小鼠淋巴细胞hprt基因突变检测的研究进展

肿瘤的发生、发展与突变事件密切相关。人类在环境中接触的突变剂的检出依赖于致突变试验。以次黄嘌呤鸟嘌呤磷酸核糖基转移酶(hypoxanthine-guaninephosphoribosyltransferase,hprt)基因为报告基因的hprt突变试验是几个重要的致突变试验之一,其优点在于不仅能检测出突变剂的致突变能力大小,还可以对突变体进行突变谱分析,并且能为多种与hprt基因突变相关的人类疾病提供研究资料。以小鼠啮齿动物为试验模型的淋巴细胞hprt基因突变试验近年来得到广泛应用,本文对其研究进展作一综述。     

AIF基因真核表达载体的构建及其在MGC-803中的表达

[目的]检测胃癌标本中AIF基因是否存在碱基突变;构建AIF基因真核表达载体AIF—GFP;并检测其在胃腺癌细胞株MGC-803中的表达。[方法]应用RT—PCR的方法分别从5例胃癌组织标本中提取总RNA,进一步扩增出全长的AIF基因;将其克隆到DMD19-T载体进行测序,并亚克隆至真核表达载体pEGFP—N2,重组载体AIF—GFP采用脂质体法瞬时转染MGC-803细胞,荧光显微镜观察AIF—GFP在细胞中的表达：[结果]由5例胃癌组织标本提取的mRNA所扩增出的AIF基因的序列与GeneBank中AIF（AF100928）序列完全一致．胃癌组织中均未发现碱基突变;构建了真核表达载体AIF—GFP;并将其转染MGC-803进行表达。[结论]胃癌中AIF基因未发生碱基突变;AIF-GFP真核表达载体,在MGC-803细胞中获得了表达,为进一步探讨AIF在胃癌诊治中的应用奠定了基础。     

ENU诱变在功能基因组研究中的应用

ENU诱变是近年来发展起来的研究功能基因的新手段.由于ENU处理后雄鼠精子基因组发生点突变,使得后代小鼠有可能出现突变表型,经筛选及遗传试验能够得到突变系小鼠.通过对突变小鼠的深入研究、对突变基因定位及位置候选法克隆突变碱基就会得到突变基因的功能信息.诱变试验可以采用不同的策略,结合近年来的研究工作,对实验规模的控制、小鼠交配程序及管理、突变表型的价值及筛查的方法等问题进行了探讨.ENU诱变能够获得功能基因研究的新材料及人类遗传性疾病的新模型,这种"表型驱动"的研究方式有可能会成为功能基因组研究最有前景的手段和捷径.     

14株肿瘤细胞P53基因突变的检测

目的：检测14株肿瘤细胞P53基因的突变情况。方法：根据P53基因5～8外显子之间的内含子序列设计引物,分别对人肺癌、脑胶质瘤、肝癌、胃癌、前列腺癌、乳腺癌、结肠癌、脉络膜恶性黑素瘤、视网膜成细胞瘤等9种肿瘤14株细胞的P53基因5、6、7、8的外显子进行PCR扩增反应。琼脂糖凝胶电泳鉴定PCR产物;DNA测序,并与正常的DNA序列比较;抽提肿瘤细胞蛋白进行Western blotting检测P53蛋白的表达。结果：14株肿瘤细胞P53基因热变区5、6、7、8外显子的扩增产物经电泳鉴定与预期相同。DNA测序结果表明,14株肿瘤细胞中8株存在P53基因5～8外显子的突变,其中肺癌细胞H1299、肝癌细胞Hep3B、肝癌细胞SMMC7721、脉络膜恶性黑素瘤细胞OCM-1检测到以前未报道过的突变;突变主要发生在外显子的编码序列,多数为单个碱基替换导致的错义突变,也有部分表现为同义突变;2株正常细胞未检测到突变。Western blotting检测显示,有基因突变的8株肿瘤细胞中仅6株有P53蛋白表达;所有P53基因未突变的肿瘤细胞株和正常细胞株均未检测到P53蛋白。结论：检测的14株肿瘤细胞中有8株细胞P53基因热变区存在突变,其中4株检测到以前未报道过的突变;2株正常细胞未发现P53基因突变。     

