16p12.2

FANCN,PNCA3

Germ-line bi-allelic and heterozygous loss-of-mutations in PALB2 are associated with different clinical disorders and outcomes. Bi-allelic inactivating mutations of PALB2 result in Fanconi anemia subtype N (FA-N, gene: FANCN), while heterozygous inheritance of a deleterious PALB2 mutation increases the lifetime risk of developing breast and pancreatic cancer. Loss of heterozygosity has not been consistently detected in tumors that develop in carriers of heterozygous PALB2 mutations (Hartley et al., 2014). How much germ-line mutations of PALB2 increase the risk of developing other malignancies, such as ovarian or lung cancers, remains to be determined (Phuah et al., 2013).

Fanconi Anemia (FA)

Just seven months after the identification and characterization of PALB2 as a novel BRCA2 binding protein (Xia et al., 2006), two independent groups identified a total of eight FA patients with biallelic mutations in PALB2: [(Xia et al., 2007): n=1, (Reid et al., 2007): n=7]. These patients exhibited a severe FA phenotype with pronounced congenital abnormalities and a high incidence of malignancies before age seven that was similar to the phenotype described for BRCA2-deficient FANCD1 patients (Alter et al., 2007; Hirsch et al., 2004; Wagner et al., 2004). Notably, these eight children developed 12 distinct malignancies (5X medulloblastoma, 3X Wilms tumors, 2X acute myeloid leukemia, 1X neuroblastoma and 1X hemangioendothelioma) before five years of age. One German patient experienced three different malignancies at 12 months of age, and there were cases of breast and pancreatic cancers present in the families (Reid et al., 2007). PALB2 was the 12^th identified FA gene, defining the FA-N complementation group.
At present, 22 FA or FA-like genes have been identified (Nepal et al., 2017). Except for the X-linked FANCB and the autosomal dominant RAD51 (FANCR), FA genes are autosomal recessive tumor suppressor genes. Based on the central activation step in the FA pathway, the monoubiquitination of the FANCD2/ FANCI protein dimer, one can distinguish the so-called early (or upstream) FA genes, FANC -A, -B, -C, -E, -F, -G, -L, -M, UBE2T (FANC-T) with no ubiquitination of FANCD2/I when mutated (Mamrak et al., 2017; Nepal et al., 2017). In contrast, late/downstream FA genes are not required for monoubiquitination of FANCD2 and FANCI. These late genes include BRCA2 (FANCD1), BRIP1 (FANCJ), FANCN/PALB2,/RAD51C (FANCO), RAD51 (FANCR), BRCA1 (FANCS), XRCC2 (FANCU), MAD2L2 (FANCV/polTheta) and RFWD3 (FANCW). Most FA patients have bi-allelic mutations in the upstream FA genes, especially FANCA, FANCC and FANCG, and show the characteristic clinical features of FA. These clinical features include progressive bone marrow failure around 7.6 years of age, various congenital anomalies, and a predisposition to acute myeloid leukemia and an assortment of solid tumors that occur in the second and third decade of life (Kutler et al., 2003). Congenital anomalies observed in FA patients can include microcephaly, short stature, skin pigmentation defects, hypogonadism, and radial ray anomalies. A significant number of FA patients also experience endocrine abnormalities (Rose et al., 2012).
In contrast to other FA complementation groups, FA patients that harbour biallelic mutations in PALB2/FANCN or its partner BRCA2/FANCD1 show clinically indistinguishable phenotypes of severe FA characterized by a very early onset and high penetrance of cancers before age seven (Reid et al., 2007). More than 90% of patients succumb to their malignancies before ten years of age. Notably, the spectrum of cancers found in FA patients from the FANCN/PALB2 and FANCD1/BRCA2 complementation groups is different than for other FA complementation groups, including frequent occurrences of medulloblastoma, Wilms tumor, neuroblastoma and hepatoblastomas (Alter et al., 2007; Tischkowitz and Xia, 2010). Bone marrow failure is usually not observed in FA-N patients (Reid et al., 2007).
On a cellular level, FA including the FA-N/PALB2 complementation group is a chromosome instability syndrome. Notably, FA patients are hypersensitive to agents which induce DNA interstrand crosslinks (ICLs). Specifically, cells from FA patients display a characteristic spontaneous and ICL-induced chromosome instability; this phenotype is typically utilized to diagnose FA (Auerbach, 2009). Recently, without functional testing, next generation sequencing based strategies have also been employed to diagnose FA in patients and at the same time identify the defective gene (De Rocco et al., 2014). Additionally, cells from FA patients display accumulation in G2-M of the cell cycle in response to ICLs (Bogliolo and Surralles, 2015). Other cellular functions affected in cells with defects in the FA pathway include sensitivity to aldehydes and oxygen, excessive cytokine production, and defects in the spindle assembly checkpoint, autophagy, cellular reprograming, unwinding of quadruplex and triplex DNA and microsatellite instability (Bogliolo and Surralles, 2015). Correction of the cellular phenotypes by expression of the appropriate FA gene can be utilized to determine the FA complementation group (Chandra et al., 2005; Hanenberg et al., 2002; Virts et al., 2015). For patients with a deficiency for FANCA, stem cell gene therapy might become a new treatment option (Hanenberg et al., 2017).

