SIMULACION DE MONTECARLO

C-myc, H-myc and L-myc are three members of the myc gene family which

encode nuclear phosphoproteins that are similar in structure and function. C-Myc is located at chromosome 8q24, N-myc is at 2p24 and L-myc is at 1p32. Expression of the myc genes is known to be involved in the regulation of cell proliferation and differentiation. Resting cells which are not proliferating exhibit low levels of myc RNA, however upon stimulation with growth inducing agent there is a transient rise in myc expression. When cells are induced to differentiate the level of myc transcript is generally down regulated (review by Spencer and Groudine 1990). Inhibition of c-myc expresion may lead to accelerated differentiation. As with other nuclear oncogenes myc genes are effective inducers of cell immortality and co-operate with activated ras

oncogenes to yield malignant transformation of rat primary embryo fibroblasts (Land et al., 1983). Myc expression is regulated at the level of transcription and at the post-transcriptional level by factors influencing stability of the proteins. Amplification of myc genes has been demonstrated in a variety of human malignant disorders suggesting a vital role for myc activation in multistage carcinogenesis (Masuda et al., 1987). Amplification is usually associated with double minutes (DMs) and homogenously staining regions (HSRs) which are indicative of gene reiteration. In general amplification is associated with more aggressive or advanced forms of cancers and may serve as a prognostic parameter in a number of human malignancies (Yokota et al., 1986).

C-myc is the cellular homologue of the viral oncogene v-myc (Colby et al., 1983) which is found in a number of retroviruses that induce leukemias and carcinomas. Both the c-myc mRNA transcript and protein product have an unusually short half life of 20-30 minutes (Rabbitts at a!., 1985). The 62Kd c-

myc protein product is thought to act as a transcriptional activator (Blackwood and Eisenman 1991) and there is evidence to suggest it may play a role in regulation of apoptosis (Arends at a!., 1993).The c-myc gene is expressed in most tissues depending on their mitotic activity (Zimmerman at a!., 1986). One exception to this however is the germinal epithelium of testicular tissue which expressed only low amounts of c-myc RNA (Stewart at a!., 1984). C-myc

amplification in general seems to have little specificity for tumour type and has been detected in a large number of neoplasms of different histogenesis (Yokota

at al., 1986). However c-myc amplification is more often a feature of solid

tumours than haematologic neoplasms. Activation of c-myc can also occur by deregulated expression e.g. in Burkitts lymphoma reciprocal translocations involving the long arm of chromosome 8 result in the juxtaposition of c-myc with

immunoglobulin genes on chromosome 14 and rarely on chromosomes 2 or 22 (Dalla-Favera at al., 1982). The breakpoints of the translocation with both the c-

myc and immunoglobulin loci vary considerably and may be internal to the c-

myc transcription unit or several hundred kb away from it. In some cases c-myc

suffers mutations and in others it does not but invariably c-myc is overexpressed.

The c-myc gene can be constitutively activated by retroviral insertion on either

side (Hayward at al., 1981). However there is no evidence that this mechanism plays a role in human neoplasms.

N-myc expression is restricted to cells derived from neuroectoderm such as neuroblastoma lung cancer and retinoblastoma (Little atal., 1983; Brodeur at al.,

1984; Lee at al., 1984). Upto 300 fold amplification of N-myc has been detected in neuroblastoma cell lines and tumours (Broduer at al., 1984). A strong correlation exists between N-myc expression and stage III and IV neuroblastomas which have a poorer prognosis than stage I and II disease.

N-myc amplification may therefore be indicative of patient outcome (Brodeur et

a/., 1984; Seeger efa/., 1985).

L-myc expression has only been detected in adult lung and correspondingly only in small cell lung cancer (Little et al., 1983; Zimmerman et a!., 1986). Amplification of c-myc and N-myc have also been demonstrated in small cell lung carcinoma (SCLC) (Johnson et al., 1988). Interestingly the three genes are alternately amplified thereby suggesting a close functional relationship. The observation has been made that c-myc amplification is more often associated with the variant form of SCLC (Little et al., 1983) which has a poorer prognosis than the classical variety so it has been suggested that amplification of c-myc is a prognostic indicator of the more aggressive form of the disease analogous to N-myc amplification in neuroblastoma.

1.5.1 Myc in TGCTs

Few studies have been reported on the status of myc genes in TGCTs. Peltomaki et al. (1991) examined the dosage of c-myc and N-myc genes in 14 TGCTs ( 8 seminomas, 1 embryonal carcinoma, 3 teratocarcinomas and 1

combined tumour) but found no consistent alterations in tumour DNA relative to normal testicular tissue. Expression of c-myc RNA was detectable in both normal and tumour testicular tissues but was not increased in the tumour sample when compared to corresponding normal testis tissue although interindividual variation in mRNA levels were observed. Expression of c-myc RNA in normal testis as well as seminoma, embryonal carcinoma and teratoma has also been reported by Misaki et al. (1989). Low levels of c-myc RNA were also reported in 4/7 embryonal carcinoma cell lines (Tesch et al., 1990). The N-myc gene however was not found to be expressed in normal testis and testicular teratomas examined by Misaki etal. (1989) but was detected in 7 of 10 seminomas and 2 of 2 embryonal carcinomas. Gene amplification was not observed in these cases. Elevated expression of N-myc in the absence of gene amplification was also reported in one seminoma by Saskela et al. (1989) and in the 7 embryonal

carcinoma cell lines of Tesch et al. (1990). Small amounts of L-myc were detectable in normal tissue but not in the seminoma whereas Misaki at al. (1989) did not observe L-myc expression in any of the cases examined.

An investigation of the c-myc oncoprotein product in archival material from testicular tumour patients showed that normal testis expressed only small amounts of p62 c-myc (Sikora at a/., 1985; Watson at al., 1986). Increased expression of c-myc was observed in all tumour cases compared with normal testis. In the nonseminoma group oncoprotein levels increased significantly with increasing teratoma differentiation. No correlation was found between p62 levels and stage of disease but patients with intermediate and undifferentiated tumours who developed recurrence had lower p62 levels than those who were disease free since their initial treatment. It is therefore suggested that quantitation of

c-myc oncoprotein product in TGCTs could give prognostic information.