Von Hippel-Lindau抑癌基因在散发性肾透明细胞癌中的突变及其意义

背景与目的：von Hippel-Lindau（VHL）基因是一种抑癌基因,其突变在肾透明细胞癌的发生发展中发挥重要的作用.本研究旨在了解VHL抑癌基因在中国人散发性.肾透明细胞癌中的突变特点,并探讨基因突变与肿瘤分期、病理分级等临床病理特征的关系.方法：应用聚合酶链反应（PCR）、双向测序方法检测72例散发性肾透明细胞癌患者肾癌组织及正常肾组织中VHL抑癌基因的突变情况.结果：72例肿瘤组织中检测出VHL抑癌基因突变25例,突变率为35%.其中移码突变13例,错义突变8例,无义突变3例,同义突变l例.第1外显子6例,第2外显子5例,第3外显子12例,内含子区2例.25例突变中,6例突变位于密码子157～166区域.VHL抑癌基因突变与肿瘤分期、分级等临床病理指标无关（P＞0.05）.结论：中国人散发性肾透明细胞癌中VHL抑癌基因突变率较低,突变主要位于第3外显子,该外显子区可能存在一个突变热点区域.VHL抑癌基因突变与肿瘤分期、分级等临床病理指标无关,是肾透明细胞癌发生发展的早期遗传事件.     

肝癌中具有高突变率位于染色体17p13.3区的新报癌基因

目的：以往研究表明肝癌中染色体17p13.3区有高频率的杂合性缺失，其最小杂合性缺失范围已被确定在D17S643至D17S1574位点间，而且其中的D17S926位点有最高的杂合性缺失率，含有该位点的基因组克隆P579已被测序分析，在P579范围共有13个新基因，这里报告其中的一个新基因（命名为肝癌抑癌基因1，HCCS1）的克隆和特性研究结果。方法：利用直接杂交筛选方法获得基因组克隆P579中的基因克隆，根据HCCS1基因克隆的cDNA序列与基因组序列进行比较确定基因的外显子与内含子，应用RT－PCR扩增组织中的HCCS1基因，序列测定检查突变，应用免疫组化检测HCCS1在组织中的表达，应用克隆形成试验和裸鼠成瘤试验检测HCCS1的生物学功能。结果：HCCS1有18个外显子，cDNA全长约2．0kb，蛋白产物定位于线粒体，HCCS1在肝癌细胞中有高频率的突变，免疫组化检测表明HCCS1在癌旁组织的表达明显高于癌组织，HCCS1转染肝癌细胞明显抑制其克隆的形成及在裸鼠体内的成瘤，结论：上述发现表明HCCS1具有肝癌抑癌基因的作用。     

非小细胞肺癌中EGFR和K-ras基因突变检测

目的:探讨不同类型非小细胞肺癌的EGFR和K-ras基因突变情况及其与肺癌相关临床病理特征的关系。方法:用厦门艾德ADxARMS试剂盒进行98例非小细胞肺癌患者肿瘤组织中EGFR（18,19,20,21外显子）基因和K-ras（12,13,61密码子）基因突变的检测。所有患者均未接受过吉非替尼的治疗。结果:98例样本中31例发生了EGFR基因突变,突变率为31.6%（31/98）,其中15例为19外显子缺失,13例为21 L858R外显子点突变,3例为20外显子突变,1例为18外显子突变。其中1例既有19外显子缺失突变,又有20外显子突变。腺癌中EGFR基因突变率较鳞癌、腺鳞癌、大细胞癌高。女性患者EGFR基因突变率较男性高。不吸烟患者EGFR基因突变率较吸烟患者高。低分化腺癌患者EGFR基因突变率较中、高分化患者高。21例发生了K-ras基因突变（21.4%）,其中12、13、61密码子均发现突变。突变率腺癌较鳞癌、腺鳞癌、大细胞癌高,与是否吸烟、患者性别、分化程度均无相关性。结论:非小细胞肺癌患者EGFR基因突变检出率较高,K-ras基因突变率较低,且两者不存在同时突变,EGFR基因突变与肺癌组织学类型、分化程度、性别等相关。K-ras基因突变与组织学类型相关。     

