Fragile X screening program in New York State

Genetic and metabolic investigations for neurodevelopmental disorders: position statement of the Canadian College of Medical Geneticists (CCMG)

PURPOSE AND SCOPE

The aim of this position statement is to provide recommendations for clinicians regarding the use of genetic and metabolic investigations for patients with neurodevelopmental disorders (NDDs), specifically, patients with global developmental delay (GDD), intellectual disability (ID) and/or autism spectrum disorder (ASD). This document also provides guidance for primary care and non-genetics specialists caring for these patients while awaiting consultation with a clinical geneticist or metabolic specialist.

METHODS OF STATEMENT DEVELOPMENT

A multidisciplinary group reviewed existing literature and guidelines on the use of genetic and metabolic investigations for the diagnosis of NDDs and synthesised the evidence to make recommendations relevant to the Canadian context. The statement was circulated for comment to the Canadian College of Medical Geneticists (CCMG) membership-at-large and to the Canadian Pediatric Society (Mental Health and Developmental Disabilities Committee); following incorporation of feedback, it was approved by the CCMG Board of Directors on 1 September 2022.

RESULTS AND CONCLUSIONS

Chromosomal microarray is recommended as a first-tier test for patients with GDD, ID or ASD. Fragile X testing should also be done as a first-tier test when there are suggestive clinical features or family history. Metabolic investigations should be done if there are clinical features suggestive of an inherited metabolic disease, while the patient awaits consultation with a metabolic physician. Exome sequencing or a comprehensive gene panel is recommended as a second-tier test for patients with GDD or ID. Genetic testing is not recommended for patients with NDDs in the absence of GDD, ID or ASD, unless accompanied by clinical features suggestive of a syndromic aetiology or inherited metabolic disease.

Pre-conception and antenatal screening for the fragile site on the X-chromosome

BACKGROUND

Fragile X is the most common cause of mental retardation after Down syndrome. It is the commonest inherited cause of mental retardation, and results from a dynamic mutation in a gene on the long arm of the X chromosome. Various strategies are used for prenatal screening.

OBJECTIVES

To determine whether pre-conceptual or antenatal screening for Fragile X carrier status in apparently low risk women confers any additional benefit over the existing practice of offering testing to women thought to be at increased risk.

SEARCH STRATEGY

We searched the Cochrane Pregnancy and Childbirth Group trials register (November 2001), the Cochrane Controlled Trials Register (The Cochrane Library Issue 3, 2001), MEDLINE (1980 to 2001), and reference lists of articles.

SELECTION CRITERIA

Randomised clinical trials comparing women being tested regardless of family history (intervention group) with women tested only when there is a family history of either fragile X and/or other undiagnosed mental illness/impairment (control group).

DATA COLLECTION AND ANALYSIS

Three reviewers independently assessed trial quality and extracted data.

MAIN RESULTS

No trials were included.

REVIEWER'S CONCLUSIONS

No information is available from randomised trials to indicate whether routine pre-conceptual or antenatal screening for fragile X carrier status confers any benefit over testing women thought to be at increased risk.

Fragile-X carrier screening and the prevalence of premutation and full-mutation carriers in Israel

Fragile-X syndrome is caused by an unstable CGG trinucleotide repeat in the FMR1 gene at Xq27. Intermediate alleles (51-200 repeats) can undergo expansion to the full mutation on transmission from mother to offspring. To evaluate the effectiveness of a fragile-X carrier-screening program, we tested 14,334 Israeli women of child-bearing age for fragile-X carrier status between 1992 and 2000. These women were either preconceptional or pregnant and had no family history of mental retardation. All those found to be carriers of premutation or full-mutation alleles were offered genetic counseling and also prenatal diagnosis, if applicable. We identified 207 carriers of an allele with >50 repeats, representing a prevalence of 1:69. There were 127 carriers with >54 repeats, representing a prevalence of 1:113. Three asymptomatic women carried the fully mutated allele. Among the premutation and full-mutation carriers, 177 prenatal diagnoses were performed. Expansion occurred in 30 fetuses, 5 of which had an expansion to the full mutation. On the basis of these results, the expected number of avoided patients born to women identified as carriers, the cost of the test in this study (U.S. $100), and the cost of lifetime care for a mentally retarded person (>$350,000), screening was calculated to be cost-effective. Because of the high prevalence of fragile-X premutation or full-mutation alleles, even in the general population, and because of the cost-effectiveness of the program, we recommend that screening to identify female carriers should be carried out on a wide scale.

