Acute lymphoblastic leukaemia

In acute lymphoblastic leukaemia (ALL) the karyotype provides important prognostic information which is beginning to have an impact on treatment. The most significant structural chromosomal changes include: the poor-risk abnormalities; t(9;22)(q34;q11), giving rise to the BCR/ABL fusion and rearrangements of the MLL gene; abnormalities previously designated as poor-risk; t(1;19)(q23;p13), producing the E2A/PBX1 and rearrangements of MYC with the immunoglobulin genes; and the probable good risk translocation t(12;21)(p13;q22), which results in the ETV6/AML1 fusion. These abnormalities occur most frequently in B-lineage leukaemias, while rearrangements of the T cell receptor genes are associated with T-lineage ALL. Abnormalities of the short arm of chromosome 9, in particular homozygous deletions involving the tumour suppressor gene (TSG) p16(INK4A), are associated with a poor outcome. Numerical chromosomal abnormalities are of particular importance in relation to prognosis. High hyperdiploidy (51-65 chromosomes) is associated with a good risk, whereas the outlook for patients with near haploidy (23-29 chromosomes) is extremely poor. In view of the introduction of risk-adjusted therapy into the UK childhood ALL treatment trials, an interphase FISH screening programme has been developed to reveal chromosomal abnormalities with prognostic significance in childhood ALL. Novel techniques in molecular cytogenetics are identifying new, cryptic abnormalities in small groups of patients which may lead to further improvements in future treatment protocols.

The predictive value of hierarchical cytogenetic classification in older adults with acute myeloid leukemia (AML): analysis of 1065 patients entered into the United Kingdom Medical Research Council AML11 trial

Acute myeloid leukemia (AML) in older adults carries a poor prognosis, and the optimum treatment remains to be determined. In younger patients, treatment stratification is frequently based upon diagnostic karyotype, which was the most important prognostic factor in the UK Medical Research Council (MRC) AML10 trial. Considered here is whether karyotype is also predictive in older adults; this is done by studying 1065 cases from MRC AML11 (median age, 66 years). Three prognostic groups were distinguished on the basis of response to induction therapy and overall survival (OS). Those with t(15;17), t(8;21), or inv(16) composed the favorable risk group. Overall, these abnormalities predicted a superior complete remission (CR) rate (72%), reflecting relatively low levels of resistant disease (RD) (8%), and lower relapse risk (RR) (56%) associated with superior OS (34% at 5 years). Normal karyotype (CR, 63%; RD, 17%; RR, 78%; OS, 15%) and other noncomplex abnormalities (CR, 53%; RD, 32%; RR, 85%; OS, 10%) composed the intermediate group; while complex karyotype predicted an extremely poor prognosis (CR, 26%; RD, 56%; RR, 91%; OS, 2%). Combining MRC AML10 and AML11 (n = 2677) revealed that the most favorable changes were rarer in older patients (younger than 55 years, 24%; 55 years or older, 7%), while complex abnormalities were more common (6% vs 13%). This study suggests that hierarchical cytogenetic classification identifies biologically distinct subsets of AML that are represented in all age groups. Furthermore, it highlights the importance of karyotype as a critical independent determinant of outcome in older patients with AML, providing a potential framework for stratified treatment approaches.

