M6P/IGF2R loss of heterozygosity occurs frequently in human breast, liver and lung cancer [6, 20–22], and the remaining allele of 30 to 50% of these tumors contains an intragenic loss-of-function point mutation in the ligand binding domains . The M6P/IGF2R is also commonly mutated in gastrointestinal and endometrial malignancies because its coding sequence contains a poly-G region that is a mutational target in tumors with mismatch repair deficiencies and microsatellite instability [23, 24]. Functional studies show that the introduction of an exogenous wild-type M6P/IGF2R into human colorectal cancer cells with a single inactivated allele significantly decreases growth rate and enhances apoptosis . Conversely, loss of M6P/IGF2R expression promotes cancer cell growth by increasing intracellular signaling from both the insulin-like growth factor I receptor and the insulin receptors .
We demonstrate herein that loss of heterozygosity at the M6P/IGF2R locus in head and neck cancer is also associated with poor patient prognosis. Loss of heterozygosity in cancer can occur either because of chromosomal deletion or somatic recombination resulting in uniparental disomy . Comparative genomic hybridization studies in head and neck cancer [10, 38] demonstrate that 6q deletion frequency (> 50%) is similar to that which we observed at the M6P/IGF2R locus. This provides evidence that M6P/IGF2R loss of heterozygosity in head and neck cancer is due primarily to either gene deletion or chromosomal loss rather than somatic recombination.
Since chromosomal deletion can affect more than one gene, M6P/IGF2R loss of heterozygosity alone does not rule out the possibility that other adjacent genes also have a tumor suppressor function in head and neck cancer. However, our previous finding, that both alleles of the M6P/IGF2R are mutated in greater than 50% of squamous cell carcinomas of the lung , supports our postulate that the M6P/IGF2R is a key head and neck cancer tumor suppressor gene at chromosome location 6q.
The clinical trial from which the tumor specimens used in this study were derived showed improvements in relapse-free survival, locoregional control, and overall survival for those patients randomized to combined modality therapy . This investigation established that patients whose tumors had M6P/IGF2R loss of heterozygosity and were treated with radiotherapy alone had a significantly worse prognosis than their counterparts with a non-mutated allele. Conversely, patients with an intact M6P/IGF2R had a similar prognosis whether they received radiotherapy alone or combined modality treatment. Thus, our findings suggest that head and neck cancer patients with tumor M6P/IGF2R loss of heterozygosity would benefit most from combined modality treatment.
The mechanism by which M6P/IGF2R allelic loss in head and neck tumors increases the effectiveness of adjuvant chemotherapy is presently unknown, but it is clearly of clinical importance. A primary function of this receptor in placental mammals involves the degradation of extracellular IGF2. M6P/IGF2R allelic loss would therefore result in enhanced cellular bioavailability of this potent growth factor, thereby potentially increasing both cell proliferation and resistance to apoptosis . The M6P/IGF2R also facilitates the activation of TGFβ, a potent cell growth inhibitor that is secreted in an inactive form (reviewed in [15, 16]). M6P/IGF2R mutation is therefore one mechanism by which cancer cells can become refractory to TGFβ's mitoinhibitory effect.
We have previously shown that M6P/IGF2R mutation in squamous cell carcinoma of the lung is highly correlated with increased TGFβ concentrations in both the tumor and patient plasma . Elevated TGFβ in the latter stages of tumor progression directly contributes to enhanced tumor angiogenesis, metastasis formation and a decreased host immune response (reviewed in ). Together these findings predict that tumors with a mutated M6P/IGF2R would be more resistant to therapy than those with an intact receptor. Moreover, if some chemotherapeutic agents are not only directly cytotoxic to cancer cells, but also reduce tumor production of growth factors, such as IGF2 and TGFβ, adjuvant chemotherapy would be more useful in treating tumors with a mutated M6P/IGF2R, as observed in this study.
M6P/IGF2R is normally imprinted in mice with only the maternal copy of the gene being expressed . In contrast, both copies of the M6P/IGF2R are expressed in humans because genomic imprinting at this locus was lost in the primate lineage approximately 75 million years ago . Importantly, restoration of biallelic M6P/IGF2R expression in mice results in a marked reduction in offspring weight late in embryonic development that persists into adulthood . This demonstrates that M6P/IGF2R allelic loss or haploid insufficiency markedly enhances cell proliferation and/or survival during fetal development. Therefore, mutation of even a single allele of the M6P/IGF2R in human somatic cells is predicted to also promote cell growth.
Haploid insufficiency for tumor suppressor genes, such as Nf2, p27
Kip 1, p53, Ptch, Pten and TGFβ, is known to promote tumor formation (reviewed in [44, 45]). Yamada et al.  demonstrated that in patients chronically infected with hepatitis B and/or hepatitis C viruses, the M6P/IGF2R is mutated not only in hepatocellular carcinomas (HCCs), but also in the phenotypically normal hepatocytes adjacent to these tumors. Interestingly, only one M6P/IGF2R allele is inactivated in the adjacent cirrhotic tissue even when both alleles are mutated in the HCC. These findings are consistent with normal appearing, preneoplastic hepatocytes forming clonal masses in the liver because M6P/IGF2R haploid insufficiency affords them with a selective growth and/or survival advantage relative to normal hepatocytes .
The liver is not the only organ in which regions of normal appearing tissue have a clonal origin. Entire lobules and large ducts of normal breast tissue can be derived from a single progenitor cell , and LOH at various chromosomal locations is frequently detectable in morphologically normal lobules adjacent to breast tumors . Lung tumors and head and neck tumors that develop in a localized region also often have a common clonal origin [3, 4, 49]. The high frequency of M6P/IGF2R loss of heterozygosity observed in the head and neck tumors in this study suggests that the phenomenon of "field cancerization", first described by Slaughter and his colleagues  in 1953, may in part result from the clonal proliferation of mucosal epithelial cells with M6P/IGF2R allelic loss.