Germline variants profiling of BRCA1 and BRCA2 in Chinese Hakka breast and ovarian cancer patients

With the development of the economy and society, women are increasingly stressed at work and in their personal lives. Additionally, and the incidence of breast cancer and ovarian cancer is on the increasing [1]. Worldwide, breast cancer has surpassed lung cancer as the most common cancer in women, and it is the leading cause of cancer death in females. Ovarian cancer is another one of the most common cancers in women and one of the leading causes of death in women [2]. China is in the stage of cancer transition. The cancer spectrum is changing from developing countries to developed countries, and the burden of breast and ovarian cancer is gradually increasing [3]. Germline mutations in breast cancer susceptibility gene 1 (BRCA1) and/or breast cancer susceptibility gene 2 (BRCA2) confer an increased risk of breast and ovarian cancers [4].

BRCA1 is located on chromosome 17, contains 24 exons and encodes a multidomain protein containing 1,863 amino acids [5]. BRCA2 is located on chromosome 13, contains 27 exons, and encodes a multidomain protein containing 3,418 amino acids [6]. The primary role of the BRCA1 and BRCA2 genes is to maintain the integrity of the genome, and they act as tumour suppressor genes [6]. Germline mutations in the BRCA1 and BRCA2 genes predispose persons to breast and ovarian cancer [4]. Mutations in the human BRCA1 and BRCA2 genes may be race-specific in a given region and region-specific in a given ethnic group [7, 8].

Hakka is a Han ethnic group with a unique genetic background and originates from the Hakka ancestors of the Han nationality in Central China. They migrated southward for many times and united with the ancient Yue residents in Guangdong, Fujian and Jiangxi [9]. Meizhou City is located in the northeastern of Guangdong Province and has a large Hakka population. However, limited information about the BRCA1 and BRCA2 mutations in this population is available in databases. This study retrospectively analysed the results of screening for genetic mutations of the BRCA1 and BRCA2 in breast and ovarian cancer patients among this population.

The BRCA genes are an important genes that determines the genetic susceptibility to cancer by participating in the regulation of DNA damage and repair, cell growth and apoptosis and by playing an indispensable role in maintaining the genetic stability of cells [26, 27]. Variants in the BRCA genes can lead to breast and ovarian cancer. Screening for BRCA gene mutations can effectively assess and predict the risk for breast and ovarian cancer. Thus, they can indicate the appropriate intervention to reduce the incidence of the disease and guide a precise treatment.

There are relatively few complete data on BRCA gene mutations in the Chinese population. At present, there is a gap in research on BRCA mutations in breast cancer and ovarian cancer patients in the Chinese population. Both of the BRCA1 and BRCA2 gene fragments are relatively long, with many diverse variants dispersed throughout the genes. Mutation types in different populations vary greatly, making it difficult to identify specific hotspot mutations. Studies have found that certain mutations are more common in certain populations, known as the founder effect, and these are called founder mutations. BRCA founder mutations have been identified in some ethnic groups worldwide. For example, BRCA1 c.68_69delAG, BRCA1 c.5266dupC and BRCA2 c.5946delT in Ashkenazi Jews [11], and BRCA1 c.5266dupC and BRCA1 c.4035delA are common in Polish patients [28]. The most common pathogenic variant in BRCA1 was c.981_982delAT, and in BRCA2 c.3195_3198delTAAT [29]. The c.303 T > G, c.1623dupG, and c.4122_4123delTG variants in BRCA1 are frequently found in the African patients with breast cancer [30]. The c.5266dupC, c.5177_5180delGAAA, and c.5251C > T variants in BRCA1 and the c.2808_2811delACAA and c.1138delA variants in BRCA2 were the most common variants among breast and ovarian cancer patients from Brazil [18]. BRCA1 ex9-12del is the most common variant in Mexican patients [31], and BRCA1 c.5095C > T is the most common variant in Arab breast and ovarian cancer patients [32]. BRCA1 c.68_69delAG is the most common variant in South Asian patients [33] and Latina patients residing in southern California [34]. BRCA2 c.3922G > T is a founder mutation in the Puerto Rican population [35]. The BRCA1 c.5266dupC mutation is recorded as the founder mutation in Italian [36], Northeastern Romanian [37], and Turkish populations [38]. BRCA1 c.5266dupC and c.181 T > G are founder mutations in the Polish population [39]. BRCA1 c.3319G > T is a founder mutation in the Western Denmark [40]. Slavic BRCA1 and BRCA2 founder mutations include BRCA1 c.5266dupC, BRCA1 c.4034delA, and BRCA1 c.68_69delAG [41]. BRCA1 c.4136_4137delCT and c.1140dupG are founder mutations in the Middle Eastern population [42]. BRCA1 c.798_799delTT is a founder mutation in the North African population [43].

