Uterine abnormalities have been reported to occur in approximately 75% of women affected with HPT-JT (Fig. 1) from 9 families with proven germline HRPT2 mutations. These women suffered from menorrhagia in their second to fourth decades, and often required hysterectomy, which revealed the presence of uterine tumours. Histological analysis revealed the occurrence of benign and malignant uterine tumours. The majority (>85%) of the uterine abnormalities were benign and consisted of adenofi- bromas, leiomyomas, adenomyosis and endometrial hyperplasia. The malignant tumours, which were found in 2 of the 15 women (i.e. <15%) from this study, consisted of adenosarco- mas. The affected women from these families often had multiple miscarriages and were found to be significantly impaired in their ability to have offspring when compared with their unaffected female relatives and to their affected male relatives.
Other tumours, including Hurthle cell thyroid adenomas, papillary thyroid carcinomas, pancreatic adenocarcinomas, colonic carcinoma, prostate carcinoma, breast cancer, lipomas and testicular mixed germ cell tumours have been reported in 1 to 3 patients with HPT-JT (Table II). An analysis of the frequencies of the occurrence of such tumours in 193 HPT-JT patients from 45 families, of which 34 had proven HRPT2 mutations, reveals that the frequencies of colorectal, prostate, breast and pancreatic cancers are 1.6%, 1.6%, 1.0% and 0.5%, respectively. These low frequencies are unlikely to be significantly above that of the normal population and it may be possible that these tumours are not associated with the HPT-JT syndrome.
Molecular genetics of the HPT-JT syndrome
Identification of the gene causing HPT-JT and causative mutations
HPT-JT is inherited as an autosomal dominant disorder and linkage studies in families mapped the gene causing HPT-JT, which is referred to as HRPT2, to chromosome 1 q21 -31.
studies refined this location to a 12cM region that contained 67 potential candidate genes. Mutations were identified in one of these genes which consisted of 17 exons and spanned 1.3Mb of genomic DNA. This gene, referred to as HRPT2, has two transcripts; one of 2.7Kb, which encodes a ubiquitously expressed and evolutionarily conserved 531 amino acid protein named PARAFIBROMIN and the other, of 4.4Kb, which has not yet been characterised. To date 63 heterozygous HRPT2 mutations have been reported (Fig. 2 and Table III). These consist of 26 different heterozygous germline HRPT2 mutations in patients with HPT-JT, FIHP and parathyroid carcinomas, and 22 different heterozygous somatic HRPT2 mutations in parathyroid adenomas and carcinomas. The mutations are scattered throughout the coding region (Fig. 2), although currently, no mutations have been reported in exons. Exons 1, 2 and 7 are more frequently involved and harbour 33%, 18% and 22%, respectively of the mutations. The over-representation of mutations in these exons is not due to their larger sizes as an examination of exons 8, 14, 15 and 16, which are of similar sizes, reveals these to contain between 0% and 7% of all mutations (Fig. 2). Over 80% of the HRPT2 mutations found in germline DNA and in somatic DNA of tumours, are nonsense or frameshift mutations (Table III) that are predicted to result in a functional loss of the PARAFIBROMIN protein because of premature truncation. Four different missense mutations have been reported: one affects the initiation methionine and is thus likely to prevent translation; two affect evolutionary conserved leucines which are replaced by a helix disruptive proline and hence likely to lead to a deleterious structural alteration of the protein; and one affects an evolutionary conserved aspartate within the Paf1 binding domain (see below) and this aspartate is replaced by an as- paragine (Table III). The substitution of the normal negatively charged aspartate residue for a polar but uncharged asparagine is predicted to disrupt the interaction between PARAFIBROMIN and the Paf1 complex that interacts with RNA polymerase II.
Figure 2 – Schematic representation of the genomic organisation of the HRPT2 gene and its mutations.
The human HRPT2 gene spans 1.3Mb of genomic DNA and encodes a 531 amino acid protein, called PARAFIBROMIN. The 1593 bp coding region is organised into 17 exons (sizes indicated) and 16 introns. The 5′-part of exon 1 and the 3′-part of exon 17 are untranslated (stippled boxes). The start (ATG) and stop (TGA) sites, in exons 1 and 17 respectively, are indicated. The locations of 63 HRPT2 mutations that have been reported in the period (6, 11, 12, 15, 17-20, 23, 24, 29, 30) 2002-2005 are shown; these include 12 different nonsense, 4 different missense, 4 different splice site and 32 different frameshift mutations. The 26 different germline mutations are indicated by solid lines, 22 different somatic mutations by short dashed lines and the 4 mutations where the status is unknown (u) by long dashed lines. Germline or somatic mutations which have been reported more than once in unrelated individuals are highlighted: a Arg9Stop, reported twice; b IVS1+1g^a, reported 3 times; c Tyr54Stop, reported twice; d Leu64Pro, reported twice; e Arg234Stop, reported 3 times and f 679insG, reported 5 times. Exons 1, 2 and 7 have significantly more mutations than the other exons, and contain 33%, 18% and 22%, respectively, of the mutations. Indeed, the mutation number per 100 bp of DNA sequence for exons 1, 2 and 7 are 16, 10 and 7, respectively, and these are significantly higher (p<0.01 for exons 1 and 2, and p<0.05 for exon 7) when compared with other exons. Thus the over-representation of mutations in exons 1, 2 and 7 is not a function of their larger sizes.
Loss of heterozygosity (LOH) involving this region of chromosome 1q has been reported in 32% of parathyroid adenomas (n=38) and 70% of parathyroid carcinomas (n=10) . In addition, LOH of chromosome 1q has been reported in 7 renal hamartomas from 2 HPT-JT patients, one renal cell carcinoma, and one pancreatic carcinoma from HPT-JT patients. These observations of LOH in tumours from HPT-JT patients and the combined occurrence of inactivating germline and somatic mutations in tumours from these patients, indicate that the HRPT2 gene acts as a tumour suppressor consistent with the Knudson ‘two-hit’ model for hereditary cancer.
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