Molecular tracking of leukemogenesis in a triplet pregnancy

The occurrence of childhood acute lymphoblastic leukemia (ALL) in 2 of 3 triplets provided a unique opportunity for the investigation of leukemogenesis and the natural history of ALL. The 2 leukemic triplets were monozygotic twins and shared an identical, acquired TEL-AML1 genomic fusion sequence indicative of a single-cell origin in utero in one fetus followed by dissemination of clonal progeny to the comonozygotic twin by intraplacental transfer. In accord with this interpretation, clonotypic TEL-AML1 fusion sequences could be amplified from the archived neonatal blood spots of the leukemic twins. The blood spot of the third, healthy, dizygotic triplet was also fusion gene positive in a single segment, though at age 3 years, his blood was found negative by sensitive polymerase chain reaction (PCR) screening for the genomic sequence and by reverse transcription-PCR. Leukemic cells in both twins had, in addition to TEL-AML1 fusion, a deletion of the normal, nonrearranged TEL allele. However, this genetic change was found by fluorescence in situ hybridization to be subclonal in both twins. Furthermore, mapping of the genomic boundaries of TEL deletions using microsatellite markers indicated that they were individually distinct in the twins and therefore must have arisen as independent and secondary events, probably after birth. These data support a multihit temporal model for the pathogenesis of the common form of childhood leukemia.

Crystal structure of an ephrin ectodomain

Eph receptor tyrosine kinases and their membrane-associated ligands, the ephrins, are essential regulators of axon guidance, cell migration, segmentation, and angiogenesis. There are two classes of vertebrate ephrin ligands which have distinct binding specificities for their cognate receptors. Multimerization of the ligands is required for receptor activation, and ephrin ligands themselves signal intracellularly upon binding Eph receptors. We have determined the structure of the extracellular domain of mouse ephrin-B2. The ephrin ectodomain is an eight-stranded beta barrel with topological similarity to plant nodulins and phytocyanins. Based on the structure, we have identified potential surface determinants of Eph/ephrin binding specificity and a ligand dimerization region. The high sequence similarity among ephrin ectodomains indicates that all ephrins may be modeled upon the ephrin-B2 structure presented here.

Modification of maternal and congenital cytomegalovirus infection by anti-glycoprotein b antibody transfer in guinea pigs

Prepregnancy human and guinea pig cytomegalovirus immunity reduces rates of congenital infection in subsequent pregnancies. Inbred JY-9 strain guinea pigs were used to study the role of hyperimmune anti-glycoprotein B (gB) serum in modification of congenital infection in early pregnancy. Significantly shorter duration of primary maternal viremia and fewer pregnancy losses occurred in passively immunized dams, compared with nonimmune dams. Placentas from recipients of negative control serum were smaller and had marked mononuclear cell infiltrates and focal necrosis and more viral foci than did those from recipients of anti-gB hyperimmune serum. Significantly higher intrauterine growth retardation occurred in pups of negative control serum recipients than in pups of passively immunized dams. Significantly higher proportions of pups and placentas from recipients of negative control serum were positive on viral culture than from passively immunized dams. Thus, anti-gB passive immunization decreased fetal infection and intrauterine growth retardation, shortened maternal viremia, and reduced pregnancy losses and placental inflammation and infection.

The wing in yeast heat shock transcription factor (HSF) DNA-binding domain is required for full activity

The yeast heat shock transcription factor (HSF) belongs to the winged helix family of proteins. HSF binds DNA as a trimer, and additional trimers can bind DNA co-operatively. Unlike other winged helix-turn-helix proteins, HSF's wing does not appear to contact DNA, as based on a previously solved crystal structure. Instead, the structure implies that the wing is involved in protein-protein interactions, possibly within a trimer or between adjacent trimers. To understand the function of the wing in the HSF DNA-binding domain, a Saccharomyces cerevisiae strain was created that expresses a wingless HSF protein. This strain grows normally at 30 degrees C, but shows a decrease in reporter gene expression during constitutive and heat-shocked conditions. Removal of the wing does not affect the stability or trimeric nature of a protein fragment containing the DNA-binding and trimerization domains. Removal of the wing does result in a decrease in DNA-binding affinity. This defect was mainly observed in the ability to form the first trimer-bound complex, as the formation of larger complexes is unaffected by the deletion. Our results suggest that the wing is not involved in the highly co-operative nature of HSF binding, but may be important in stabilizing the first trimer bound to DNA.