In 2016, BRCA1/2 germline mutations were screened in 5,931 unselected Chinese women with breast cancer, and this study found that the BRCA1 c.5470_5477del was the most common variant in this population [44]. In 2017, Lang et al. enrolled 2,991 breast cancer patients and 1,043 healthy individuals in their study. They found that the most common BRCA1 mutation was c.5470_5477del, and the most common BRCA2 mutations were c.470_474del and c.3109C > T [45]. Wang et al. also found that BRCA1 c.5470_5477del was highly prevalent in a population of Chinese women population [46]. Studies have shown that BRCA1 c.5470_5477del was a founder mutation in Chinese Han ovarian cancer patients [47] and Chinese Han breast cancer patients [48]. A meta-analysis conducted by Kim et al. on population samples from mainland China in 2016 found that BRCA1 c.981_982delAT and BRCA2 c.3195_3198delTAAT were highly prevalent in mainland Chinese population [29]. In 2018, Kwong et al. analysed more than 600 samples from breast cancer patients in Hong Kong and more than 80 samples from Chinese patients who were overseas and found that the BRCA1 c.964delG and BRCA2 c.3109C > T mutations are common in the local population of Hong Kong [49]. In a recent meta-analysis of BRCA1 and BRCA2 gene variations in Chinese individuals, c.5470_5477del, c.2612C > T, and c.3548A > G in BRCA1, and c.3109C > T, c.2806_2809delAAAC, and c.5164_5165delAG in BRCA2 were the most common variants in this population [25]. In general, BRCA1 c.5470_5477del is considered to be a hotspot and founder mutation in the Chinese population.

The BRCA1 c.5470_5477del variant is not found in the Hakka population. Among the Hakka population in this population, the most common BRCA1 pathogenic variant is c.2635G > T (p.Glu879*) in this study. This variant is predicted to encode a truncated nonfunctional protein. BRCA1 c.2635G > T, a reported mutation among Hong Kong Chinese patients [50, 51], patients with breast cancer from Malaysia [52], and breast and/or ovarian cancer patients from Singapore [53, 54]. However, this variant is relatively rare in these populations and is not a common variant. This variant is not seen in other populations. Another common mutation BRCA1 c.3756_3759delGTCT has been detected in some populations, such as Thai [55], Polish [56], Belarusian [57], Italian [58], French-Canadian [59], and Czech populations [60]. BRCA1 c.4065_4068del has been detected in some populations [61‐63]. Another study showed that c.4065_4068del is one of the three most common BRCA1 variants in Chinese ovarian cancer patients [47]. In the BRCA2 gene, c.5164_5165delAG has been detected in the Chinese Han population [64], Macau population [65], and Taiwanese populations [66]. BRCA2 c.2339C > G has been detected in Taiwanese [67], and Japanese [68] individuals. BRCA2 c.2806_2809del has been detected in Mexican individuals [69].

In addition, there were 3 male breast cancer patients, accounting for 0.21% (3/1430) of the breast cancer patients in this study. Male breast cancer is a rare malignancy that accounts for less than 1% of all breast cancers [70] in some populations. It accounts for 0.48% of cases in the South Korean populations [71], 0.6% in the Australian population [72], 0.9% in the American population [73], and 0.55% in the Danish population [74]. Of course, there are some populations with higher rates of breast cancer in men. For example, the male breast rate is 1.1% in Northern India [75], and it is higher in some populations in Africa (6.2% in North Uganda [76], 2.6% in Burkina Faso [77], and 3.2% in 27 African countries [78]). Epidemiological differences between different groups of people may be related to region, race and living environment. Studies have shown that the major risk factors for the development of male breast cancer include advancing age, hormonal imbalance, radiation exposure, and a family history of breast cancer, but the most relevant risk factor is mutations in the BRCA2 gene [79, 80]. None of the three male breast cancer patients in this study had BRCA mutations. Understanding of the biology, clinical manifestations, genetics and treatment of male breast cancer is evolving, but due to the rarity of the disease, it is not well understood at present. More in-depth research is needed.

In general, the prevalence and spectrum of the BRCA1 and BRCA2 genes in the Hakka patients with breast cancer and ovarian cancer from southern China are different from those in other ethnic groups. This study provides a basis and serves as a reference for clinical counselling and the prevention and treatment strategies of breast cancer and ovarian cancer based on genetic screening. Identifying hotspot variants is an effective way to improve genetic counselling because molecular testing can target the hotspot variants, thereby enabling faster and cheaper testing. Clinical BRCA1 and BRCA2 testing enables the identification of individuals at elevated risk for hereditary breast and ovarian cancer. The results of this study can provide local patients with more information about pretest and post test genetic testing. Such information includes why it is indicated, possible test outcomes, implications of the test results for family, economic wellbeing, psychosocial wellbeing, and cancer surveillance and prevention options. Thus, genetic counselling was provided to patients.

Although this study has identified some hotspot variants in the Hakka population, we cannot rule out the possibility that other hotspot variants may exist in a larger Hakka patient population. This is one of the limitations of this study. In addition, participants were identified as Hakka through questionnaires, and no population genetic information was collected and analysed on these participants in this study. This is another shortcoming of this study. Finally, in clinical treatment, although the mutation of BRCA gene was taken into consideration when choosing treatment options, the correlation between the BRCA gene mutation and the prognosis of different treatment options was not analysed. This is one of the deficiencies of this study. In the future, BRCA gene mutation studies with a larger sample size should be carried out in China, including multiethnic studies, and unified standards should be adopted to establish a more complete BRCA gene mutation database that is consistent with the characteristics of the Chinese population. We believe that this study can complement the BRCA gene mutation information in the Chinese population.

In this study, the BRCA gene mutations accounted for a certain proportion of the patients with breast cancer and ovarian cancer in the Hakka population of southern China. In this population, the most common pathogenic variant in the BRCA1 gene was c.2635G > T, and the most common pathogenic variant in the BRCA2 gene was c.5164_5165delAG in BRCA2 gene in this population. The prevalence and spectrum of variants in the BRCA1 and BRCA2 genes in the Hakka patients from southern China are different from those in other ethnic groups.