TMPRSS2‐ERG fusions linked to prostate cancer racial health disparities: A focus on Africa

Abstract Background The androgen‐regulated gene TMPRSS2 to the ETS transcription factor gene ERG fusion is the most common genomic alteration acquired during prostate tumorigenesis and biased toward men of European ancestry. In contrast, African American men present with more advanced disease, yet their tumors are less likely to acquire TMPRSS2‐ERG. Data for Africa is scarce. Methods RNA was made available for genomic analyses from 181 prostate tissue biopsy cores from Black South African men, 94 with and 87 without pathological evidence for prostate cancer. Reverse transcription polymerase chain reaction was used to screen for the TMPRSS2‐ERG fusion, while transcript junction coordinates and isoform frequencies, including novel gene fusions, were determined using targeted RNA sequencing. Results Here we report a frequency of 13% for TMPRSS2‐ERG in tumors from Black South Africans. Present in 12/94 positive versus 1/87 cancer negative prostate tissue cores, this suggests a 92.62% predictivity for a positive cancer diagnosis (P = 0.0031). At a frequency of almost half that reported for African Americans and roughly a quarter of that reported for men of European ancestry, acquisition of TMPRSS2‐ERG appears to be inversely associated with aggressive prostate cancer. Further support was provided by linking the presence of TMPRSS2‐ERG to low‐grade disease in younger patients (P = 0.0466), with higher expressing distal ERG fusion junction coordinates. Conclusions Only the second study of its kind for the African continent, we support a link between TMPRSS2‐ERG status and prostate cancer racial health disparity beyond the borders of the United States. We call for urgent evaluation of androgen deprivation therapy within Africa.


| INTRODUCTION
The most frequent somatic alteration in prostate cancer (PCa) involves an intrachromosomal translocation or deletion event on the long arm of chromosome 21. This genomic alteration appears to occur early in tumourigenesis, resulting in fusion of the androgen-regulated transmembrane protease serine 2 gene (TMPRSS2) to a member of the transcription factor erythroblastosis virus E26 transforming sequence family (ERG) 1 , leading to androgen-dependent overexpression of ERG. 2 The frequency of TMPRSS2-ERG fusions in PCa shows a notable racial disparity. A recent meta-analysis confirmed the highest prevalence rates in men of European ancestry (49%), with lower levels reported for men of Asian (27%) and African (25%) ancestries. 3 The latter includes three African American studies (73/260; 28%) and a single study from West Africa, specifically Ghana (47/262; 18%). There also appears to be racial discordance in the mechanism of TMPRSS2-ERG occurrence, with African Americans most commonly exhibiting fusion through deletion, and European and Asian Americans through translocation. 4 Intriguingly, African ancestry is significantly associated with high-grade (Gleason score ≥ 8) PCa, 5 which questions the role of TMPRSS2-ERG fusions in tumor development in African populations.

Reporting a 2.1-fold increase in high-grade PCa presentation in Black
South African men compared with African Americans, 6 we sought to determine the frequency, composition, and clinicopathological association of acquired TMPRSS2-ERG fusions within non-American men of African ancestry.

| Clinicopathological overview and ethics
Patients were enrolled at time of diagnosis as part of the Southern African Prostate Cancer Study (SAPCS) 6 and presented at one of the participating urological clinics within the Provinces of Limpopo or Gauteng of South Africa. All self-identified ethnically as Black South African, including a single Black Zimbabwean, from one of 11 ethnic groupings, with age and prostate specific antigen levels recorded at time of diagnosis (Table 1)

| TMPRSS2-ERG expression
Expression of the TMPRSS2-ERG fusion gene in each patient was calculated by normalizing the total number of fusion supporting reads to individual sequencing library sizes.

| Statistics
Variables between groups were analyzed using Fisher's exact test.
P < 0.05 were considered statistically significant.

| Frequency and distribution of TMPRSS2-ERG fusion
The frequency of TMPRSS2-ERG fusion gene occurrence was 12.8%  Though not statistically significant due to small sample size, there was a striking difference in the expression levels of patients presenting with either the proximal or distal ERG fusion coordinates, with higher expression detected from the distal isoforms ( Figure 2C).

| TMPRSS2-ERG validation, isoforms and ERG expression
Additionally, patients possessing the higher expressing distal ERG fusion coordinates presented earlier at diagnosis, with a median age of 69 years (range 55 to 73 years) compared with the three patients with the lower expressing proximal coordinates, at 72, 78, and 81 years ( Figure 2D).  While androgen deprivation therapy (ADT) administered through hormone blockers are traditionally used for treating advanced PCa, in Southern Africa orchidectomy is still common practice. Either way, the goal is to inhibit androgen receptor (AR) signaling. 16 Combining ADT with docetaxel, a taxane chemotherapy, has been shown to provide further survival benefit to patients. 17 The positive effect of docetaxel is believed to act, at least in part, through impairing AR activity. 18 In turn, TMPRSS2-ERG fusion positive patients appear to respond well to ADT and neoadjuvant docetaxel, compared with untreated patients. 19 More recently, a large study out of China suggests that TMPRSS2-ERG fusion is a biomarker for predicting response to ADT. 20 The lack of androgen-regulated TMPRSS2-ERG fusion within African populations biased toward aggressive disease presentation, calls for immediate assessment of the true benefit of ADT, taking into consideration significant long-term side effects of the more commonly practiced orchidectomy, within the continent.

| SUMMARY
To the best of our knowledge, this is the second only PCa study of its kind for Africa and the first for southern Africa. Built on observations made within African Americans and combined with the single study from Ghana, it is becoming evident that TMPRSS2-ERG fusions are significantly less common in driving prostate tumourigenesis within populations from Africa. This is further supported by an inverse association between presence and adverse outcomes, as well as our observed link with younger-onset low-risk disease.