Daily or weekly therapy with resiquimod (R-848) reduces genital recurrences in herpes simplex virus-infected guinea pigs during and after treatment

The effect of resiquimod (R-848), an immune-response modifier that is similar to imiquimod, on recurrent herpes simplex virus (HSV) was evaluated using the guinea pig model of genital herpes. Guinea pigs were intravaginally infected with HSV-2 and then were randomized on day 14 to receive nothing or 0.1 mL/kg per dose of subcutaneous resiquimod, given either daily, every other day, or weekly from days 15-35. During a 3-week course of therapy, recurrences in all 3 treated groups were reduced by >80%, compared with the control group. After therapy, recurrences remained significantly (P<.05) decreased in all 3 groups for the next 3 weeks. The group treated weekly developed the fewest recurrences. Significant increases in interleukin-2 levels, produced by incubation of mononuclear cells with HSV-2 antigens, but not in circulating antibody also were detected in the treated groups. Resiquimod treatment may offer significant advantages to present antiviral therapies for the control of recurrent genital herpes.

The detection and significance of chromosomal abnormalities in childhood acute lymphoblastic leukaemia

In childhood acute lymphoblastic leukaemia (ALL), cytogenetics plays an essential role in diagnosis and prediction of outcome. Conventional cytogenetic analysis, complemented by fluorescence in situ hybridization (FISH), is highly effective in the accurate detection of chromosomal abnormalities. For the precise identification of specific genetic changes, molecular techniques may be applied. Chromosomal changes in ALL may be of structural or numerical type. A large number of established structural chromosomal rearrangements have now been described for which the genetic alterations and effect on prognosis are well known. These include t(9;22)(q34;q11) and BCR/ABL, rearrangements of 11q23 involving MLL, t(12;21)(p13;q22) with the ETV6/AML1 fusion, t(1;19)(q23;p13) with E2A/PBX1, t(8;14)(q24;q32) and the immunoglobulin genes. Genetic changes associated with T ALL are also known, although their effect on outcome is less pronounced. Rare chromosomal abnormalities are continually being discovered in small patient subgroups leading to the identification of new ALL associated genetic changes. Alterations in chromosome number have a strong impact on outcome in childhood ALL. The association of a high hyperdiploid karyotype (51-65 chromosomes) with a good prognosis has been known for more than 20 years. Conversely, the loss of chromosomes in the near-haploid group (23-28 chromosomes) indicates a poor outcome. New methods of cancer classification involving gene expression profiling may eventually supercede cytogenetic analysis in the diagnosis and prediction of outcome in leukaemia. It is more likely that they will be used in a complementary approach alongside cytogenetic, FISH and molecular analysis to guide patient management in childhood ALL.

Reduction of recurrent HSV disease using imiquimod alone or combined with a glycoprotein vaccine

The aim of this study is to evaluate the effects of an immune modulator, imiquimod, given alone or in combination with an HSV vaccine on HSV immune responses and as immunotherapy of a genital recurrence model. After recovery from primary genital HSV infection, animals were randomized to placebo, 21 days of imiquimod plus a placebo vaccine, or 21 days of imiquimod plus an HSV-2 glycoprotein vaccine. Placebo or HSV vaccine was given in the footpad on days 16 and 37 after HSV-2 genital inoculation. Daily imiquimod or placebo was given subcutaneously in the shoulder on days 16 through 37. Genital recurrences were monitored and HSV specific NK activity, IL-2 response and ELISA antibody were assayed. For the entire 15 week observation period, imiquimod alone reduced recurrences 62.6%, while addition of HSV vaccine to imiquimod reduced recurrences 80.6% compared to placebo/placebo. The duration of significant recurrence reduction was more notable with the addition of vaccine. Imiquimod alone significantly reduced the weekly HSV recurrent disease in the first 10 weeks (53-94% reduction, mean 75.9%), as did imiquimod plus HSV-vaccine (71-98% reduction, mean 89.5%). In weeks 10-15, imiquimod alone reduced recurrences significantly only in week 10 (20-53% reduction, mean 33%), whereas the addition of vaccine extended the significant recurrence reduction to 14 weeks (44-71% reduction, mean 56.8%). The recurrence reduction is correlated to an increased HSV-induced in vitro IL-2 response and NK activity against HSV targets in treated groups. Both imiquimod and the imiquimod/vaccine combination significantly reduced genital HSV recurrences, but the combination extended the duration and extent of protection for recurrences compared to imiquimod alone. Enhanced HSV specific immune responses correlated to the protection.

