Indian Journal of Cancer Medknow Publications on behalf of Indian Cancer Society ISSN: 0019-509X EISSN: 1998-4774 Vol. 43, Num. 3, 2006, pp. 122-126

Indian Journal of Cancer, Vol. 43, No. 3, July-September, 2006, pp. 122-126

Original Article

Study of the RET gene and his implication in thyroid cancer: Morocco case family

Ainahi A, Kebbou M, Timinouni M, Benabdeljalil N, Fechtali T, Oufara S, El Antri S

Laboratoire de Physiologie et Pharmacologie "UFR Environnement et Santé" FST, Mohammedia, Université Hassan II, Maroc
Correspondence Address:Laboratoire de Physiologie et Pharmacologie "UFR Environnement et Santé" FST, Mohammedia, Université Hassan II, Maroc, abdelhakim.ainahi@pasteur.ma

Code Number: cn06019

Abstract

BACKGROUND: Multiple endocrine neoplasia type 2A (MEN 2A) is an autosomal dominant inherited cancer syndrome that affects multiple tissues derived from the neural crest. Inheritance of MTC is related to the presence of specific mutations in the RET proto-oncogene. Almost all mutations in MEN 2A involve one of the cysteines in the extracellular domain of the RET receptor.
AIMS: The objective of the present study was the biochemical and molecular characterization of the first Moroccan clinically established MEN 2A patient and at-risk family members.
SETTINGS AND DESIGN: This is a study on a family presented with MTC referred to our institute in 2004.
MATERIALS AND METHODS: Peripheral blood leukocyte DNA samples were isolated and amplified by polymerase chain reaction followed by restriction enzyme analysis and DNA sequencing.
RESULTS: We identified a heterozygous germ line missense mutation at codon 634 of exon 11 in the RET gene that causes a cysteine to arginine amino acid substitution in the DNA of the proband; this mutation was not found in the DNA of the parents or relatives.
CONCLUSIONS: The detection of mutated MEN 2A gene carriers enables us to differentiate high-risk members from those who bear the wild-type gene. Occasionally, application of RET proto-oncogene testing may lead to the detection of unexpected de novo mutation that could be transmitted to children.

Keywords: Calcitonin, de novo , diagnosis, medullary thyroid carcinoma, RET proto-oncogene

Multiple endocrine neoplasia type 2A (MEN 2A) is an autosomal dominant inherited cancer syndrome that affects multiple tissues derived from the neural crest.[1],[2] The syndrome is characterized by the association of medullary thyroid carcinoma (MTC), pheochromocytoma in 50% of cases and / or primary hyperparathyroidism in 20% of cases.[3],[4] The specific germ line point mutations in the RET gene have been found to be associated with the inheritance of the MEN 2A phenotype. The human RET proto-oncogene, located on chromosome 10q11.2, consists of 21 exons and encodes a transmembrane receptor tyrosine kinase that plays a role in the normal development, differentiation and neoplastic growth of neural crest lineages.[1],[4] RET protein has an extracellular domain including regions with homology to the cadherin family and a large cysteine-rich region, transmembrane domain and intracellular domain functions in the phosphorylation of tyrosine residues involved in the interaction with downstream targets and activation of signaling pathways.[4],[5] Under normal conditions, RET receptor is activated by a multicomponent complex involving one of its ligands (glial cell line-derived neurtrophic factor, neurturin, artemin and persephin) and one of their cell surface bound coreceptors (respectively, GFRa-1, GFRa-2, GFRa-3 and GFRa-4).[5] Almost all mutations in MEN 2A involve one of the cysteines in the extracellular domain of RET receptor encoded by exon 11 (codon 634) or exon 10 (codons 609, 611, 618 and 620).[3],[4] The detection of germ line mutations in the RET gene has important diagnostic and therapeutic impacts: First, genetic screening of patients at risk allows to identify disease gene carriers with very high specificity and sensitivity. Second, total thyroidectomy can be performed based on mutation carrier status in a prophylactic attempt, ideally in a premalignant stage of disease.[6],[7],[8] In this paper, we describe a MEN 2A kindred with de novo C634R RET proto-oncogene germ line mutation in exon 11. Although this phenomenon is not novel, the finding is the first of its kind in Morocco and since de novo mutation of MEN 2A has not often been described in the literature, this case could be added to the database.

