We describe a patient who is a homozygous carrier of a germline TP53 mutation, p.R337H, having inherited the same mutant allele from both parents. The patient, currently aged 9 years, developed ACC at age 11 months and has been under clinical follow-up for other health outcomes, including cancers of the Li-Fraumeni spectrum, since that time. At age 9 years, the patient was healthy, with normal development, normal cardiopulmonary/exercising capacity and no suspicious proliferative lesions detectable using standard whole-body MRI.
In western Europe, it has been estimated that germline TP53 mutations spontaneously occur at a rate of 1:5,000 births . Approximately 50% of mutation carriers develop cancer by age 30. Carriage of two mutant alleles in the classic DNA-binding domain region of the gene (either through de novo mutation on both alleles or through inheritance of a mutant from both paternal and maternal sides) is therefore predicted to be an extremely rare event; indeed, there have been no reports of such an event. In Brazil, however, the existence of a founder mutation, p.R337H, which is present in approximately 0.3% of the dense population in the southern and southeastern regions of the country, make the occurrence of homozygosity for germline TP53 mutations more likely than anywhere else in the world. Prior to this report, homozygosity for p.R337H had been observed in one subject in a study of 55 Brazilian paediatric and adult patients with apparently sporadic ACC. That patient, a girl diagnosed with ACC at 12 months of age and whose parents were unaffected carriers, was healthy and had not developed another cancer at age 10 years , Latronico and Fragoso, personal communication 2011].
Several studies have shown that germline mutations in cancer predisposition genes may have a dose effect, resulting in a more severe phenotype in homozygotes than in heterozygotes. For example, the heterozygous state of the CHEK2 1100delC variant, which predisposes carriers to breast cancer, is associated with an OR of 1.5-3.0 (corresponding to a lifetime risk of 20-25%), while the homozygous state is associated with a greater than fourfold increase in the lifetime risk compared with the general population [31, 32]. Furthermore, several cancer-predisposing mutations in tumour-suppressor genes display different phenotypes in heterozygotes and homozygotes. This is the case for mutations in the PALB2, BRIP1 and ATM genes, where heterozygotes have an increased lifetime risk of breast cancer, and homozygotes are diagnosed with multisystemic genetic syndromes (Fanconi’s anaemia for the first 2 genes and ataxia-telangiectasia for the latter). There are also situations where homozygous and heterozygous states for a mutation in the same gene are each associated with different genetic syndromes, such as the Lynch (LS) and Childhood Cancer (CCS) syndromes associated with heterozygous and homozygous germline mutations in the MMR genes [33, 34].
In the present case, our observations do not support the hypothesis that inheritance of two mutant TP53 alleles may lead to a compound phenotype with increased risk for early onset cancer. It is possible that the absence of cumulative effects is due to the particular structural properties of the p.R337H mutant protein. Based on the structural analysis of a peptide encoding the oligomerisation domain, it has been shown that replacement of arginine by histidine at position 377 perturbs the formation of a hydrogen bond that links R377 on one p53 monomer to D352 on another p53 monomer, thus forming a dimer . At pH 7, histidine at position 377 retains the capacity to donate a H-bond. However, at pH 8, this capacity is lost, thus preventing dimerisation at slightly elevated pH levels. In the present case, we speculate that the proteins encoded by the two mutant alleles can form dimers in neutral pH conditions, which also occurs between one mutant and one wild-type monomer in patients who inherit only one mutant p.R337H allele. However, upon a small pH increase, the hydrogen bonds between the monomers, which form the homodimer consisting of the two mutant proteins, would be expected to break.
Recent experimental studies have shown that p53 plays a critical role in controlling cell bioenergetics and, specifically, mitochondrial metabolism . In the mouse, lack of p53 leads to impaired cardiorespiratory fitness and loss of aerobic competence. Mice lacking functional p53 show a decrease in maximum exercise capacity and are less responsive to the effects of training than their p53-competent counterparts [16, 17]. However, such effects have not been documented in humans thus far. The results of the cardiopulmonary exercise test performed here indicate that, despite carrying two TP53 mutant alleles, our patient has preserved functional capacity, as demonstrated by peak power output, peak oxygen uptake, and an anaerobic threshold within the limits of normality . Moreover, ventilatory efficiency and hemodynamic responses to exercise were also normal. If mitochondrial abnormalities were to be present in our patient, we would expect to observe an early anaerobic threshold, reduced peak oxygen uptake, and ventilatory inefficiency. Because individuals with metabolic abnormalities, such as patients with McArdle’s disease, may present hyperventilation during exercise without blood lactate accumulation , a preserved ventilatory anaerobic threshold, as observed in our patient, may not assure normal muscle oxidative metabolism. However, normal maximal exercise capacity and a normal anaerobic threshold are strong indicators of preserved muscular oxidative capacity. In healthy individuals, there is a strong association between muscle respiratory capacity and the anaerobic threshold . Although an influence of previous or current androgen excess on energy metabolism in this patient cannot be excluded, such an influence is unlikely because cardiovascular function was assessed many years after the normalisation of the hormone levels and bone age results were normal. Therefore, the cardiopulmonary exercise test results for our patient indicate that inheritance of two mutant TP53 p.R337H alleles does not appear to affect energy metabolism in humans by the age of 10 years.