In 1959, Lejeune, Gautier, and Turpin discovered the association between Down’s syndrome and a third chromosome 21 [1] . Down syndrome (DS or DNS), also known as trisomy 21, is a genetic disorder caused by the presence of all, or part of a third copy of chromosome 21 [2] . It is typically associated with physical growth delays, characteristic facial features, and mild to moderate intellectual disability‎ [3] . For example, the average IQ of a young adult with Down syndrome is 50 which is the same as mental age of an 8- or 9-year-old child, but this can vary widely. There is no known behavior or environmental factor that changes the risk. We can diagnose Down syndrome by prenatal tests and by direct observation and genetic testing after birth. The diagnosis of Down’s syndrome is made by chromosome analysis, which can be initiated prenatally due to identified risk factors, or postnatally due to the characteristic appearance of the infant‎ [4] . Prenatal diagnosis for chromosomal anomalies was first introduced in the 1970s, and was initially restricted to amniocentesis in the second trimester [5] . Down syndrome can be identified during pregnancy by prenatal screening followed by diagnostic testing, or after birth by direct observation and genetic testing‎ [2] . Research efforts now focus on improvement of the sensitivity and specificity of screening, to reduce or eliminate the number of women needing an invasive diagnostic test, such as chorionic-villus sampling or amniocentesis‎ [1] . Use of fetal cells in the maternal circulation for prenatal diagnosis could eliminate the need for amniocentesis in diagnostic testing, however, isolation of fetal cells from maternal blood is still associated with several technical and biological difficulties‎ [6] . Down syndrome is caused by having three copies of the genes on chromosome 21 [7] ‎. Approximately 90% - 95% have free trisomy 21’, and in about 95% of these the extra chromosome is of maternal origin, as determined by DNA-poly- morphism analysis‎ [8] . Among maternal errors, about 75% are a result of non-dis- junction in the first meiotic division and 25% of non-disjunction in the second meiotic division of the oocyte [9] . We still know little about the causes of non-disjunction that lead to Down’s syndrome. Calculation of the frequency of Down’s syndrome depends on whether maternal age, gestational timing of diagnosis, and case loss due to prenatal diagnosis and termination of pregnancy are taken into account‎ [10] . Maternal age affects the chances of having a pregnancy with Down syndrome and advanced maternal age remains the only well documented risk factor in DS‎ [11] . There is no clear evidence of paternal age effect. The number of terminated pregnancies with Down’s syndrome has increased, and the prevalence of Down’s syndrome births has decreased from one in 700 to about one in 1000 [12] . Many (15%) who live 40 years or longer develop Alzheimer disease‎ [13] . Apolipoprotein E (apoE) is a plasma protein involved in cholesterol transport and metabolism. The apoE gene is located on chromosome 19, and the three most common alleles are e2, e3, and E4 [14] . The apoE allele e4 has been identified as a risk factor for early-onset and late-onset Alzheimer’s disease (AD) in both familial and sporadic cases [15] [16] . Increased frequency of Down syndrome (DS) births in the families of individuals with AD and increased frequency of AD in relatives of DS probands suggest a shared genetic susceptibility to DS and AD‎ [17] . An increased risk of AD in mothers of adults with Down syndrome (DS) has been reported. A shared genetic susceptibility to DS and AD leading to an accelerated ageing process had been hypothesized. Subsequently, an increased e4 allele frequency has been found in mothers (£32 years of age) of DS children arising from a meiosis II (MII) error in the Danish population‎ [18] . We hypothesized that young mothers of DS children could have an increased frequency of apoE allele E4, and studied apoE allele distribution in cases of trisomy 21 and their parents. Wehypothesized that young mother of DS children could have an increased frequency of Apolipoprotein E (apoE) allele E4. Apolipoprotein E4 allele distribution in parents of Down’s syndrome children as a risk factor in KhorasanRazaviprovince, Iran was studied in this paper. The present study is to investigate association of 112 codon of APOE gene C/T and 158 codon of APOE gene C/T with Down syndrome in population of Northeast Iran (Khorasan Razavi).

NCBI SNP database (dbSNP) was explored to discover frequency of these polymorphisms (112 codon of APOE gene C/T and 158 codon of APOE gene C/T) (http://www.ncbi.nlm.nih.gov/projects/SNP). No frequency data was registered for these variations in dbSNP. To determine power of study, information about frequency is essentials. There for 99 individuals (33 families with Down syndrome “trio study”) with Persian origin from Northeast of Iran was selected and genotyped for mentioned SNPs in order to find out allele frequency in our population. 33 mothers, and their corresponding fathers and DS children (due to free trisomy 21) from the population of North-east of Iran have been included in this study. All samples were collected after informed consent during the period between 2013 and 2015.Basic demographic data of these participators are about age, sex, race. The average age of Down syndrome children was 6 years (range 0 - 35). Average maternal age was 33.2 years. Inclusion criteria is thatall patients with Down syndrome who are already using Karyotyping for detecting trisomy 21 has been confirmed in them. In addition, 270 subjects from the general population (paternity testing cases with an a Exclusion criteria is all patients who do not have Down syndrome or, there is no evidence of a definite diagnosis. verage age of 36 years; 90 males, 90 females and 90 children) chosen as control group. Control individuals have no clinical evidence for Down syndrome. The study received the approval of the hospital Ethics Research Committee and all the investigated participants provided informed consent before sampling.

Allele frequencies for 112 and 158 codons obtained from 99 individuals in case group and 270 individuals in control group Persians of northeastern part of Iran. 99 cases and 270 controls were genotyped for SNPs loci 112 and 158 codon of APOE gene by ARMS- PCR assay. Genotype and allele frequencies in both groups of patient and controls were compared separatelyas shown in Table 4 and Table 5. In 112 codon of APOE gene it seems having T allele reduces the risk of nondisjunction and in contrast C allele may be a risk factor in happening of nondisjunction. (p-value = 0.000006, OR = 2.66, 95% CI = 1.74 - 4.06) (Table 6). In 158 codon of APOE gene it seems having T allele reduces the risk of nondisjunction and in contrast C allele may be a risk factor in happening of nondisjunction. (p-value = 0.00 OR = 3.89, 95% CI = 2.38 - 6.34) (Table 7). E4 allele frequency in mothers of Down syndrome is about 14 percent more than those in

control group as shown in Table 8 and Table 9. According to results of the present study, 112 codon of APOE gene and 158 codon of APOE gene can be considered as susceptibility genetic factors for nondisjunction in Northeast of Iran. E4 allele has main role in nondisjunction.