as familial adenomatous polyposis.

Familial Adenomatous Polyposis

Approximately 1% of persons who develop colorectal cancer do so through inheritance of an autosomal dominant disorder known as familial adenomatous polyposis (FAP). Affected persons develop numerous polyps of the large bowel, which can involve its entirety (Figure 14.11). There is a high risk of carcinomatous change taking place in these polyps, with more than 90% of persons with FAP eventually developing bowel cancer.

FIGURE 14.11 Large bowel from a person with polyposis coli opened up to show multiple polyps throughout the colon.

(Courtesy Mr. P. Finan, Department of Surgery, General Infirmary, Leeds.)

The identification of an individual with FAP and an interstitial deletion of a particular region of the long arm of chromosome 5 (5q21) led to the demonstration of linkage of FAP to DNA markers in that region. Subsequent studies led to the isolation of the adenomatous polyposis coli (APC) gene. Analyses of the markers linked to the APC gene in cancers from persons who have inherited the gene for this disorder have shown LOH, suggesting a similar mechanism of gene action in the development of this type of bowel cancer.

Studies in the common, non-hereditary form of bowel cancer have shown similar LOH at 5q in the tumor material, with the FAP gene being deleted in 40% and 70% of sporadically occurring adenomas and carcinomas of the colon. LOH has also been reported at a number of different sites in colonic cancer tumors that include the regions 18q21-qter and 17p12-p13, the latter region including the TP53 gene, as well as another gene at 5q21 known as the ‘mutated in colorectal cancer’ (MCC) gene, consistent with the development of the common form of colonic cancer being a multistage process.

‘Deleted in Colorectal Cancer’

Allele loss on chromosome 18q is seen in more than 70% of colorectal carcinomas. The original candidate gene for this region, called DCC, has been identified and cloned; it has a high degree of homology with the family of genes encoding cell adhesion molecules. The DCC gene is expressed in normal colonic mucosa but is either reduced or absent in colorectal carcinomas. As with TP53, somatic mutations in the DCC gene of the remaining allele occur in some cancers where gene expression is absent. The known homology suggests that loss of DCC plays a role in cell–cell and cell–basement membrane interactions, features that are lost in overt malignancy. However, mutations in the DCC gene have been found in only a small proportion of colonic cancers. Other genes deleted in this region in colorectal tumors include SMAD4 and SMAD2.

Hereditary Non-Polyposis Colorectal Cancer—Lynch Syndrome

A proportion of individuals with familial colonic cancer may have a small number of polyps, and the cancers occur more frequently in the proximal, or right side, of the colon, which is sometimes called ‘site-specific’ colonic cancer. The average age of onset for colonic cancer in this condition is the mid-forties. This familial cancer-predisposing syndrome is inherited as an autosomal dominant disorder and has been known as hereditary non-polyposis colorectal cancer (HNPCC)—even though polyps may be present (the name helps to distinguish the condition from FAP). There is now a preference to return to the original eponymous designation of Lynch syndrome, specifically Lynch syndrome type I (site-specific, e.g., colorectal). There is also a risk of small intestinal cancers, including stomach, endometrial cancer, and a variety of others (see Table 14.5).

MYH Polyposis

In a large study, nearly 20% of familial polyposis cases showed neither dominant inheritance nor evidence of an APC gene mutation. Of these families, more than 20% were found to have mutations in the MYH gene, and affected individuals were compound heterozygotes. In contrast to the other polyposis conditions described in the following section, MYH polyposis is an autosomal recessive trait, thus significantly affecting genetic counseling as well as the need for screening in the wider family. The gene, located on chromosome band 1p33, is the human homolog of mutY in E. coli. This bacterial mismatch repair operates in conjunction with mutM to correct A/G and A/C base-pair mismatches. In tumors studied, an excess of G:C to T:A transversions was observed in the APC gene. Mutations that effectively knock out the MYH gene, therefore, lead to defects in the base excision-repair pathway; this is a form of DNA mismatch repair that, unusually, follows autosomal recessive inheritance.

Juvenile Polyposis Syndrome

Autosomal dominant transmission is well described for a rare form of juvenile polyposis that may present in variety of ways, including bleeding with anemia, pain, intussusception, and failure to thrive. The polyps carry an approximate 13-fold increased cancer risk and, once diagnosed, regular surveillance and polypectomy should be undertaken. The average age at diagnosis of cancer is in the third decade, so that colectomy in adult life may be advisable. Two genes have been identified as causative: SMAD4 (18q) and BMPR1A (10q22). Both are components of the TGF-β signaling pathway (p. 85) and SMAD4 mutations, which account for about 60% of cases, appear to carry a higher malignancy potential and the possibility of large numbers of gastric polyps.

Cowden Disease

Also known as multiple hamartoma syndrome, Cowden disease is autosomal dominant but very variable. Gastrointestinal polyps are found in about half of the cases and are generally benign hamartomas or adenomas. Multiple lipomas occur with similar frequency and the oral mucosa may have a ‘cobblestone’ appearance. Significant macrocephaly is very common in this condition. Importantly, however, there is a high incidence (50%) of breast cancer in females, usually occurring at a young age, and papillary thyroid carcinoma affects about 7% of the patients. Testicular seminoma can occur in males. Mutations in the tumor suppressor PTEN gene on chromosome 10q23, encoding a tyrosine phosphatase, cause Cowden disease. A related phenotype with many overlapping features, which glories in the eponymous name Bannayan-Riley-Ruvalcaba syndrome, has also been shown to be due to mutations in PTEN in a large proportion of cases.