53例国人肾透明细胞癌中PBRM 1基因的突变分析

目的 分析Polybromo 1（PBRM 1）基因在国人肾透明细胞癌中的突变特点,探讨基因突变与临床病理特征和预后的关系。方法 选取53例肾透明细胞癌患者,利用PCR结合直接测序的方法检测PBRM 1基因2～5号外显子、15～17号外显子的突变情况。分析PBRM 1基因突变与临床病理特征和预后的相关性。结果 53例样本中检测出PBRM 1基因突变14例,突变率为26％。突变较集中分布在第15、17外显子,第15外显子5例,第17外显子6例。PBRM 1基因突变组和无突变组的中位无进展生存期分别为5个月（95％CI：1.6～8.4个月）和14个月（95％CI：9.7～18.3个月）,中位生存期分别为7个月（95％CI：2.6～11.4个月）和18个月（95％CI：5.6～30.4个月）,差异均有统计学意义（P＜0.05）。 PBRM 1基因突变与性别、年龄、发病部位、分期及疾病控制率等无关（P＞0.05）。结论 中国人肾透明细胞癌中PBRM 1基因突变率较高,突变主要位于第15、17外显子。PBRM 1基因突变可能是肾透明细胞癌患者预后不良的危险因素。     

非小细胞肺癌患者DNA修复基因XPD多态性与p53基因突变的相关性研究

[目的]研究非小细胞肺癌(NSCLC)患者DNA修复基因XPD多态性与p53基因突变的相关性。[方法]以PCR-RFLP方法分析60例NSCLC患者外周血XPD基因Lys751Gln多态;运用PCR-SCCP银染结合DNA序列分析法检测癌组织p53基因第5、6、7和8外显子突变情况。[结果]19例患者至少携带1个XPD751Gln等位基因;25例发生p53基因突变,突变率为42%;携带至少1个XPD751Gln等位基因的患者中p53基因突变率为52.6%,XPD751野生基因型中突变率为36.6%(P=0.27)。所有p53基因突变患者DNA序列分析结果显示,颠换27个,转换12个;突变类型分析显示,携带至少1个XPD751Gln等位基因的患者颠换发生明显占优势,颠换∶转换为14∶2,XPD751野生基因型颠换∶转换为13∶10(P=0.08)。[结论]NSCLC患者中DNA修复基因XPD多态性与p53基因突变发生无明显相关性。     

中国非小细胞肺癌患者K-Ras和EGFR基因突变与临床病理特征的关系

目的 探讨中国非小细胞肺癌（NSCLC）患者中K-Ras和表皮生长因子受体（EGFR）基因突变情况及其与临床病理特征的关系。方法 回顾性分析2011年7月至2013年8月广州医科大学附属第一医院收治的381例NSCLC患者的临床病理特征,并应用扩增突变阻滞系统（ARMS）检测其癌组织中EGFR基因18、19、20、21外显子共21个点突变和K-Ras基因12、13密码子共6个点突变,分析其突变情况及与临床病理特征的相关性。结果 21例（5.5％）存在K-Ras基因突变,其中20例12密码子,1例13密码子Asp突变;146例（38.3％）存在EGFR突变,其中4例18外显子突变（G719S）,52例19号外显子序列缺失突变,3例20外显子序列缺失突变,85例21外显子突变（81例L858R,4例L861Q）,2例双突变。男性患者K-Ras基因突变率高于女性患者,差异有统计学意义（6.8％ vs. 2.5％, P=0.018）。EGFR基因突变与性别、吸烟史、临床分期、全身转移、病理类型均有关（P＜0.05）。二分类Logistic回归分析显示,病理类型和性别与EGFR基因突变密切相关。结论 中国NSCLC患者中EGFR突变常见,该突变与腺癌有关;K-Ras基因突变率较低,多见于男性,其他相关因素尚需进一步研究。     

Differential locus sensitivity to mutation induction by ionizing radiations of different LETs in Chinese hamster ovary K1 cells