Convalescent phase outpatient parenteral antiinfective therapy for children with complicated appendicitis

BACKGROUND

Children with a perforated or gangrenous appendix become clinically stable after medical and/or surgical therapy but often remain in the hospital solely to complete parenteral antibiotic therapy. This prospective study investigates the outcomes when children who meet specified criteria are discharged to complete parenteral antibiotic therapy at home.

METHODS

Children age 1 to 17 years with appendicitis complicated by generalized peritonitis or intraabdominal abscess were eligible to participate. Subjects whose fever was decreasing, who were able to tolerate oral liquids and for whom further parenteral antibiotic therapy was deemed necessary were discharged from the hospital to receive outpatient parenteral antiinfective therapy (OPAT) with meropenem. Therapy was administered by a family member and supervised by home care nurses. Study personnel visited the home daily to collect data on adverse events, compliance and resource utilization. Pa tients served as their own controls in models of reduced hospitalization and net cost savings.

RESULTS

Discharged on average on the fourth postoperative day, 87 children received 4.5 +/- 2.1 days of OPAT. Six (7%) children were subsequently readmitted for complications including bowel obstruction (4 children), intraabdominal abscess (1 child) and pleural effusion (1 child). Another child developed a viral syndrome during OPAT. All other patients recovered uneventfully. Six (7%) children discontinued meropenem prematurely because of rash (4 patients) or diarrhea (2 patients). According to models in which each day of OPAT replaced a day of inpatient care, discharge to OPAT reduced hospitalization by 42 +/- 15% and saved a median of $2908 (10th to 90th percentile range, $1,077 to $4,707) per patient.

CONCLUSION

Convalescent phase OPAT is a cost-effective alternative to continued hospitalization for children with complicated appendicitis who are clinically stable yet require further parenteral antibiotic therapy.

Cross-species color banding in ten cases of myeloid malignancies with complex karyotypes

Cross-species color banding is a multiple-color fluorescence in situ hybridization (FISH) technique using probes developed from other animal species. Hybridization to human metaphases produces color banding patterns specific for each homologous chromosome pair. The technique has been evaluated in a complementary manner with G-banding and chromosome painting in a series of 10 myeloid malignancies with complex or unresolved karyotypes. Color banding detected the majority of chromosomal abnormalities, which had been identified by G-banding and in each case revealed chromosomal changes that G-banding had not identified. Painting was necessary to confirm these abnormalities due to the limitation of only seven colors in the color-banded karyotype. At the same time, painting fortuitously uncovered cryptic abnormalities in 6 of 10 cases that had not been detected by color banding. Insertions were visible by painting only. This study has demonstrated that in the analysis of complex karyotypes, the application of color banding revealed the involvement of the long arm of chromosome 3, indicating a poor risk, in two cases not identified by G-banding. Therefore, these techniques applied together have revealed cryptic chromosomal abnormalities with prognostic significance, which in some cases may have implications for patient management.

t(7;12)(q36;p13), a new recurrent translocation involving ETV6 in infant leukemia