Peutz-Jegher Syndrome

Also autosomal dominant, this condition is characterized by the presence of dark melanin spots on the lips, around the mouth (Figure 14.12), on the palms and plantar areas, and other extremities. These are usually present in childhood and can fade in adult life. Patients often present with colicky abdominal pain from childhood from the development of multiple polyps that occur throughout the gastrointestinal tract, although they are most common in the small intestine. These are hamartomas but there is a significant risk of malignant transformation. There is an increased risk of cancers at other sites, particularly breast, uterus, ovary, and testis, and these tend to occur in early adult life. Regular screening for these cancers throughout life, from early adulthood, is warranted. Mutations in a serine threonine kinase gene, STK11, located on chromosome 19p, cause Peutz-Jegher syndrome.

FIGURE 14.12 Pigmented melanin spots affecting the oral mucosa of a child with Peutz-Jegher syndrome, which are usually more prominent in childhood compared with adult life. Affected individuals are at risk of multiple polypoid hamartomas throughout the gastrointestinal tract, which may undergo malignant change.

BRCA1 and BRCA2 Genes

Family studies of early-onset or premenopausal breast cancer showed that it behaved like a dominant trait in many families. Linkage analysis in these families showed that the tendency to develop breast cancer mapped to the long arm of chromosome 17, eventually leading to identification of the BRCA1 gene. A proportion of families with early-onset breast cancer that did not show linkage to this region showed linkage to the long arm of chromosome 13, resulting in the identification of the BRCA2 gene.

Approximately 40% to 50% of families with early-onset autosomal dominant breast cancer have a mutation in the BRCA1 gene and have been shown to have a 60% to 85% lifetime risk of developing breast cancer. Females with a BRCA1 mutation have an increased risk of developing ovarian cancer, and males an increased risk of developing prostate cancer. Mutations in the BRCA2 gene account for 30% to 40% of families with early-onset autosomal dominant breast cancer, and the lifetime risk of developing breast cancer is similar. Although initially mutations in the BRCA2 gene were not thought to be associated with an increased risk of other cancers, women heterozygous for a mutation also have an increased risk of developing ovarian cancer, and males an increased risk of prostate cancer. In some of the original familial breast cancer families recruited for linkage studies, a number had males who developed breast cancer. Although breast cancer in males is very rare, males with mutations in the BRCA2 gene have a 6% lifetime risk of developing breast cancer, approximately a 100-fold increase in the population risk of breast cancer in males.

Familial Cancer-Predisposing Syndromes

Many familial cancer-predisposing syndromes are inherited as autosomal dominant traits that are fully penetrant, with the consequent risk for heterozygotes of developing cancer approaching 100%. This level of risk means that more invasive means of screening with more frequent and earlier initiation of screening protocols are justified than would be acceptable for the population in general (Table 14.10).

Table 14.10 Conditions in which Prophylactic Surgery is an Accepted Treatment, and Treatments that are under Evaluation, as an Option for the Familial Cancer-Predisposing Syndromes or Individuals at Increased Risk for the Common Cancers

Inherited Susceptibility for the Common Cancers

Screening for the common cancers arising from inherited susceptibility has only relatively recently been established and by its very nature involves a long-term undertaking for the individual at risk as well as his or her physician or surgeon. It is important to emphasize that the natural enthusiasm for screening needs to be balanced with the paucity of hard data in many instances on the relative benefits and risks. However, recommended screening protocols are increasingly evidence based as more data become available (Box 14.2).

Who Should Be Screened?

In the case of the rare, dominantly inherited, single-gene familial cancer-predisposing syndromes such as FAP, von Hippel–Lindau, and multiple endocrine neoplasia (MEN), those who should be screened can be identified on a simple mendelian basis. However, for Rb, for example, the situation is more complex. If no RB1 mutation has been identified, presymptomatic genetic testing cannot be offered. Some individuals with the non-hereditary form have bilateral tumors, whereas some with the hereditary form have no tumor (i.e., the condition is non-penetrant) or a unilateral tumor. It may be impossible to distinguish which form is present, and screening of second-degree, as well as first-degree, relatives may be appropriate given that early detection can successfully prevent blindness. For those with a family history of the common cancers, such as bowel or breast cancer, the risk levels at which screening is recommended, and below which screening is not likely to be of benefit, will vary. At each extreme of risk the decision is usually straightforward, but with intermediate-level risks there can be doubt as to relative benefits and risks of screening.

What Age and How Often?

Cancer in persons with a familial cancer-predisposing syndrome tends to occur at a relatively earlier age than in the general population and screening programs must reflect this. With the exception of FAP, in which it is recommended that sigmoidoscopy to detect rectal polyps should start in the teenage years, most cancer screening programs do not start until 25 years of age or later. The highest-risk age band for most inherited susceptibilities is 35 to 50 years, but because cancer can still develop in those at risk at a later age, screening is usually continued thereafter. In some families the age of onset of cancer can be especially early and it is recommended that screening of at-risk individuals in these families commences 5 years before the age of onset in the earliest affected member. Again, Rb is an exception to the usual rule because, as it is a cancer of early childhood, screening starts in the postnatal period with frequent ophthalmic examination.

The recommended interval between repeated screening procedures should be determined from the natural history of the particular cancer. The development of colorectal cancer from an adenoma is believed to take place over a number of years, and as a result it is thought that 5-year screening intervals will suffice. If, however, a polyp is found, the interval between screening procedures is usually brought down to 3 years. Breast cancer is not detectable in a premalignant stage and early diagnosis is critical if there is to be a good prognosis. Annual mammography for females at high risk is therefore recommended from the age of 35 years.

What Sites Should Be Screened?

Having decided who is at risk within a family, one has to judge which types of cancer are most likely to occur and which systems of the body should be screened.

Inherited Susceptibility for the Common Cancers