Materials and Methods

Case presentation

This is a study on a family from the northwest of Morocco referred to our institute in 2004 and in which one member presented clinical MTC disease. This family has an apparently negative family history for relevant thyroid disorders and hypertension. The study included a total of six individuals from two generations. The index patient [II.3,[Figure - 1] was an 18-year-old girl. At the time of initial presentation, she presented general symptoms including palpitations, flush and a palpable thyroid nodule. In 2003, she underwent a total thyroidectomy with the lymph node surgery. Histological findings showed the presence of microscopic foci MTC in both lobes. Later, diagnostic imaging investigation comprising metaiodobenzylguanidine (MIBG) scintigraphy revealed intra-adrenal pheochromocytoma. Subsequently, the patient underwent adrenalectomy and histopathology confirmed the diagnosis of pheochromocytoma. The patient had no children and her parents were both alive and did not present any sign related to MEN 2A. A pro forma was completed for the index patient, including patient age, sex, date of diagnosis, therapeutic modalities, clinical status, previous hormonal investigation and histological findings. Classification of the index patient was done based on clinical data according to the international RET mutation consortium definitions.[3] As routinely performed in all MTC patients, the index patient and at-risk family members were submitted to RET mutation genetic screening. Prior to this, all individuals included in the study were fully informed and their informed consents were obtained.

Endocrine testing

For detection of MTC, basal plasma calcitonin concentrations (bCT) were measured by two-site immunoradiometric assay (IRMA) using the ELSA-hCT commercial kit (CIS Bio International, Gif sur Yvette, France). To assess parathyroid function, total calcium was determined using routine diagnostic testing and serum parathyroid hormone level (PTH 1-84) was determined by IRMA method using the ELSA-PTH commercial kit (CIS Bio International, Gif sur Yvette, France). To detect adrenal pheochromocytoma, 24-hour urine specimens were collected for estimation of the extraction of metanephrines. The follow-up of MTC, based on the determination of the bCT level and the carcinoembryonic antigen (CEA) value measured by IRMA method using the ELSA2-CEA commercial kit (CIS Bio International, Gif sur Yvette, France), was done 1 year after surgery to detect persistent or recurrent disease.

DNA analysis for mutations of the RET proto-oncogene

High molecular weight DNA was isolated from peripheral blood leukocyte samples and collected on ethylenediamine tetraacetate according to standard protocols.[9] After measurement of DNA concentration and DNA/protein ratio, the DNA was used for polymerase chain reaction (PCR) amplification. The amplification of the DNA segment containing codon 634 in exon 11 was done as described by Donis-Keller et al using the following primers (11F, 5′-CCTCTGGCGGTGCCAAGCCTC-3′; 11R, 5′-CACCGGAAGAGGAGTAGCTG-3′).[6] One hundred nanograms of genomic DNA was amplified in a final volume of 50 ml using 10 mM Tris-HCl (pH 8.3), 50 mM KCl, 1.5 mM MgCl2, 200 mM dNTPs, 1.25 units of Taq polymerase and 0.5 mM of each primer. Genomic DNA was denatured for 5 min at 94°C prior to 35 cycles at 94, annealing at 54°C, elongation at 72°C for 40 s at each temperature followed by 10 min 72°C polishing step. A negative control without DNA was applied in each reaction. The amplified DNA was analyzed on a 2% agarose gel. The presence of mutation was detected by digesting an aliquot of the PCR product with the restriction enzymes HhaI, RsaI and Hae III at 37°C for 3 h. Then, the product was examined on a 2.5% agarose gel and the bands visualized by ethidium bromide staining. The only informative digestion was that with HhaI. A new site for this enzyme is, in fact, generated by the Cys 634 Arg mutation; as a consequence, the amplified band of 235 bp is digested in two lower bands of 174 and 61 bp. The 235-bp band, corresponding to the nonmutated allele, was still detectable, indicating that the mutation was present in a heterozygous state [Figure - 2]. The presence of the mutation was confirmed by direct sequencing using the same primers as for PCR amplification. Thus, PCR product of exon 11 was purified using exonuclease I and shrimp alkaline phosphatase (Amersham Life Science, Cleveland, OH) to remove the excess of primers and deoxyribonucleotides. The purified products were subjected to 25 cycles (96°C for 20 s, 50°C for 10 s and 60°C for 4 min) with sense or antisense primer using fluorescence-based dideoxyterminator cycle sequencing (ABI PRISM big dye terminator cycle sequencing ready reaction kit with AmpliTaq Polymerase, FS, PE applied bio system, Warrington, United Kingdom). The products were eluted through a Centri-Sep spin column and subjected to gel electrophoresis. Data collection and analysis were performed on an automate 3130 DNA sequencer (ABI PRISM 3130 genetic analyzer, applied bio system Inc, Foster City, CA, USA). A heterozygous mutation appeared: A transition occurred at position 634, replacing a T with a C resulting in the substitution of a cysteine with an arginine [Figure - 3]. The presence of mutations in RET in the hot spot exons has been reported in a number of publications. Thus, to ensure that no other relevant mutations have been overlooked, the exons 10, 13, 14, 15 and 16 were amplified and sequenced from genomic DNA using primers and conditions described by Brendt et al.[5] No mutations were detectable in these exons.