The ETV6 gene is rearranged as a result of translocations involving a wide variety of chromosomal partners. To date, 12 partner genes for ETV6 have been cloned, and a further 23 chromosomal regions have been described. We previously identified a cryptic t(7;12) with ETV6 involvement in two cases of infant leukemia. The finding of a third case of t(7;12), also in an infant, prompted a more focussed search based on the common features found in these patients and those reported in the literature. The selection criteria were age at diagnosis < 20 months and the presence of +19 and/or +8 in the karyotype; cases with abnormalities of 7q and/or 12p were also considered. FISH studies using whole chromosome paints and probes for the ETV6 gene revealed a t(7;12) in 10 out of 23 cases studied. Seven of these had evidence of ETV6 rearrangement. Of those with ETV6 involvement, six had a 7q36 and one a 7q22 breakpoint. Importantly, in three cases the 7q36 breakpoint was within the same PAC, suggesting the existence of a new nonrandom translocation. However, in at least one patient the 7q36 breakpoint was different. The identification of the 7q partner genes will determine whether it is the disruption of ETV6 alone, or the formation of fusion genes, that is important for leukemogenesis in these patients. As both 7q36 and 7q22 are critical regions of gene loss in del(7q) leukemias, the identification of partner genes from these regions may also be important in understanding the pathogenesis of these diseases.

Analysis of ETV6/AML1 abnormalities in acute lymphoblastic leukaemia: incidence, alternative spliced forms and minimal residual disease value

The t(12;21)(p13;q22) translocation, resulting in the fusion of the ETV6 and AML1 genes, occurs in 20-25% of paediatric B-lineage acute lymphoblastic leukaemias (ALL). The identification of the fusion product has important prognostic and therapeutic implications as the translocation has been associated with a favourable clinical outcome. The aim of this study was threefold: (i) to assess the frequency and clinical association of the fusion gene in patients with and without a cytogenetically detectable chromosome 12 and/or 21 abnormality or failed cytogenetic results, (ii) to characterize alternative forms of ETV6/AML1 transcripts, and (iii) to use ETV6/AML1 as a molecular marker for the investigation of minimal residual disease (MRD). ETV6/AML1 fusion was detected in 22 (39%) of 56 cases studied by reverse transcriptase polymerase chain reaction (RT-PCR). ETV6/AML1 fusion was found in nine out of 16 (56%) cases with a cytogenetically visible chromosome 12 abnormality, but also in nine out of 29 patients (31%) without a chromosome 12 abnormality or patients with failed cytogenetics (four out of 11 patients, 36%), making this the most common cytogenetic abnormality in childhood ALL. Alternatively spliced ETV6/AML1 forms were investigated in 14 of the positive patients. Exon 5 of ETV6 was fused in frame to all AML1 exons, except exon 4. Fusion to exon 6 of AML1 resulted in one amino acid change. The presence of ETV6/AML1 was associated with a lower white blood cell count (Student's t-test; P = 0.009) and common (c)ALL phenotype (chi(2) test; P > 0.001), but no better disease-free survival. Our study shows that (i) RT-PCR is the most effective approach for the detection of t(12;21) in childhood ALL, (ii) the association of ETV6/AML1 and chromosome 12 and/or 21, seen in 56% of our cases, further confirms existing data, (iii) overall survival of patients with t(12;21) was not better than other cytogenetics groups, and (d) MRD analysis using ETV6/AML1 fusion is specific, but not sensitive enough to avoid false negative results.

The genetics of childhood acute lymphoblastic leukaemia

In childhood acute lymphoblastic leukaemia (ALL) a number of genetic changes have been identified which provide diagnostic and prognostic information with a direct impact on patient management. The most significant abnormalities include the translocation, t(12;21)(p13;q22), giving rise to the ETV6/AML1 gene fusion; BCR/ABL arising from t(9;22)(q34;q11); re-arrangements of the MLL gene; the E2A/PBX1 from the t(1;19)(q23;p13); re-arrangements of MYC with the immunoglobulin genes and re-arrangements of the T cell receptor genes. Chromosomal deletions, particularly those of the short arms of chromosomes 9 and 12 and the long arm of chromosome 6, have been postulated to be the sites of tumour suppressor genes (TSG). Numerical chromosomal abnormalities are of particular importance in relation to prognosis. High hyperdiploidy (50-65 chromosomes) is associated with a good risk, whereas the outlook for patients with near haploidy (23-29 chromosomes) is extremely poor. In view of the introduction of risk-adjusted therapy into the UK childhood ALL treatment trials, an interphase FISH screening programme has been developed to reveal chromosomal abnormalities with prognostic significance in childhood ALL.