Prenatal diagnosis of fragile X syndrome and the risk of expansion of a premutation

The aim of the present study was to evaluate prospectively the dynamics of the FMR1 gene. The risk of full mutation among pregnant women and the carriers, and the risk of expansion of a premutation allele to a full mutation were estimated. We identified 89 pregnant women with an expanded FMR1 gene seeking prenatal diagnosis. Amniocentesis or chorion villus sampling (CVS) was offered and a DNA test of the FMR1 gene was carried out in such pregnancies. The overall risk of full mutation among women (N = 21) with a repeat size between 60 and 80 was 4.8% (one fetus with mosaicism), and the risk of expansion of the premutation allele to a full mutation was 14% in those offspring to whom the premutation allele was transmitted. The risk of full mutation among the carriers (N = 13) with a repeat size between 81 and 100 was 61.5% (8/13), and the risk of expansion of a premutation allele to a full mutation was 89%. Only one case fell into the category of 101-200 repeats, and expansion to a full mutation was recorded. Fetuses of full mutation mothers inherited the larger allele in 64% (14/22) of the cases. The range of 40-59 repeats was safe: there were no fetal full mutations. The risk of full mutation was also low among the subjects with a repeat size between 60 and 80, whereas the risk increased significantly after 80 repeats. Maternal premutation size was positively correlated with the risk of having a full mutation offspring.

Physical characteristics of young boys with fragile X syndrome: reasons for difficulties in making a diagnosis in young males

Fragile X syndrome is the leading form of hereditary mental retardation, but the condition is still underdiagnosed in young children. Because of concern that the fragile X phenotype is subtle in young boys and therefore contributes to underdiagnosis of the disorder, we evaluated 73 boys (36 with fragile X and 37 same-age boys who were fragile X negative) using a checklist that we devised to learn which characteristics might be the most useful for alerting professionals to this diagnosis. After a multiple comparisons adjustment, only 4 of 42 characteristics differed significantly in their distributions between the two groups of boys (P < 0.0012), but 10 other items may also have predictive value for fragile X syndrome (P < 0.01). Four additional items occurred in at least 80% of boys with fragile X and may also be helpful for the clinician. Professionals who work with developmentally delayed children should be aware of these 18 clinical characteristics and some of the behavior characteristics commonly seen in boys with fragile X so that they can readily diagnose patients.

Fragile X syndrome. Clinical and cytogenetic studies

Three families with the fragile X syndrome were studied with the aim to establish the most frequent clinical signs in the affected individuals and heterozygous women. The clinical evaluation, IQ level measurements and cytogenetic studies were performed in 40 subjects, 20 males and 20 females. The fragile X diagnosis was confirmed in all the male individuals with mental retardation. In the postpubertal subjects the most frequent clinical signs were inner canthal distance < 3.5 cm, macro-orchidism, long and narrow face and high arched palate while in the prepubertal subjects the behavioral characteristics as hyperactivity and poor eye contact were the most frequent and were observed in all patients. Twenty six percent of the heterozygous women presented with mental retardation and showed clinical signs rather than behavioral ones. All male individuals with mental retardation were observed as having fragile X [fra(X)] in lymphocytes culture. Sixty three percent of women showed fra(X). There was a positive correlation between the frequency of fra(X) and the clinical characteristics. We emphasize the importance of the clinical evaluation in the study of familial mental retardation and in the screening of isolated cases with suspect of having the fragile X syndrome.

Applicability of a checklist for clinical screening of the fragile X syndrome

In a population of 340,000 in Southern Häme, Finland, there were 541 intellectually disabled adult males (> 16 years) known to the District Organisation for the Care of the Mentally Retarded in August 1993. Of these, 197 already had a confirmed etiological diagnosis, with 20 having the fragile X syndrome. The other 344 males were screened for the fragile X syndrome using a three-step method: a clinical checklist used by a specialist nurse, a clinical examination by a physician who was very familiar with the fragile X syndrome, and the FRAXA-locus gene test. Six new fragile X males were found. The minimum prevalence of the fragile X syndrome in the district was calculated to be 1:4400.

A simplified six-item checklist for screening for fragile X syndrome in the pediatric population

OBJECTIVE

In our general experience, about 2% of samples referred for fragile X testing showed positive results on Southern blot analysis. The aim of this project was to determine whether screening criteria could be developed to increase the proportion of positive test results without sacrificing sensitivity.

STUDY DESIGN

We retrospectively analyzed nine clinical characteristics from patient records of 273 male and 62 female pediatric probands (average age, 5.7 years) referred for fragile X testing. The characteristics included mental retardation, family history of mental retardation, large or prominent ears, elongated face, attention deficit hyperactivity disorder, autistic-like behavior, simian crease, macroorchidism, and hyperextensible joints. These were scored as 2 if present, 1 If borderline present, and 0 if absent.

RESULTS

Analysis of the nine characteristics identified three (simian crease, macroorchidism, and hyperextensible joints) with low frequency and statistical insignificance, which were therefore eliminated. With the use of the remaining six characteristics, If a score of 5 or more was used as the criterion for requesting fragle X testing, then close to 60% of those tests from our patient population could have been eliminated without missing any positive cases. The validity of our threshold score of 5 was subsequently confirmed among an additional six cases of fragile X syndrome.

CONCLUSION

With our simplified six-item clinical checklist, 60% of testing could have been eliminated, thereby improving the cost-effectiveness of fragile X testing and increasing the proportion of cases with positive results by threefold.

Carrier diagnosis of the fragile X syndrome--a challenge in antenatal clinics

OBJECTIVE

The fragile X syndrome, a common cause of mental retardation, is poorly recognized even in families at risk. The aims of our study were to evaluate the possibility of finding previously unidentified carriers of the genetic defect in fragile X families, to use this information in antenatal diagnosis, and to study the attitudes of these families to genetic screening.

