Tumour morphology predicts PALB2 germline mutation status

PALB2, a partner and localiser of BRCA2, is crucial for proficient homologous recombination repair of DNA double-strand breaks through its regulation of BRCA2 and its interaction with BRCA1 (Xia et al, 2006; Sy et al, 2009; Zhang et al, 2009). Bi-allelic inactivating mutations in PALB2 underlie Fanconi anaemia subtype N and have been shown to be associated with high risk of childhood cancers (Reid et al, 2007; Xia et al, 2007). Heterozygous germline loss-of-function mutations in PALB2 have been associated with increased risk of breast cancer (Rahman et al, 2007). The first study that reported an association between PALB2 mutations and breast cancer risk involved familial breast cancer cases and unaffected controls from the United Kingdom. Using only some information obtained from just 10 families, and under strong modelling assumptions, the average relative risk associated with 5 protein-truncating PALB2 mutations was estimated indirectly to be 2.3-fold (95% confidence interval (CI)=1.4–3.9) (Rahman et al, 2007). Subsequent population-based studies estimated the risk associated with PALB2 mutations to be higher (Erkko et al, 2008; Southey et al, 2010). For example, PALB2 c.1592delT was identified in 18 out of 1918 (0.9%) Finnish breast cancer cases unselected for family history compared with 6 out of 2501 (0.2%) unaffected controls (odds ratio (OR)=3.94; 95% CI=1.5–12.1). Using the family histories of the case carriers, PALB2 c.1592delT was estimated to be associated with a 40% (95% CI=17–77%) risk of breast cancer to the age of 70 years (Erkko et al, 2008). Similarly, PALB2 c.3113G>A was identified in 5 out of 1403 (0.4%) unselected Australian breast cancer cases and 0 out of 764 (0%) unaffected controls (Southey et al, 2010). Using the family histories of the five carrier cases, the estimated cumulative risk for PALB2 c.3113G>A was 91% (95% CI=44–100%) to the age of 70 years. Therefore, population-based studies of breast cancer that have directly used the family history data have estimated that at least some PALB2 mutations are associated with a breast cancer risk (penetrance) comparable to that of the average pathogenic mutation in BRCA2: 45% (95% CI=31–56%) (Antoniou et al, 2003). Mutations in PALB2 are rare (varying from 0.1% to 1.5% depending upon the population) (Foulkes et al, 2007; Rahman et al, 2007; Tischkowitz et al, 2007; Dansonka-Mieszkowska et al, 2010; Papi et al, 2010; Southey et al, 2010; Bogdanova et al, 2011; Casadei et al, 2011; Ding et al, 2011; Hellebrand et al, 2011; Teo et al, 2013a, 2013b) but for women carrying them, and their relatives who might also be mutation carriers, knowing their mutation status has the potential to be clinically important as carriers are at high risk of breast cancer. Identified mutation carriers could be informed of optimal, risk appropriate clinical screening and treatment. Potential therapies could include those that target homologous DNA repair dysfunction (Buisson et al, 2010). As PALB2 mutations have also been associated with increased risk of developing a second breast cancer (Tischkowitz et al, 2012), risk reducing surgery and treatment might also be considered by PALB2 mutation carriers. The integration of PALB2 mutation testing into clinical practice is still in progress and strategies that effectively identify potential PALB2 mutation carriers could help facilitate this important process. Characterisation of the morphology of breast cancers arising in PALB2 mutation carriers and non-carriers offers the possibility of identifying tumour morphological features predictive of an underlying germline PALB2 mutation, as they have been shown for underlying BRCA1 mutations (Lakhani et al, 1998; Southey et al, 2011; Hopper et al, 2012). This could be conducted at the time of diagnosis and therefore, be used to facilitate personalised treatment strategies, as well as enabling identification of those relatives who have also inherited a similar high breast cancer risk. Breast cancer tumour morphology can be suggestive of underlying familial, if not heritable, risk. We recently reported that, in a population-based sample of 375 women with early-onset breast cancer cases with no known high-risk mutation in a breast cancer susceptibility gene, minimal sclerosis, presence of circumscribed growth, extensive intraductal carcinoma and lobular growth patterns were independent predictors of increased breast cancer risk for their first-degree female relatives (2.0-fold to 3.3-fold increased risk for relatives, P<0.02 for all listed features). Relatives of the 128 (34%) index cases with none of these 4 features were at population risk (standardised incidence ratio=1.03, 95% CI=0.57–1.85), while relatives of the 37 (10%) index cases with two or more features were at high risk (standardised incidence ratio=5.18, 95% CI=3.22–8.33) (Dite et al, 2012). Breast cancer morphological features can also be used to identify women most likely to carry germline mutations in breast cancer susceptibility genes. It has been known for some time that some morphological features are more common in cancers arising in BRCA1 mutation carriers (Lakhani et al, 1998). These features have been identified by studying carriers across a wide range of ages at diagnosis and ascertained either because of their strong family cancer history or through population-based sampling. Lack of oestrogen receptor (ER) and progesterone receptor (PR) expression has also been reported to improve prediction of BRCA1 mutation status based on family history (Lakhani et al, 2002; James et al, 2006; Mavaddat et al, 2010). Using a population-based sample of 452 young women with breast cancer, we found that just two breast tumour morphological features (trabecular growth pattern and high mitotic index) were sufficient to identify 28 of the 29 BRCA1 mutation carriers in the study (Southey et al, 2011). Moreover, prediction of mutation status using these two features was more sensitive and specific than using family history alone, and when combined, the area under the receiver operator curve was in excess of 0.9. A detailed analysis of the morphological features of PALB2 mutation-associated breast cancers has not been previously conducted. Some information about the general morphology of breast tumours arising in PALB2 mutation carriers is available from work studying breast tumours carrying the Finnish founder mutation PALB2 c.1592delT. Mutation carriers with a family history of breast cancer were more likely to have ‘triple negative' tumours (negative for ER, PR, and human epidermal growth factor receptor 2 (HER2) expression) when compared with familial non-PALB2 mutation-associated breast cancers (54.5% and 12.2%, respectively; P<0.0001). The PALB2 c.1592delT-associated tumours were reported to be more often of higher grade and to have greater expression of Ki67, which is a cellular marker for proliferation than tumours arising in non-carriers of the mutation. Carrying this PALB2 mutation was also reported to be associated with reduced survival; comparing affected PALB2 mutation carriers, negative for HER2 expression, with a family history of breast cancer with affected non-carriers of BRCA1, BRCA2, or PALB2 mutations, the hazard ratio was estimated to be 4.57 (95% CI=1.96–10.64; P=0.0004) (Heikkinen et al, 2009). In this study, we conducted a standardised pathology review of 28 invasive breast cancers arising in women who carry a germline loss-of-function PALB2 mutation. The morphological characteristics of these 28 tumours were compared with those of a population-based sample of 770 unselected breast tumours that had undergone the same standard pathology review.