Mutation induction after exposures to 250 kVp X-rays, alpha-particles from the radon daughter 212Bi, and fission-spectrum neutrons from the JANUS reactor was studied in Chinese hamster ovary (CHO) K1 cells and in CHO-10T5, a K1 derivative containing the bacterial gene xanthine-guanine phosphoribosyl transferase (gpt). Mutation induction was analyzed at three genetic loci: the gpt locus, the hypoxanthine-guanine phosphoribosyl transferase (hprt) locus, and the thymidine kinase (tk) locus. After X-irradiation, mutants were induced at the tk loci at approximately 8-9 times the rate of mutant induction at the hprt locus, and the rate of mutant induction at the gpt locus was 8-10 times greater than that at the hprt locus. Neutron and alpha-radiation were more effective mutagenic agents. Mutant frequencies were approximately 4- to 6-fold higher than for X-rays at the hprt and gpt loci and greater than 12-fold greater than X-rays at the tk locus. The greater sensitivity of the tk locus to mutation induction by ionizing radiation (especially neutron and alpha-particle radiation) compared to the hprt locus is likely to be due to the recovery of an additional class of mutants, possibly ones containing larger-sized mutational events. Approximately half of the X-ray-induced tk-1- mutants were small-colony mutants, and 75% of the alpha- and neutron-induced tk-1- mutants were small-colony mutants. The increase in the proportion of small-colony mutants seen with increasing radiation linear energy transfer (LET) suggests that the radiation quality influenced the type of mutation recovered at this locus. There is probably a different reason for the hypersensitivity of the gpt locus because the frequency of gpt mutants, compared to the hprt locus, was independent of radiation quality. Therefore, the LET dependence of mutant induction is gene specific and not necessarily related to the size of deletion recoverable.     

甲氧化三丁基锡的急性毒性和遗传毒性研究

目的:研究甲氧化三丁基锡(tributyltin methoxide,TBTMO)对小鼠的急性毒性和遗传毒性。方法:选取昆明种小鼠110只,随机分为11组(TBTMO 420、286、194、180、132、90、73、61、41.5、28 mg/kg及阴性对照组),一次性灌胃给药进行急性毒性测试;另取昆明种小鼠50只随机分成5组(TBTMO 80、40、20 mg/kg组,阴性对照组和阳性对照组),各剂量组和阴性对照组均连续给药2 d,阳性对照组连续给药5 d,均连续观察35d,处死小鼠取双侧附睾制片进行精子畸形试验;并取昆明种小鼠50只随机分成5组(TBTMO 80、40、20 mg/kg组,阴性对照组和阳性对照组),灌胃给药30 h后处死小鼠取胸骨骨髓进行微核试验;取GFP转基因小鼠50只随机分成5组(TBTMO 80、40、20 mg/kg组,阴性对照组和阳性对照组),收集每只小鼠的脾淋巴细胞进行hprt基因突变试验。然后用不同浓度TBTMO(69.4、48.6、34.0、23.8、16.6、0μg/ml)分别处理中国仓鼠肺成纤维细胞(Chinesehamster lung,CHL)进行染色体畸变试验;用不同浓度TBTMO(每皿5×10~5、5×10~4、5×10~3、5×10~2、50、5、0.5、0.05、5×10~-3、5×10~-4μg)分别处理TA97、TA98、TA100、TA102 4种菌株进行Ames试验。结果:TBTMO对小鼠的半数致死量(LD_(50))为98 mg/kg。与阴性对照组比较,TBTMO可致小鼠精子畸形(P0.05或P0.01),且有剂量依赖趋势,其余4项遗传毒性试验结果均为阴性。结论:在本实验条件下,TBTMO对小鼠的急性经口毒性属于中等毒性,可能具有一定的遗传毒性。     

Carcinogen-induced lymphomagenesis in pim-1 transgenic mice: dose dependence and involvement of myc and ras

Transgenic mice overexpressing the pim-1 oncogene in their lymphoid compartments are predisposed to T-cell lymphomagenesis but only to the extent that approximately 10% of the transgenic mice develop lymphomas within 34 weeks after birth. Recently, we have shown that lymphomagenesis in pim-1 transgenic mice can be accelerated by infecting pim-1 transgenic mice with murine leukemia viruses or by treating the mice with a relatively low dose of 60 mg of the carcinogen N-ethyl-N-nitrosourea (ENU) per kg of body weight. Here we describe the incidence of tumors as a function of the dose of ENU. Either 200, 15, 4, 1, or 0.1 mg/kg ENU was injected into transgenic and control mice and the tumor incidence was monitored. T-cell lymphomas developed in 100 and 70% of the pim-1 transgenic mice treated with 200 and 15 mg/kg ENU, respectively. Approximately 20% of the Emu-pim-1 transgenic mice developed lymphomas after treatment with either 4, 1, or 0.1 mg/kg ENU. The nontransgenic mice developed lymphomas only after injection with 200 mg/kg (45%). The data show that Emu-pim-1 transgenic mice are approximately 25-fold more susceptible to ENU-induced lymphomagenesis than control mice. In most tumors the expression of c-myc was strongly elevated, probably as a direct or indirect effect of ENU. Analysis of the lymphomas for ras mutations revealed that approximately 10% of the lymphomas bear a ras mutation. The data indicate that at least some of these mutations are not the direct result of alkylation by ENU but rather represent spontaneous mutations that occurred later in the tumorigenic process.     