Results

We found by endocrine screening test an elevated serum bCT. The evaluation for suspected pheochromocytoma demonstrated increased urinary metanephrines [Table - 1]. These laboratory results confirmed the clinical diagnosis of MEN 2A with a clinical picture characterized by MTC and pheochromocytoma. One year after surgery, the serum CEA level was within the normal range, whereas serum bCT level remained elevated; the value was 119 pg/ml. The normal serum calcium and serum intact parathyroid hormone values indicated that the parathyroid glands were unaffected.

By DNA testing, we identified a heterozygous germ line missense mutation at codon 634 of exon 11 in the RET gene that causes a cysteine to arginine amino acid substitution in the DNA of the proband and then confirmed the diagnosis established from clinical and biochemical parameters. Next, presymptomatic identification of the carriage of mutated RET proto-oncogene was attempted in the remaining family members: Three sisters, one brother and both parents. They were shown to bear the wild-type gene [Figure - 4] and they could therefore be excluded from further clinical screening. Finally, using a parental test (data not shown), we ruled out the possibility of no paternity; therefore, the finding was a case of de novo RET gene mutation associated with MEN 2A that could be transmitted to the children. In addition, results of known RET gene polymorphisms analyses were negative in our MTC case (data not shown).

Discussion

Mutations in the hot spot exons and newly detected risk exons 5 and 8 of the RET proto-oncogene have been well characterized and several groups have studied the disease phenotype-genotype, allowing prediction of the clinical manifestations of specific mutations in MEN 2A kindred.[10],[11],[12] Many studies of RET mutations in inheritable MTC have been published in different countries. Here, we report, for the first time in our country, the identification of a case of MEN 2A associated with de novo C634R RET sequence alteration. Missense mutations at codon 634 coding for one of five cysteine residues of the extracellular part of the protein are frequent in MEN 2A; Cys ⁶³⁴sub Arg, in particular, is the one most frequently associated with this syndrome.[3] The mutation detected in our case, as well as most other studies, is concentrated in the cysteine residues of the RET receptor. This mutation results in ligand-independent receptor dimerization and autophosphorylation, converting the mutated allele to a dominant transforming gene.[7],[13],[14],[15],[16]

In classical MEN 2A cases, the substitution of cysteine by arginine in the mutations involving codon 634 is significantly predictive for the development of pheochromocytoma and hyperparathyroidism disease.[17],[18] However, in the kindred that we have described here, the parathyroid glands seem to be unaffected. An explanation for this clinical presentation is that the patient was young and hyperparathyroidism could develop later in advanced age. This fact is in agreement with some reports indicating that disease penetrance, age at onset and clinical manifestation of the disease can be quite variable within carriers of the same RET mutation.[17],[18] It also possible that early thyroidectomy with removal of some parathyroid glands had altered the natural course of the parathyroid disease. Furthermore, the results derived from RET gene polymorphisms analyses do not permit us to exclude the possible role of the other variants in RET or other related genes in the final presentation of the disease.[19]

Elevated bCT levels in MEN 2A patient are indicative of the presence of MTC; persistent high values after total thyroidectomy predict usually residual lymph node metastases or distant tissues. Calcitonin production by the pheochromocytoma has also been described in a few reports, but actually it is an exceptional rarity case report.[20] In our case, the persistent high calcitonin level during the first postoperative year with normal CEA level is in fact consistent with incompleteness of the first surgical procedure. This clinical finding is quite in line with data emphasizing the need for a systematic locoregional lymphadenectomy in addition to thyroidectomy.[21],[22]

In the literature, rare cases of de novo RET mutations have been described in which germ line alterations are found in one affected person and his offspring but not in his parents. Usually, the mutated allele was of paternal origin, suggesting that paternal RET proto-oncogene may be sensitive to mutation during spermatogenesis or during cell division after early fertilization.[23] Deeper investigations would be needed to elucidate the underlying mechanisms for this relationship. Unfortunately, we could not establish whether the parental alleles were involved in the transmission of the disease because our laboratory suffers from lack of different intragenic polymorphic markers.

Conclusion

Acknowledgment

References

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