Edelliselle sivulle

Biology of mouse yolk sac development and pediatric yolk sac tumors

Susanna Siltanen, MD

Supervisor: Markku Heikinheimo

The yolk sac in rodents plays a vital role in nutrition prior to formation of the placenta. It is composed of two different cell layers, parietal and visceral endoderm, that have a common precursor, the primitive endoderm. The yolk sac is also the origin of germ cells, the precursors for gametes. Germ cells migrate early in development to the abdominal cavity where they combine with the somatic components of the gonad. Disruption of this migration or imbalance in the hormonal milieu of the developing gonad may cause germ cells to give rise to germ cell tumors (GCTs). Due to the pluripotent nature of germ cells these tumors may contain any embryonic or extraembryonic tissues. Teratomas that include mature or immature tissues derived from all three embryonal disc layers are benign by histology but can be harmful due to their location, proliferativity or invasion to adjacent tissues. Malignant GCTs include e.g. yolk sac tumors (YSTs), chorioncarcinomas and embryonal carcinoma. One tumor may contain several tissue types mixed together making histological differentiation of GCTs difficult. Thus, new methods for diagnostics are needed.

Abnormal transcription has been linked to variety of human diseases, from neoplasia to birth defects. Transcription factors are proteins that recognize specific sequences of DNA and control DNA transcription. The GATA-transcription factor family consists of six members (GATA-1 –6). The DNA-binding domain of GATA-protein consists of two homologous, conserved zinc fingers. GATA-4 and GATA-6 have been shown by knock-out experiments to be essential for normal yolk sac development. Recently, two cofactors for GATA-proteins, Friend-of-GATA (FOG-1) and FOG-2 have been found.

Mutations in the genes that directly control cell birth or cell death are widely accepted as the basis of cancer. The number of pediatric YSTs that have been genetically analysed is, due to rarity of these tumors, limited. Gain of 12p, characteristic of adolescent germ cell tumor, is rare on pediatric GCTs. Instead, deletions of chromosomes 1p and 6q have been reported.

We have surveyed GATA-4 expression in mouse yolk sac and human GCTs and cell lines using immunohistochemistry, in situ hybridization and northern blot. Based on the results, we conclude that GATA-4 is a clinically useful marker of human YSTs, and speculate that it may play a role in maintenance of the malignant phenotype.

GATA-6 has been shown to be expressed in early stage mouse yolk sac, in several developing organs and has also been found in several malignancies. We have studied GATA-6 expression in later stage mouse yolk sac and pediatric GCTs by immunohistochemistry and in situ hybridization. The results of this study will soon be submitted. The same methods will be used to study whether FOG-1 or -2 and their regulating actions on GATA factors play any role in development of yolk sac or pediatric YSTs.

The role of uPA and its reseptors in GCTs are being studied by immunohistochemsitry. Our preliminary experiments have shown uPA and uPAR expression in selected pediatric YSTs. We will now study the expression of these factors in more detail in collaboration with prof. A. Vaheri and Dr. S. Mustjoki, University of Helsinki.

The gene for transcription factor GATA-4 in human is located in chromosome 8. To see whether the frequent expression of GATA-4 in pediatric YSTs is due to changes on chromosomal level, we are conducting fluorescense in situ hybridization (FISH) on fresh tumor samples, in collaboration with Dr. M. Wessman, University of Helsinki.

Siltanen S, Anttonen M, Heikkila P, Narita N, Laitinen M, Ritvos O, Wilson DB, Heikinheimo M. Transcription factor GATA-4 is expressed in pediatric yolk sac tumors. Am J Pathol. 1999 Dec;155(6):1823-9.