放射治疗对宫颈癌病人DNA损伤及hprt基因突变的初步研究

目的：评价放射治疗对宫颈癌患者造成的遗传学损伤。方法：取15例宫颈鳞癌放疗患者放疗前及放疗累积剂量为0Gy、10Gy、20Gy、30Gy、40Gy、50Gy、60Gy时的外周静脉血,以彗星实验检测淋巴细胞的DNA损伤,采用多核细胞法测hprt基因位点突变率。结果：各累积剂量组淋巴细胞DNA拖尾率比照射前显著升高（P〈0.01）,并且在0-60Gy之间,拖尾率与累积照射剂量之间成线性关系。各累积剂量组尾长比照射前均增加（P〈0.01）,在照射剂量累积30Gy时,尾长均值达最大。尾长与累积剂量间不成线性关系。hprt基因位点突变率在照射后升高,与照射前相比,在照射累积剂量为30Gy、40Gy和50Gy时,显示有统计学意义差别（P〈0.05）。hprt基因突变率与累积剂量间呈现较好的剂量-效应关系。结论：宫颈癌患者放疗后造成外周血淋巴细胞DNA损伤及hprt基因突变,hprt基因突变和彗星实验用于评价放疗造成的损伤,具有较高的敏感性。     

Different Mutation Frequencies and Spectra among Organs by N-Methyl-N-nitrosourea in rpsL (strA) Transgenic Mice

The frequencies and spectra of N -methyl- N -nitrosourea (MNU)-induced in vivo somatic mutations were determined in rpsL (strA) transgenic mice. The wild-type rpsL gene, which exhibits a streptomycin-sensitive (Sm^s) phenotype, was used as the rescue marker gene. Studies of mutation spectra among different organs and tissues were simplified using this system because of the short coding sequence (375 bp) of the rpsL gene. MNU administration to transgenic mice significantly elevated the mutation frequencies in various adult organs. Two distinctive patterns of mutation spectrum were observed, depending on the organs tested. Mutations derived from labile organs (spleen and thymus) were predominantly G:C to A:T transitions, as expected for MNU mutagenesis. Stable organs like the liver and brain, however, carried many fewer G:C to A:T transitions but significantly more single base deletions, of which the spectrum was very similar to that of background mutations in the rpsL transgenic mice. This spectrum difference among more and less proliferating organs was confirmed by the predominant occurrence of G:C to A:T transitions in fetal liver cells exposed to transplacental MNU treatment. In addition, most (approximately 90%) of the G:C to A:T transitions induced by MNU were detected in the first nucleotide of some 5'-G-(C or G)-3' sequences, many of which corresponded to the middle guanine residue of 5'-purine-G-(C or G)-3' sequences. It is thus suggested that at particular sites, the neighboring bases in both the 5' side and 3' side seem to influence either the susceptibility to DNA damage or the ability to repair MNUinduced lesions.     

Different mutation frequencies and spectra among organs by N-methyl-N-nitrosourea in rpsL (strA) transgenic mice.

The frequencies and spectra of N-methyl-N-nitrosourea (MNU)-induced in vivo somatic mutations were determined in rpsL (strA) transgenic mice. The wild-type rpsL gene, which exhibits a streptomycin-sensitive (Sm(S)) phenotype, was used as the rescue marker gene. Studies of mutation spectra among different organs and tissues were simplified using this system because of the short coding sequence (375 bp) of the rpsL gene. MNU administration to transgenic mice significantly elevated the mutation frequencies in various adult organs. Two distinctive patterns of mutation spectrum were observed, depending on the organs tested. Mutations derived from labile organs (spleen and thymus) were predominantly G:C to A:T transitions, as expected for MNU mutagenesis. Stable organs like the liver and brain, however, carried many fewer G:C to A:T transitions but significantly more single base deletions, of which the spectrum was very similar to that of background mutations in the rpsL transgenic mice. This spectrum difference among more and less proliferating organs was confirmed by the predominant occurrence of G:C to A:T transitions in fetal liver cells exposed to transplacental MNU treatment. In addition, most (approximately 90%) of the G:C to A:T transitions induced by MNU were detected in the first nucleotide of some 5'-G-(C or G)-3' sequences, many of which corresponded to the middle guanine residue of 5'-purine-G-(C or G)-3' sequences. It is thus suggested that at particular sites, the neighboring bases in both the 5' side and 3' side seem to influence either the susceptibility to DNA damage or the ability to repair MNU-induced lesions.     