STUDY DESIGN

We identified 59 fragile X families living in a population of 900,000 inhabitants. A deoxyribonucleic acid test on the FMR1 gene was offered to 1071 persons in these families who had a risk of at least 12.5% of having the fragile X premutation or full mutation.

RESULTS

A total of 48.1% of the persons who were offered the test accepted it. A diagnosis was made in 20 male and 66 female subjects with the full mutation and in 30 male and 133 female subjects with a premutation. All 21 pregnant carriers of this mutation accepted chorionic villus biopsy.

CONCLUSION

Pregnant relatives should be informed of the availability of screening for fragile X carrier status in families with a member having clinical fragile X syndrome. Antenatal clinics offer a good gateway for approaching families with this inherited developmental defect.

Evaluation of school children at high risk for fragile X syndrome utilizing buccal cell FMR-1 testing

We describe a pilot project utilizing saliva to identify the FMR-1 mutation in high-risk special education students from four public school districts in Colorado. The program included presentations to special education teachers regarding fragile X syndrome, parental consent for testing, completion of a behavior checklist by the teachers, identification of special education students at high risk for fragile X syndrome, subsequent brief examination of face and hands, collection of a saliva sample by either Gatorade swish or brushing of the inside of the cheek, and analysis for the FMR-1 mutation by PCR. Equivocal samples were studied by direct DNA testing using Southern blot analysis, and abnormal results were confirmed by a blood analysis for the FMR-1 mutation. Mutant individuals received genetic counseling and medical and educational assessments to optimize treatment and intervention. This pilot project was met with enthusiasm by the schools. Of the first 439 students evaluated, 68% were male with an average age of 7.75 years; 13% were mentally retarded or autistic. Most students referred for the evaluation were learning disabled (51%) and/or had an Attention Deficit Hyperactivity Disorder (ADHD) (35%). The overall prevalence of the FMR-1 mutation was 5 of 439 or 1.1%. This relatively low yield is probably due to the high number of non-retarded but learning disabled students tested. Of the mentally retarded patients tested, 3.5% were positive for the FMR-1 mutation; however, of the non-retarded or learning disabled patients, only 0.79% were FMR-1 positive.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of the child with idiopathic mental retardation

A review of the advances in diagnostic techniques for evaluation of children with idiopathic mental retardation is presented. The current status of the use of clinical genetics, cytogenetics, molecular genetics, and neuroimaging in evaluating children with mental retardation is emphasized. Special attention is given to the evaluation of children with mental retardation and "autism" or "cerebral palsy."

New York State screening program for fragile X syndrome: a progress report

New York State has established a program to screen post-pubertal mentally retarded males for the fragile X [fra(X)] syndrome. The goal of the program is to identify affected males and inform their families of the diagnosis. Females in these families who are at risk for inheriting the mutation will then be able to determine their carrier status and consider that information in making reproductive decisions. Males were evaluated for 10 features of the syndrome by physicians and nurses throughout the state; cytogenetic analysis was carried out on a subset of this population. A total of 1332 males has been screened and chromosome studies have been completed for 489. Forty-three (9%) were positive for fra(X), and an additional 11 other chromosome abnormalities were identified. The 43 patients belonged to 38 families. Of the 24 families who were informed of the diagnosis, 12 consulted genetic counseling centers for follow-up studies and 12 did not.

Fragile X screening program in a Spanish region

In a Spanish region with a population of one million, we screened 371 mentally retarded males, who had no previous diagnosis for fragile X [fra(X))] syndrome. Fifty-three of the 371 males were fra(X) positive. Of these 44 of 362 or 12.1% were unrelated. Family studies identified a large number of obligate carriers and women at risk for being carriers who were given genetic counseling including prenatal diagnostic information. Considering the age of the carriers and the fertility rate, 23 affected males could be born to these women. The prevention potential of this program suggests that it is highly cost-effective.

Fra(X)(q27.2), the common fragile site, observed in only one of 760 cases studied for the fragile X syndrome

Cell cultures from 760 whole blood, amniotic fluid, chorionic villus sample, and peripheral umbilical blood sample specimens were exposed to multiple fra(X)(q27.3) induction systems (none had aphidicolin). Fifty-three exhibited the rare fragile site, fra(X)(q27.3) or FRAXA, none of which demonstrated the common fragile site or FRAXD at band Xq27.2. Only one cell in one of the negative whole blood FUdR-treated cultures from a mentally retarded male showed FRAXD. Therefore, it appears that FRAXD occurs very rarely in cultures treated to induce FRAXA since only one positive cell was observed in over 88,000 analyzed. It appears that very low frequencies of fra(X)(q27) can be accounted for only in part by the presence of the common fragile site since only one of 9 cases, each with one fra(X)(q27) positive cell, exhibited FRAXD and the others were FRAXA. After confirmation of FRAXA with direct DNA testing in a large number of low frequency cases, it should be possible to rely on the detection of very low frequencies of fra(X)(q27.3), e.g., 1% with at least 2 positive cells.