Impaired Planar Germ Cell Division in the Testis, Caused by Dissociation of RHAMM from the Spindle, Results in Hypofertility and Seminoma

Abstract

Hypofertility is a risk factor for the development of testicular germ cell tumors (TGCT), but the initiating event linking these pathologies is unknown. We hypothesized that excessive planar division of undifferentiated germ cells promotes their self-renewal and TGCT development. However, our results obtained from mouse models and seminoma patients demonstrated the opposite. Defective planar divisions of undifferentiated germ cells caused their premature exit from the seminiferous tubule niche, resulting in germ cell depletion, hypofertility, intratubular germ cell neoplasias, and seminoma development. Oriented divisions of germ cells, which determine their fate, were regulated by spindle-associated RHAMM-a function we found to be abolished in 96% of human seminomas. Mechanistically, RHAMM expression is regulated by the testis-specific polyadenylation protein CFIm25, which is downregulated in the human seminomas. These results suggested that spindle misorientation is oncogenic, not by promoting selfrenewing germ cell divisions within the niche, but by prematurely displacing proliferating cells from their normal epithelial milieu. Furthermore, they suggested RHAMM loss-of-function and spindle misorientation as an initiating event underlying both hypofertility and TGCT initiation. These findings identify spindle-associated RHAMM as an intrinsic regulator of male germ cell fate and as a gatekeeper preventing initiation of TGCTs. Cancer Res; 76(21); 6382-95. Ó2016 AACR.

cd44 deletion suppresses atypia in the precancerous mouse testis

“…Atrophy was diagnosed by observation of several seminiferous tubules devoid of germ cells (Figures B, 2C cf . 2A), but the consequences of this pathology on testis size were also macroscopically evident (Figure D) . This result is rather surprising for two reasons.…”

“…Second, the double mutant mice not only lack CD44 but express p53 (Figures N, 1M cf . 1O), which exacerbates atrophy …”

“…This hypothesis is supported by the testicular phenotype of the hmmr m/m mouse . In this model, deletion of the RHAMM C‐terminus abolishes spindle association of the protein and disrupts the oriented division of undifferentiated spermatogonia, causing their premature displacement from the basal compartment of the seminiferous tubule niche, with two outcomes: (i) germ cell depletion, testicular atrophy and hypofertility; (ii) the formation of precancerous lesions (expressing placental alkaline phosphatase—PLAP—a marker of GCNIS) and appearance of cells with atypical morphology that could progress to seminoma …”

“…RHAMM, a mitotic spindle‐associated protein, highly expressed in testicular germ cells, is downregulated in ca. 90% of human seminomas .…”

“…RHAMM, a mitotic spindle‐associated protein, highly expressed in testicular germ cells, is downregulated in ca. 90% of human seminomas . Genomic alterations in locus 5q34 (where RHAMM is encoded) of TGCTs, led to the suggestion that it bears a candidate TGCT suppressor …”