Breast Cancer

Back-to-back with the identification of PALB2 as the 12^th FA gene, Rahman et al. reported the identification of 10 out of 923 individuals (1.1%) from familial breast cancer pedigrees with mono-allelic loss-of-function germ-line mutations in PALB2 (Rahman et al., 2007). A lower frequency of PALB2 germ-line mutation (0.5 to 1%) was found in patients with or without a positive family history (Erkko et al., 2007; Foulkes et al., 2007). Further, founder mutations were detected in Finland (Erkko et al., 2007) and Canada (Foulkes et al., 2007).
The largest study to date included 311 women and 51 men from 154 families with loss-of-function germ-line PALB2 mutations, of whom 229 women and 7 men developed breast cancer. Biostatistical analyses revealed that the risk of developing breast cancer for PALB2 mutation carriers was increased by a factor of 9.07 (95% CI, 5.72 to 14.39) when compared to the breast cancer incidence in the general population (Antoniou et al., 2014). The cumulative risk of female heterozygous PALB2 germ-line mutation carriers to develop breast cancer by the age of 70 was as high as 35%. Thus, along with BRCA1 and BRCA2, PALB2 is among the genes that confer the highest breast cancer risk when mutated. In a recent German study of 5589 breast cancer patients without mutations in BRCA1/2, loss-of-function mutations in PALB2 accounted for 1.15% of cases and were also significantly associated with bilateral breast cancer occurrence (Hauke et al., 2018). Only 8 out of 40 patents with PALB2 germ-line mutations belonged to the triple negative breast cancer subtype (Hauke et al., 2018).
Breast cancer causing mutations of PALB2 include established nonsense, frameshift and splice site mutations, which all are thought to compromise the role of PALB2 in cellular responses to DNA damage. However, it is noteworthy that almost 50% of the PALB2 sequence alterations listed in ClinVar are missense variations of unknown clinical and functional significance. Importantly, loss of function of PALB2 is synthetically lethal with radiation, PARP inhibitors, and cisplatin and related compounds. While tumors, which are driven by mutations in PALB2, typically have loss of function of PALB2, normal tissues retain a functional copy of this gene. As such, radiation and/or PARP inhibitors or platinum compounds may be particularly effective against tumor cells with bi-allelic PALB2 mutations.
Interestingly, the relative risk for ovarian cancer was only increased non-significantly to 2.31 for PALB2 mutation carriers (Antoniou et al., 2014). This is surprising, as the PALB2 protein physically interacts with the products of other ovarian cancer susceptibility genes, specifically BRCA1, BRCA2 and RAD51C (Park et al., 2014b).

Pancreatic Cancer

While BRCA2, which encodes a partner of the PALB2 protein, is the most frequently mutated gene in hereditary pancreatic cancer (Shindo et al., 2017; Zhen et al., 2015), mutation of PALB2 is also an important cause of this disease (Jones et al., 2009). To date, truncating loss-of-function germ-line mutations in PALB2 have been associated with the development of pancreatic cancer. In some pedigrees of families with inherited pancreatic cancer, breast and other cancers have also been observed (Blanco et al., 2013; Zhen et al., 2015).