27. Molecular spectra of acrylamide-induced mutation at hprt locus inhuman promyelocytic leukemia HL-60 and NB4. cell lines

The genotoxicity of acrylamide was investigated by methods of single cell clone culturing, two-way screening count, multiplex PCR amplification and electrophoresis technique. Acrylamide only showed clear mutagenesis until dose raised 700 mg*L-1 in HL-60 cells. The most frequent spontaneous mutation were point mutation (≥90.0%) and acrylamide-induced mutation mainly included partial deletion and point mutation (respectively 40.0%～66.7%/33.3%～60.0%). Total gene deletion wasn't discovered in both of cells. There were deletion mutation in all exons of hprt gene(except 7/8 exon), and toward the 3' end of the hprt gene. The most frequent acrylamide-induced mutation were point mutation and single exon deletion (93.3%/86.1%). There were not clear difference in both of cells. The results suggest that the spectra of spontaneous and acrylamide-induced mutants were different. and the smaller changes in genetic structure have something to do with mechanism.     

Tumors arising in DNA mismatch repair-deficient mice show a wide variation in mutation frequency as assessed by a transgenic reporter gene

We reported previously that thymic lymphomas arising in mice lacking the DNA mismatch repair (MMR) gene, Msh2(-/-), exhibited striking elevations in the mutation frequency of a transgenic lacI reporter gene when compared with normal Msh2(-/-) tissues. To investigate whether hypermutation was a feature of all tumors arising in MMR-deficient mice, lacI transgene mutation frequencies were obtained from several different mouse tumors deficient for PMS2 and/or MSH2. While lacI gene hypermutation was again clearly evident in Msh2 +/- ms2(-/-) and Msh2(-/-)Pms2(-/-) thymic lymphomas, three non-thymic MSH2-deficient tumors failed to show lacI gene mutation frequency elevations when compared with a normal tissue of MMR-deficient mice. The elevated mutation frequencies in the lymphoid tumors, and the finding of multiple clustered mutations in lacI genes rescued from these tumors, suggest that they are possibly generated by a lymphoma-specific hypermutational mechanism.     

DNA sequence analysis of 1-nitrosopyrene-induced mutations in the hprt gene of Chinese hamster ovary cells.

DNA sequence was determined in 21 mutants induced at the hprt locus of Chinese hamster ovary (CHO) cells by 1-nitrosopyrene, a metabolite of the tumorigenic environmental pollutant 1-nitropyrene. Following cDNA synthesis using RNA from each of the mutants, the hprt protein-coding region was amplified by the polymerase chain reaction (PCR) and subjected to direct DNA sequence analysis. Sixteen primary mutations were found: seven were G:C----T:A transversions, five were G:C----A:T transitions, two were single basepair insertions, one was a single basepair deletion, and one was a complex mutation involving substitutions at two A:T basepairs. The simple basepair substitution mutations preferentially occurred with one or two purines 3' to the mutated dG, and mutations in exons 1-4 disproportionately occurred with the mutated dG on the nontranscribed DNA strand. In addition, 12 of the mutants produced one or more cDNA PCR products with partial or complete exon deletions. Seven mutants with multiple PCR products had point mutations in one of the products; exon deletions in the other product(s) removed these point mutations. A group of solvent control mutants had a different distribution of basepair substitution mutations and a lower proportion of cDNAs with exon deletions than that found for the 1-nitrosopyrene-induced mutants. The results indicate a specificity for the induction of mutations in the hprt gene of CHO cells by 1-nitrosopyrene with respect to both the types of mutations produced and their location in the hprt gene. Also, the elimination of point mutations in many of the cDNA PCR products with exon deletions suggests that mutations in the protein-coding sequence affect hprt mRNA processing.