Econstudentlog

Sexually Transmitted Viral Pathogens

The stuff below covers material from the last half of part V in Holmes et al. This previous post also dealt with this topic (the title of this post is the title of part 5 of the book, which if not hidden away in a 2000+ page textbook might well have been a book of its own; I’m quite sure you can find entire books on these topics which go into much less detail than does part 5 of this book). Some of the stuff was really hard to read; I’ve tried to include in this post mainly stuff people who have not read the rest of the coverage might be expected to understand without too much difficulty.

“Antibody to EBV [Epstein-Barr Virus] can be detected in 90-95% of the population by adulthood.10 Primary exposure often occurs in the first years of life, with seroconversion evident before the age of 5 years in 50% of children studied in the United States and Great Britain.22,23 In economically advantaged communities, primary infection may be delayed until adolescence or early adulthood,24 at which time acquisition of virus produces the clinical syndrome acute infectious mononucleosis. […] Primary EBV infection in infancy or early childhood is usually subclinical, but when delayed until the second decade of life, it manifests as infectious mononucleosis in up to 50% of patients. A self-limiting lymphoproliferative disease, the syndrome consists of fever, headache, pharyngitis, lymphadenopathy, and general malaise. Resolution of symptoms may take weeks to months, but primary infection is always followed by the establishment of a permanent viral carrier state.”

“a highly significant correlation between seropositivity for EBV, sexual intercourse, and an increasing number of sexual partners was found in a cross-sectional analysis of 1006 new University students.50 In this study, two-thirds of infectious mononucleosis cases were statistically attributable to sexual intercourse, whereas only a tenth of asymptomatic primary infections were linked to sexual activity.” [This is of course just a cross-sectional analysis, but even so – ‘kissing disease‘ may perhaps be a slightly inaccurate term…]

“The overall size of the EBV-infected B cell reservoir is largely controlled by CD8+, HLA class I-restricted, EBV-specific cytotoxic T lymphocytes (CTLs). Up to 5% of the total circulating CD8+ T cell pool may be committed to this single virus in the EBV-carrier state,110 indicating the critical role for T-cell surveillance in maintaining the host: virus balance. Impaired CTL responses in immunosuppressed patients such as transplant recipients or HIV-1-infected individuals32,111 leads to an expansion of the infected B cell population and potentially fatal lymphoproliferative disease. The contribution of virus-specific CTLs in immune regulation of EBV-induced lymphoproliferation has been made clear by recent therapeutic interventions involving adoptive transfer of virus-specific T lymphocytes to restore immunity against EBV infection in bone marrow transplant recipients.112, 113, 114

Non-Hodgkin’s lymphoma, an AIDS-defining cancer some 60 times more common in AIDS patients than in the general population,122 can be divided morphologically into Burkitt-like and immunoblastic lymphomas […]. Both have a higher association with EBV (30-40% and 75-80%, respectively) in AIDS than in non-AIDS groups.123,124 […] Burkitt’s lymphomas appear early in the course of AIDS, prior to profound immunosuppression, whereas immunoblastic lymphomas typically occur in late-stage AIDS when cellular immunity is compromised. […] Hodgkin’s lymphomas in the setting of AIDS contain EBV in 75-90% of the cases,140,141 reflecting the unusually high frequency in the HIV-infected population of mixed-cellularity and lymphocyte-depletion subtypes142 known to be most closely associated with EBV in the general population.15,16 Although not considered an AIDS-defining illness, Hodgkin’s lymphoma occurs with increased frequency in the setting of HIV infection. […] Our understanding of the precise role of EBV in the biology of each malignancy remains rudimentary.”

“Papillomaviruses are a group of small DNA viruses that primarily induce epithelial cell proliferation, or papillomas, in higher vertebrates. Infections are strictly genus or species specific and there is considerable tropism among the viruses for particular anatomic sites. […] The genomes of over 100 human papillomavirus (HPV) types have been molecularly cloned and completely sequenced and partial sequences for potentially up to another hundred types have been detected by PCR-based assays.9,10

[You can skip this part without missing out on anything important:] “A basal level of transcription from the p97/p105 promoter is regulated by the keratinocyte-dependent enhancer in the NCR and can be repressed by binding of E2 to its cognate recognition sequences located adjacent to the p97/p105 TATA boxes.82,87 E2 has a role in repressing transcription from the viral promoter and its loss on viral DNA integration allows increased expression of the oncogenic proteins E6 and E7. E2 also activates transcription under some circumstances.88 Variations in binding to four different E2 binding sites may contribute to transcriptional regulation.89 Interestingly, the E8E2C protein of HPV-31 had an ability to repress transcription from a single promoter-distal E2 binding site. This activity was lacking in the full-length E2, suggesting that E8E2C has a role in regulating transcription.90 E2 binds the general transcription factor TFIIB91 and is thought to directly inhibit HPV transcription at a step subsequent to binding of TBP or TFIID to the TATA box.92 It interacts with numerous other transcription factors and chromatin modification factors including CBP,93 p/CAF,94 C/EBP,95 nucleosome assembly protein 1,96 and Top BP1.97 An interaction between bromodomain protein 4 and E2 is required for transcriptional activation by E2.98 Two crystal structure studies of the amino terminal transcription activation domain of E2 suggested that dimerization of E2 helps recruit distal transcription factors to the viral transcription complex99 …” [I decided to include a quote like this as well to indicate what kind of stuff the book is also full of, and illustrate why it was hard to read. There’s a lot of stuff in some of these chapters which I had a really hard time following.]