Deletion of 6q16-q21 in human lymphoid malignancies: a mapping and deletion analysis

Two distinct regions of minimal deletion (RMD) have been identified at 6q25-q27 in non-Hodgkin's lymphoma (RMD-1), and at 6q21-q23 in acute lymphoblastic leukemia (ALL; RMD-2) by loss of heterozygosity and fluorescence in situ hybridization studies. In this study, 30 overlapping yeast artificial chromosomes (YACs), 1 expressed sequence tag, and 11 novel YAC ends were identified using bidirectional YAC walks between markers D6S447 (proximal) and D6S246 (distal) in RMD-2. The genes AF6q21, human homologue of the Drosophila tailless (HTLX), CD24 antigen, the Kruppel-like zinc finger BLIMP1, and cyclin C (CCNC), previously mapped to 6q21, were accurately positioned in a telomere-to-centromere orientation. Approximately 3.5 Mb were found to separate the BLIMP1 (adjacent to D6S447) and AF6q21 genes (telomeric to D6S246). Deletions of 6q were investigated in 21 cases of ALL using the newly characterized YAC clones in dual-color fluorescence in situ hybridization studies. A region centromeric to D6S447 (containing marker D6S283) and a region telomeric to marker CHLC.GGAT16CO2 (and containing marker D6S268) were identified as distinct and nonoverlapping regions of deletion in ALL.

The rug3 locus of pea encodes plastidial phosphoglucomutase

Two cDNA clones were isolated from pea (Pisum sativum L.) and their deduced amino acid sequences shown to have significant homology to phosphoglucomutases from eukaryotic and prokaryotic sources. The longer cDNA contained a putative transit-peptide-encoding sequence, supporting the hypothesis that the isolated clones represent the cytosolic and plastidial isoforms of phosphoglucomutase in pea. Plastid protein import assays confirmed that the putative plastidial isoform was targeted to the plastid stroma where it was proteolytically processed. Expression, co-segregation, linkage, and molecular analyses have confirmed that the rug3 locus of pea encodes plastidial phosphoglucomutase. Mutations at this locus result in a near-starchless phenotype of the plant.

Correlation of maternal and pup NK-like activity and TNF responses against cytomegalovirus to pregnancy outcome in inbred guinea pigs

Immunologic causes for poor outcome of pregnancy complicated by primary cytomegalovirus (CMV) infection are only partially understood. Maternal and pup tumor necrosis factor (TNF) and natural killer cell (NK)-like activity associated with primary gestational CMV infection initiated in either the first or third trimester equivalent in the inbred guinea pig model were investigated. Poor pregnancy outcome defined as fetal resorptions, premature delivery, stillbirths, and intrauterine growth retardation occurred with infection at either gestational time. Induction of TNF and NK activity by CMV infection during pregnancy correlated with resorptions in early pregnancy infection and with premature labor in late pregnancy infection. Stillbirths occurred with CMV infection at either time. Regardless of the gestational time of CMV acquisition, poor outcome correlated with higher maternal NK and TNF responses during the first weeks after maternal virus acquisition. Furthermore, CMV infected dams with loss of >/= 50% of conceptus had higher TNF responses than infected dams with < 50% conceptus loss. Likewise, pups born in litters from CMV-infected dams with resorptions and/or premature labor also had enhanced NK activity and TNF response to CMV compared with pups born to dams not having resorptions or premature labor. TNF responses in the delivered pups of infected dams were higher than from pups of uninfected dams regardless of litter outcome, whereas pup NK responses were enhanced only in pups from litters of dams with premature labor or resorptions. Enhanced NK and TNF activity appear to be associated with premature delivery and other poor outcomes of pregnancy.

