High Frequency of Copy Number Variations and Sequence Variants at CYP21A2 Locus: Implication for the Genetic Diagnosis of 21-Hydroxylase Deficiency

Abstract

BackgroundThe systematic study of the human genome indicates that the inter-individual variability is greater than expected and it is not only related to sequence polymorphisms but also to gene copy number variants (CNVs). Congenital Adrenal Hyperplasia due to 21-hydroxylase deficiency (21OHD) is the most common autosomal recessive disorder with a carrier frequency of 1∶25 to 1∶10. The gene that encodes 21-hydroxylase enzyme, CYP21A2, is considered to be one of the most polymorphic human genes. Copy number variations, such as deletions, which are severe mutations common in 21OHD patients, or gene duplications, which have been reported as rare events, have also been described. The correct characterization of 21OHD alleles is important for disease carrier detection and genetic counsellingMethodology and Findings CYP21A2 genotyping by sequencing has been performed in a random sample of the Spanish population, where 144 individuals recruited from university students and employees of the hospital were studied. The frequency of CYP21A2 mutated alleles in our sample was 15.3% (77.3% were mild mutations, 9% were severe mutations and 13.6% were novel variants). Gene dosage assessment was also performed when CYP21A2 gene duplication was suspected. This analysis showed that 7% of individuals bore a chromosome with a duplicated CYP21A2 gene, where one of the copies was mutated.ConclusionsAs far as we know, the present study has shown the highest frequency of 21OHD carriers reported by a genotyping analysis. In addition, a high frequency of alleles with CYP21A2 duplications, which could be misinterpreted as 21OHD alleles, was found. Moreover, a high frequency of novel genetic variations with an unknown effect on 21-hydroxylase activity was also found. The high frequency of gene duplications, as well as novel variations, should be considered since they have an important involvement in carrier testing and genetic counseling.

Mutation Analysis of theCYP21A2Gene in the Iranian Population

“…Gene duplication was responsible for about 5% (4/88 allele) of the mutations, which could also be considered common in comparison with other mutations. This kind of gene lesion was a higher frequency than in the Netherlands (Koppens et al, 2002) and was similar to values published in studies from Spain (Parajes et al, 2008). A higher duplicated version of the pseudogene (only upstream of the CYP21A1P was available) as compared with the CYP21A2 gene was observed in our study, similar to a study by Lobato et al (1998).…”

“…About 20%-25% of the recombinations are due to unequal crossing over, causing large gene conversions (GCs), duplications, and deletions (Werkmeister et al, 1986). In fact, unequal crossing over may generate duplication (Koppens et al, 2002); there is variability of gene duplication frequency depending on the population ethnicity (Haglund-Stengler et al, 1991;Wedell et al, 1994;Lobato et al, 1998;Parajes et al, 2008). Rare mutations account for *5% of the disorder and for mechanisms other than GC (White and Speiser, 2000).…”

The prevalence of the non-classical form of congenital adrenal hyperplasia as exemplified (by the population of the Moscow region)

“…В 2008 г. изучение распространенности НК21ОН продолжила группа авторов из Испании, которые провели молекулярно-генетическое исследование (секвенирование) образцов крови 144 добровольцев (95 женщин, 49 мужчин), в ходе которого мутации, характерные для НК21ОН, были выявлены в 17 из 144 образцов; частота носительства составила 12% или 1:8, а распространенность заболевания, вычис-ленная по закону Харди-Вайнберга, -1:255. Важ-но отметить, что в 16 (94%) из 17 образцов была вы-явлена мутация V281L [19].…”

“…[17]. Ни в одном из 938 образцов носи-тельства мутации P30L обнаружено не было, что, в соответствии с данными трех зарубежных исследо-ваний, описанных выше, доказывает, что частота встречаемости мутации V281L значительно превы-шает частоту встречаемости мутации P30L [17][18][19].…”

Problems of neonatal screening on congenital adrenal hyperplasia

“…Òàê, äëÿ ñ³ëü-âòðàòíî¿ ôîðìè õàðàêòåðíîþ º íàÿâ-í³ñòü äåëåö³é ³ çíà÷íèõ ãåííèõ êîíâåðñ³é (Q318X, R356W, 12 spl), äëÿ â³ðèëüíî¿ ôîðìè -òî÷êîâî¿ ìóòàö³¿ I172N. Òî÷êîâ³ ìóòàö³¿ V281L, P30L, P453S º õàðàêòåðíèìè òàêîae äëÿ íåêëàñè÷íî¿ ôîðìè ÂÃÊÍÇ [1,2,3,5,9,10,22,23,35,45]. Àëå ç îãëÿäó íà âåëèêó ê³ëüê³ñòü ôåðìåíò³â, ùî áå-ðóòü ó÷àñòü ó ñòåðî¿äî´åíåç³, çíà÷íó ê³ëüê³ñòü âàð³àíò³â ìóòàö³é òà ¿õ êîìá³íàö³é íå äèâíî, ùî ó ñâ³ò³ êîaeí³ 2-3 ðîêè â÷åí³ â³äêðèâàþòü íîâ³ ãåíí³ ìóòàö³¿, ÿê³ ïðèçâîäÿòü äî óðàaeåííÿ êîðè ÍÇ ³ç ðîçâèòêîì ð³çíèõ ôîðì ¿¿ äèñôóíêö³¿.…”