“Predictions about the role of HPV in neoplasia obtained from experimental studies are consistent with the natural history of cervical cancer. Among women who develop squamous intraepithelial lesions, the time from first detection of viral DNA to lesions is short, ~2 years, though factors like number of partners and infection with other STDs may influence the interval […]. Even mildly dysplastic lesions show an increase in proliferation and polyploidization. These changes result as a direct consequence of expression of E6/E7. The median age of carcinoma in situ (CIS) in the United States is 29,223,224 indicating that approximately a decade has elapsed between the initial infection and severe dysplasia. CIS lesions are characterized by their aneuploid DNA content and thus reflect the genetic instability that accompanies prolonged expression of E6/E7. The preneoplastic lesions can regress spontaneously. One principle explanation for their regression is likely to be an effective immune response, and generalized T-cell deficiency has been associated with increased dysplasia […]. Other explanations may include the fact that some, perhaps most, alterations will be deleterious, resulting in cell death. The median age of invasive cervical cancer in the United States is 49, indicating that additional changes required for invasion and metastasis are acquired slowly over time.”

“All HPV types are linked to the development of low-grade cervical SILs [squamous intraepithelial lesions],16 whereas high-grade cervical SILs are usually positive for oncogenic HPV types.272 In a cohort of young women in the United States, Moscicki et al.155 reported that 15% developed LSIL within 3 years after initial HPV infection, and in the UK, Woodman et al.70 reported that the 3-year cumulative incidence of any cytologic abnormality after initial infection was 33%. In another cohort of young women in the United States, Winer et al.132 reported that approximately 50% developed a low-grade lesion within 3 years after initial HPV infection […]. The latter study also reported that rates of lesion detection tend to increase with increased screening frequency, due to spontaneous regression of most low-grade lesions. Therefore, the shorter interval between follow-up visits (4 vs. 6 months) may explain why that study detected LSIL in a higher proportion of women. Half of newly detected low-grade lesions appear to regress within 6-9 months.70,132,273 It also appears that vaginal SIL is not uncommon among women with incident HPV infections. While less commonly detected than cervical SIL, one study reported that almost 30% of women with incident HPV infections developed vaginal SIL within 3 years (Fig. 28-3B), and the median duration of these lesions was less than 5 months.132

Results from natural history studies suggest that LSILs are transient manifestations of productive HPV infections, whereas HSILs are cervical cancer precursor lesions. This is in contrast to the previously held theory that cervical carcinogenesis always follows a progression from persistent HPV infection to development of low- and then high-grade lesions. Several recent studies have shown that cervical highgrade lesions are a relatively early manifestation of HPV infections in young women. Woodman et al.70 reported that the risk of high-grade lesions was highest in the first 6 months after initial HPV infection. It has also been shown that histologically confirmed high-grade lesions are common after infection with HPV 16 and 18 infections,132,274 with one study reporting that 27% of women with incident HPV 16 or 18 infections developed histologically confirmed CIN grade 2 or 3 within 3 years […] and that the median time to development was 14 months.132

“Although viral hepatitis is unquestionably an ancient disease, it is only in the past 40 years that an appreciation has emerged of the diversity of infectious agents capable of causing the clinical syndrome of acute hepatitis. […] at least five distinctly different human viruses (classified as hepatitis A through E) are now generally recognized to be causative agents of acute and/or chronic viral hepatitis […].

Hepatitis A virus (HAV), hepatitis B virus (HBV), hepatitis C virus (HCV), hepatitis delta virus (HDV), and hepatitis E virus (HEV) all share a remarkable tropism for the liver despite profound differences in their physical structure, pathobiology, and epidemiology. Each of these viruses is a cause of clinically overt acute hepatitis associated with frank jaundice. The severity of the liver disease, which frequently accompanies acute infection with these viruses, generally distinguishes them from cytomegalovirus and Epstein-Barr virus, which typically cause much milder liver dysfunction during primary infections. Within the United States, almost all cases of acute hepatitis are caused by infection with HAV (51%), HBV (40%), or HCV (9%).3 These acute hepatitis virus infections cannot be distinguished from each other without serologic testing. Acute hepatitis represents a considerable disease burden within the United States, with an estimated 40,000-60,000 clinical cases occurring annually during the past 5 years. Only a fraction of these cases are reported to public health authorities, and a substantial number of additional infections do not come to medical attention because they are asymptomatic. Fulminant hepatic failure and death occur in a very small proportion of patients with acute hepatitis A or B, but these clinical endpoints are rarely associated with acute HCV infection in the United States.4,5