Monosomy 20 as a pointer to dicentric (9;20) in acute lymphoblastic leukemia

Twenty new cases of acute lymphoblastic leukemia (ALL) with the dicentric chromosome dic(9;20)(p1113;q11) are presented. This chromosomal abnormality is difficult to identify from G-banding alone. It masquerades as monosomy 20 and is only accurately identified by fluorescence in situ hybridization (FISH). Monosomy 20 was found in 59/2790 patients with successful karyotypes entered to the Leukaemia Research Fund/UK Cancer Cytogenetics Group Karyotype Database in ALL (LRF/UKCCG Karyotype Database). FISH revealed dic(9;20) in 20/25 cases with available material. Extra copies of chromosome 21 were found in 8 of the 20 cases. Patients were 14 females and six males, aged 1-32 years (median 4 years), with leukocyte counts of 2-536 (median 23) x 109/l and immunophenotypes of common or pre-B ALL (17 cases), T-ALL (one case) or unknown (two cases). Four patients relapsed at 2, 22, 28 and 47 months and two died at 49 and 63 months (median follow-up 37 months). FISH studies on the remaining five patients showed one with monosomy 20 and four with other rearrangements of the chromosome. This study has increased the number of reported cases of dic(9;20) from 17 to 37. It has identified dic(9;20) in one case of T-ALL and shows an association of this translocation with trisomy 21.

Multiplex fluorescence in situ hybridization and cross species color banding of a case of chronic myeloid leukemia in blastic crisis with a complex Philadelphia translocation

Exciting new techniques in molecular cytogenetics--namely, spectral karyotyping, multiplex fluorescence in situ hybridization (M-FISH), and cross species color banding--have been recently developed. An increasing number of reports demonstrate the success of these procedures in providing additional cytogenetic information--identifying marker chromosomes and revealing the presence of previously undetected chromosomal changes. However, these procedures have their limitations, and their absolute sensitivity in the accurate identification of subtle chromosomal abnormalities remains to be established. M-FISH and color banding have been applied to a case of chronic myeloid leukemia with a complex Philadelphia translocation involving chromosomes 9, 17, and 22, which had initially been identified from G-banded chromosome analysis. The abnormalities were confirmed by chromosome "painting" and specific probes. Although M-FISH and color banding revealed no additional cryptic chromosomal changes, this study has clearly demonstrated the success of these multiple color FISH approaches in the accurate characterization of a complex rearrangement with subtle abnormalities.

Antivirals other than for HIV therapy

This review concentrates on drugs directed at viruses other than HIV and does not include anti-HIV drugs. Many antivirals are nucleoside analogues, although new classes of enzyme inhibitors are being developed for treatment of HIV (proteases) and enteroviruses. Antisense drugs and receptor blockers have recently been approved. Regardless of mechanism, antivirals are predominantly virustatic, not virucidal, and usually require help from the host immune system to either completely eliminate pathogens (e.g., influenza) or to provide sufficient immune surveillance to prevent a latent pathogen from reactivating (e.g., the herpesviruses). Viruses by nature are intracellular pathogens, using the host cell to produce progeny. Often the viral pathogen uses the host cell machinery or enzyme system to facilitate replication. At times the virus usurps nearly the entire intracellular system for replication and may shut down all functional host cell output for host-determined products. Because of the intracellular nature of viruses and because viruses use many "normal" host cell factors, functions, and systems for replication, antivirals can have notable toxicities to host cells. As antivirals inhibit viral activity in the viral-infected host cells (these encompass a minority of the total host cell numbers), they may also negatively affect functions in the virus-uninfected host cells. These alterations may be biochemical, detected only if sought specifically, or clinically obvious toxicities. Either may limit the use of some antivirals to severe or life-threatening situations.