“The major burden of disease due to hepatitis virus infections stems from the chronic liver damage that occurs in individuals who develop persistent infections. The proportion of persons who become persistently infected is highly dependent on the infecting virus. Persistent infections with HAV are not well documented and may never occur. On the other hand, more than 50% of persons infected with HCV fail to clear the virus and most eventually develop biochemical and histologic evidence of chronic liver disease.6 HBV has an intermediate tendency to establish persistence, with the risk of persistent infection being highly dependent on the age at the time of infection and immunologic competence of the individual.

It is appropriate to focus attention on the hepatitis viruses in a textbook concerned with sexually transmitted diseases (STDs). Although these are systemic infections that are also commonly transmitted by other means, HBV is a sexually transmitted infection, and sexual activity may profoundly influence the risks for acquisition of HAV. To a lesser extent, sexual behavior may also influence the risk of infection with HCV and HDV.”

“Several factors account for the high risk of HBV infection among MSM [Males who have Sex with Males], which was noted in […] early studies. One of the most important factors was the number of sexual partners. The typical homosexually active male frequenting Denver’s steam baths in the late 1970s had eight different male sexual contacts per month.206 These contacts were largely anonymous and could total as many as 1000 over the lifetime of a gay man.”

“The endemicity of HBV infection varies greatly worldwide and is influenced primarily by the predominant age at which infection occurs.121,190 Endemicity of infection is considered high in those parts of the world where at least 8% of the population is HBsAg positive, and 70-90% of the population has serological evidence of previous HBV infection. Almost all infections occur during either the perinatal period or early in childhood, which accounts for the high rates of chronic HBV infection in these populations. Risk of HBV infection continues after the first 5 years of life, but its eventual contribution to the high rate of chronic infection is less significant. Chronic infection with HBV is strongly associated with HCC [liver cancer], and areas with a high endemicity of chronic HBV infection have the highest death rates from this neoplasm. […]

In most developed parts of the world, including the United States and Western Europe, the prevalence of chronic HBV infection is <1%, and the overall infection rate is 5-7%. The highest incidence of acute hepatitis B is among young adults, and high-risk sexual activity and injectable drug use account for most cases of newly acquired hepatitis B.191, 192, 193, 194 In the United States, heterosexual activity accounts for 40% of new hepatitis B cases, while MSM represent 15% of new cases.”

“HCV infection accounts for 15% of acute viral hepatitis cases within the United States. However, it is by far the leading cause of chronic viral hepatitis and is present in over 40% of persons with chronic liver disease. The morbidity and mortality associated with HCV infection are due to its unique propensity to cause persistent infection in most persons, a feature that distinguishes this virus from other hepatitis viruses. The specific mechanisms underlying viral persistence are not known.

Although it has been controversial, the balance of evidence now favors the occasional sexual transmission of HCV. The risk of infection with HCV, like HBV, has been independently related to numbers of sexual partners in some STD clinic studies.226,285 Although the risk of HCV infection has been shown to correlate with numbers of partners and/or specific sexual practices in some studies of homosexual men,286,287 the risk of infection is overwhelmingly more closely tied to injection drug use. […] About 60-85% of all infections lead to virus persistence, and this is often associated with evidence of chronic liver disease […]. After many years, this process may culminate in cirrhosis and liver failure, or the development of hepatocellular carcinoma. These end-stage events in chronic hepatitis C may claim as many as 10,000 lives annually in the United States.326 […] As many as a third of patients found to have chronic hepatitis C will ultimately develop cirrhosis6,284,333,334; although it is not well defined, the fraction of all patients who are infected with the virus and progress to cirrhosis is undoubtedly far lower. Cirrhosis may be present within as little as 60 months of the initial infection but is identified typically in persons who have been infected for decades. Factors associated with disease progression include age at infection, regular alcohol consumption, coinfection with HIV or HBV, and more recently obesity and insulin resistance.335, 336 Some patients, usually with well-established cirrhosis, develop primary hepatocellular carcinoma.283 Chronic HCV infection is the most important etiology of hepatocellular carcinoma in western countries.

The major challenge to clinical investigators has been the discovery of specific markers that are predictive of progression of chronic hepatitis C to a clinically significant disease state. This remains an exceptionally difficult problem. There is no good correlation between biochemical markers and the extent of fibrosis or the presence or absence of cirrhosis. Indeed, many patients with cirrhosis have no obvious laboratory abnormalities.333 In addition, quantitative measurements of the viremia (“virus load”) are not useful in determining the extent of disease resistance.335, 336

 

July 20, 2014 - Posted by | books, cancer, medicine

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